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THE ADVANCE OF SCIENCE IN THE LAST HALF-CENTURY

BY

T.H. HUXLEY, F.R.S.

NEW YORK

D. APPLETON AND COMPANY

1889




THE ADVANCE OF SCIENCE IN THE LAST HALF-CENTURY

[Sidenote: Recent industrial progress]

The most obvious and the most distinctive features of the History of
Civilisation, during the last fifty years, is the wonderful increase
of industrial production by the application of machinery, the
improvement of old technical processes and the invention of new ones,
accompanied by an even more remarkable development of old and new
means of locomotion and intercommunication. By this rapid and vast
multiplication of the commodities and conveniences of existence, the
general standard of comfort has been raised, the ravages of pestilence
and famine have been checked, and the natural obstacles, which time
and space offer to mutual intercourse, have been reduced in a manner,
and to an extent, unknown to former ages. The diminution or removal of
local ignorance and prejudice, the creation of common interests among
the most widely separated peoples, and the strengthening of the forces
of the organisation of the commonwealth against those of political or
social anarchy, thus effected, have exerted an influence on the
present and future fortunes of mankind the full significance of which
may be divined, but cannot, as yet, be estimated at its full value.

[Sidenote: caused by the increase of physical science]

This revolution--for it is nothing less--in the political and social
aspects of modern civilisation has been preceded, accompanied, and in
great measure caused, by a less obvious, but no less marvellous,
increase of natural knowledge, and especially of that part of it which
is known as Physical Science, in consequence of the application of
scientific method to the investigation of the phenomena of the
material world. Not that the growth of physical science is an
exclusive prerogative of the Victorian age. Its present strength and
volume merely indicate the highest level of a stream which took its
rise, alongside of the primal founts of Philosophy, Literature, and
Art, in ancient Greece; and, after being dammed up for a thousand
years, once more began to flow three centuries ago.

[Sidenote: Greek and mediæval science.]

It may be doubted if even-handed justice, as free from fulsome
panegyric as from captious depreciation, has ever yet been dealt out
to the sages of antiquity who, for eight centuries, from the time of
Thales to that of Galen, toiled at the foundations of physical
science. But, without entering into the discussion of that large
question, it is certain that the labors of these early workers in the
field of natural knowledge were brought to a standstill by the decay
and disruption of the Roman Empire, the consequent disorganisation of
society, and the diversion of men's thoughts from sublunary matters to
the problems of the supernatural world suggested by Christian dogma in
the Middle Ages. And, notwithstanding sporadic attempts to recall men
to the investigation of nature, here and there, it was not until the
fifteenth and sixteenth centuries that physical science made a new
start, founding itself, at first, altogether upon that which had been
done by the Greeks. Indeed, it must be admitted that the men of the
Renaissance, though standing on the shoulders of the old philosophers,
were a long time before they saw as much as their forerunners had
done.

The first serious attempts to carry further the unfinished work of
Archimedes, Hipparchus, and Ptolemy, of Aristotle and of Galen,
naturally enough arose among the astronomers and the physicians. For
the imperious necessity of seeking some remedy for the physical ills
of life had insured the preservation of more or less of the wisdom of
Hippocrates and his successors, and, by a happy conjunction of
circumstances, the Jewish and the Arabian physicians and philosophers
escaped many of the influences which, at that time, blighted natural
knowledge in the Christian world. On the other hand, the superstitious
hopes and fears which afforded countenance to astrology and to alchemy
also sheltered astronomy and the germs of chemistry. Whether for this,
or for some better reason, the founders of the schools of the Middle
Ages included astronomy, along with geometry, arithmetic, and music,
as one of the four branches of advanced education; and, in this
respect, it is only just to them to observe that they were far in
advance of those who sit in their seats. The school men considered no
one to be properly educated unless he were acquainted with, at any
rate, one branch of physical science. We have not, even yet, reached
that stage of enlightenment.

[Sidenote: Further advance after Renaissance.]

In the early decades of the seventeenth century, the men of the
Renaissance could show that they had already put out to good interest
the treasure bequeathed to them by the Greeks. They had produced the
astronomical system of Copernicus, with Kepler's great additions; the
astronomical discoveries and the physical investigations of Galileo;
the mechanics of Stevinus and the 'De Magnete' of Gilbert; the anatomy
of the great French and Italian schools and the physiology of Harvey.
In Italy, which had succeeded Greece in the hegemony of the scientific
world, the Accademia dei Lyncei and sundry other such associations for
the investigation of nature, the models of all subsequent academies
and scientific societies, had been founded, while the literary skill
and biting wit of Galileo had made the great scientific questions of
the day not only intelligible, but attractive, to the general public.

[Sidenote: Francis Bacon.]

In our own country, Francis Bacon, had essayed to sum up the past of
physical science, and to indicate the path which it must follow if its
great destinies were to be fulfilled. And though the attempt was just
such a magnificent failure as might have been expected from a man of
great endowments, who was so singularly devoid of scientific insight
that he could not understand the value of the work already achieved by
the true instaurators of physical science; yet the majestic eloquence
and the fervid vaticinations of one who was conspicuous alike by the
greatness of his rise and the depth of his fall, drew the attention of
all the world to the 'new birth of Time.'

[Sidenote: The defect of his method.]

But it is not easy to discover satisfactory evidence that the 'Novum
Organum' had any direct beneficial influence on the advancement of
natural knowledge. No delusion is greater than the notion that method
and industry can make up for lack of motherwit, either in science or
in practical life; and it is strange that, with his knowledge of
mankind, Bacon should have dreamed that his, or any other, 'via
inveniendi scientias' would 'level men's wits' and leave little scope
for that inborn capacity which is called genius. As a matter of fact,
Bacon's 'via' has proved hopelessly impracticable; while the
'anticipation of nature' by the invention of hypotheses based on
incomplete inductions, which he specially condemns, has proved itself
to be a most efficient, indeed an indispensable, instrument of
scientific progress. Finally, that transcendental alchemy--the
superinducement of new forms on matter--which Bacon declares to be the
supreme aim of science, has been wholly ignored by those who have
created the physical knowledge of the present day.

Even the eloquent advocacy of the Chancellor brought no unmixed good
to physical science. It was natural enough that the man who, in his
better moments, took 'all knowledge for his patrimony,' but, in his
worse, sold that birthright for the mess of pottage of Court favor and
professional success, for pomp and show, should be led to attach an
undue value to the practical advantages which he foresaw, as Roger
Bacon and, indeed, Seneca had foreseen, long before his time, must
follow in the train of the advancement of natural knowledge. The
burden of Bacon's pleadings for science is the gathering of
fruit'--the importance of winning solid material advantages by the
investigation of Nature and the desirableness of limiting the
application of scientific methods of inquiry to that field.

[Sidenote: Hobbes.]

[Sidenote: Descartes.]

Bacon's younger contemporary, Hobbes, casting aside the prudent
reserve of his predecessor in regard to those matters about which the
Crown or the Church might have something to say, extended scientific
methods of inquiry to the phenomena of mind and the problems of social
organisation; while, at the same time, he indicated the boundary
between the province of real, and that of imaginary, knowledge. The
'Principles of Philosophy' and the 'Leviathan' embody a coherent
system of purely scientific thought in language which is a model of
clear and vigorous English style. At the same time, in France, a man
of far greater scientific capacity than either Bacon or Hobbes, René
Descartes, not only in his immortal 'Discours de la Méthode' and
elsewhere, went down to the foundations of scientific certainty, but,
in his 'Principes de Philosophie,' indicated where the goal of
physical science really lay. However, Descartes was an eminent
mathematician, and it would seem that the bent of his mind led him to
overestimate the value of deductive reasoning from general principles,
as much as Bacon had underestimated it. The progress of physical
science has been effected neither by Baconians nor by Cartesians, as
such, but by men like Galileo and Harvey, Boyle and Newton, who would
have done their work just as well if neither Bacon nor Descartes had
ever propounded their views respecting the manner in which scientific
investigation should be pursued.

[Sidenote: For a time the progress without fruits.]

The progress of science, during the first century after Bacon's death,
by means verified his sanguine prediction of the fruits which it would
yield. For, though the revived and renewed study of nature had spread
and grown to an extent which surpassed reasonable expectation, the
practical results--the 'good to men's estate'--were, at first, by no
means apparent. Sixty years after Bacon's death, Newton had crowned
the long labors of the astronomers and the physicists, by coordinating
the phenomena of solar motion throughout the visible universe into one
vast system; but the 'Principia' helped no man to either wealth or
comfort. Descartes, Newton, and Leibnitz had opened up new worlds to
the mathematician, but the acquisitions of their genius enriched only
man's ideal estate. Descartes had laid the foundations of rational
cosmogony and of physiological psychology; Boyle had produced models
of experimentation in various branches of physics and chemistry;
Pascal and Torricelli had weighed the air; Malpighi and Grew, Ray and
Willoughby had done work of no less importance in the biological
sciences; but weaving and spinning were carried on with the old
appliances; nobody could travel faster by sea or by land than at any
previous time in the world's history, and King George could send a
message from London to York no faster than King John might have done.
Metals were worked from their ores by immemorial rule of thumb, and
the centre of the iron trade of these islands was still among the oak
forests of Sussex. The utmost skill of our mechanicians did not get
beyond the production of a coarse watch.

The middle of the eighteenth century is illustrated by a host of great
names in science--English, French, German, and Italian--especially in
the fields of chemistry, geology, and biology; but this deepening and
broadening of natural knowledge produced next to no immediate
practical benefits. Even if, at this time, Francis Bacon could have
returned to the scene of his greatness and of his littleness, he must
have regarded the philosophic world which praised and disregarded his
precepts with great disfavor. If ghosts are consistent, he would have
said, 'These people are all wasting their time, just as Gilbert and
Kepler and Galileo and my worthy physician Harvey did in my day. Where
are the fruits of the restoration of science which I promised? This
accumulation of bare knowledge is all very well, but _cui bono_? Not
one of these people is doing what I told him specially to do, and
seeking that secret of the cause of forms which will enable men to
deal, at will, with matter, and superinduce new natures upon the old
foundations.'

[Sidenote: Its recent effect on life.]

But, a little later, that growth of knowledge beyond imaginable
utilitarian ends, which is the condition precedent of its practical
utility, began to produce some effect upon practical life; and the
operation of that part of nature we call human upon the rest began to
create, not 'new natures,' in Bacon's sense, but a new Nature, the
existence of which is dependent upon men's efforts, which is
subservient to their wants, and which would disappear if man's
shaping and guiding hand were withdrawn. Every mechanical artifice,
every chemically pure substance employed in manufacture, every
abnormally fertile race of plants, or rapidly growing and fattening
breed of animals, is a part of the new Nature created by science.
Without it, the most densely populated regions of modern Europe and
America must retain their primitive, sparsely inhabited, agricultural
or pastoral condition; it is the foundation of our wealth and the
condition of our safety from submergence by another flood of barbarous
hordes; it is the bond which unites into a solid political whole,
regions larger than any empire of antiquity; it secures us from the
recurrence of the pestilences and famines of former times; it is the
source of endless comforts and conveniences, which are not mere
luxuries, but conduce to physical and moral well-being. During the
last fifty years, this new birth of time, this new Nature begotten by
science upon fact, has pressed itself daily and hourly upon our
attention, and has worked miracles which have modified the whole
fashion of our lives.

[Sidenote: These results often too much regarded;]

What wonder, then, if these astonishing fruits of the tree of
knowledge are too often regarded by both friends and enemies as the
be-all and end-all of science? What wonder if some eulogise, and
others revile, the new philosophy for its utilitarian ends and its
merely material triumphs?

[Sidenote: for scientific research rarely directed to practical ends]

In truth, the new philosophy deserves neither the praise of its
eulogists, nor the blame of its slanderers. As I have pointed out, its
disciples were guided by no search after practical fruits, during the
great period of its growth, and it reached adolescence without being
stimulated by any rewards of that nature. The bare enumeration of the
names of the men who were the great lights of science in the latter
part of the eighteenth and the first decade of the nineteenth
century, of Herschel, of Laplace, of Young, of Fresnel, of Oersted, of
Cavendish, of Lavoisier, of Davy, of Lamarck, of Cuvier, of Jussieu,
of Decandolle, of Werner and of Hutton, suffices to indicate the
strength of physical science in the age immediately preceding that of
which I have to treat. But of which of these great men can it be said
that their labors were directed to practical ends? I do not call to
mind even an invention of practical utility which we owe to any of
them, except the safety lamp of Davy. Werner certainly paid attention
to mining, and I have not forgotten James Watt. But, though some of
the most important of the improvements by which Watt converted the
steam-engine, invented long before his time, into the obedient slave
of man, were suggested and guided by his acquaintance with scientific
principles, his skill as a practical mechanician, and the efficiency
of Bolton's workmen had quite as much to do with the realisation of
his projects.

[Sidenote: but instigated by love of knowledge]

In fact, the history of physical science teaches (and we cannot too
carefully take the lesson to heart) that the practical advantages,
attainable through its agency, never have been, and never will be,
sufficiently attractive to men inspired by the inborn genius of the
interpreter of nature, to give them courage to undergo the toils and
make the sacrifices which that calling requires from its votaries.
That which stirs their pulses is the love of knowledge and the joy of
the discovery of the causes of things sung by the old poets--the
supreme delight of extending the realm of law and order ever farther
towards the unattainable goals of the infinitely great and the
infinitely small, between which our little race of life is run. In the
course of this work, the physical philosopher, sometimes
intentionally, much more often unintentionally, lights upon something
which proves to be of practical value. Great is the rejoicing of those
who are benefited thereby; and, for the moment, science is the Diana
of all the craftsmen. But, even while the cries of jubilation resound
and this floatsam and jetsam of the tide of investigation is being
turned into the wages of workmen and the wealth of capitalists, the
crest of the wave of scientific investigation is far away on its
course over the illimitable ocean of the unknown.

[Sidenote: It is in its turn assisted by industrial improvements.]

Far be it from me to depreciate the value of the gifts of science to
practical life, or to cast a doubt upon the propriety of the course of
action of those who follow science in the hope of finding wealth
alongside truth, or even wealth alone. Such a profession is as
respectable as any other. And quite as little do I desire to ignore
the fact that, if industry owes a heavy debt to science, it has
largely repaid the loan by the important aid which it has, in its
turn, rendered to the advancement of science. In considering the
causes which hindered the progress of physical knowledge in the
schools of Athens and of Alexandria, it has often struck me[A] that
where the Greeks did wonders was in just those branches of science,
such as geometry, astronomy, and anatomy, which are susceptible of
very considerable development without any, or any but the simplest,
appliances. It is a curious speculation to think what would have
become of modern physical science if glass and alcohol had not been
easily obtainable; and if the gradual perfection of mechanical skill
for industrial ends had not enabled investigators to obtain, at
comparatively little cost, microscopes, telescopes, and all the
exquisitely delicate apparatus for determining weight and measure and
for estimating the lapse of time with exactness, which they now
command. If science has rendered the colossal development of modern
industry possible, beyond a doubt industry has done no less for modern
physics and chemistry, and for a great deal of modern biology. And as
the captains of industry have, at last, begun to be aware that the
condition of success in that warfare, under the forms of peace, which
is known as industrial competition lies in the discipline of the
troops and the use of arms of precision, just as much as it does in
the warfare which is called war, their demand for that discipline,
which is technical education, is reacting upon science in a manner
which will, assuredly, stimulate its future growth to an incalculable
extent. It has become obvious that the interests of science and of
industry are identical, that science cannot make a step forward
without, sooner or later, opening up new channels for industry, and,
on the other hand, that every advance of industry facilitates those
experimental investigations, upon which the growth of science depends.
We may hope that, at last, the weary misunderstanding between the
practical men who professed to despise science, and the high and dry
philosophers who professed to despise practical results, is at an end.

Nevertheless, that which is true of the infancy of physical science in
the Greek world, that which is true of its adolescence in the
seventeenth and eighteenth centuries, remains true of its riper age in
these latter days of the nineteenth century. The great steps in its
progress have been made, are made, and will be made, by men who seek
knowledge simply because they crave for it. They have their
weaknesses, their follies, their vanities, and their rivalries, like
the rest of the world; but whatever by-ends may mar their dignity and
impede their usefulness, this chief end redeems them.[B] Nothing great
in science has ever been done by men, whatever their powers, in whom
the divine afflatus of the truth-seeker was wanting. Men of moderate
capacity have done great things because it animated them; and men of
great natural gifts have failed, absolutely or relatively, because
they lacked this one thing needful.

[Sidenote: True aim and method of research.]

To anyone who knows the business of investigation practically, Bacon's
notion of establishing a company of investigators to work for
'fruits,' as if the pursuit of knowledge were a kind of mining
operation and only required well-directed picks and shovels, seems
very strange.[C] In science, as in art, and, as I believe, in every
other sphere of human activity, there may be wisdom in a multitude of
counsellors, but it is only in one or two of them. And, in scientific
inquiry, at any rate, it is to that one or two that we must look for
light and guidance. Newton said that he made his discoveries by
'intending' his mind on the subject; no doubt truly. But to equal his
success one must have the mind which he 'intended.' Forty lesser men
might have intended their minds till they cracked, without any like
result. It would be idle either to affirm or to deny that the last
half-century has produced men of science of the calibre of Newton. It
is sufficient that it can show a few capacities of the first rank,
competent not only to deal profitably with the inheritance bequeathed
by their scientific forefathers, but to pass on to their successors
physical truths of a higher order than any yet reached by the human
race. And if they have succeeded as Newton succeeded, it is because
they have sought truth as he sought it, with no other object than the
finding it.

       *       *       *       *       *

[Sidenote: Progress from 1837 to 1887.]

I am conscious that in undertaking to progress give even the briefest
sketch of the progress of physical science, in all its branches,
during the last half-century, I may be thought to have exhibited more
courage than discretion, and perhaps more presumption than either. So
far as physical science is concerned, the days of Admirable Crichtons
have long been over, and the most indefatigable of hard workers may
think he has done well if he has mastered one of its minor
subdivisions. Nevertheless, it is possible for anyone, who has
familiarised himself with the operations of science in one department,
to comprehend the significance, and even to form a general estimate of
the value, of the achievements of specialists in other departments.

Nor is their any lack either of guidance, or of aids to ignorance. By
a happy chance, the first edition of Whewell's 'History of the
Inductive Sciences' was published in 1837, and it affords a very
useful view of the state of things at the commencement of the
Victorian epoch. As to subsequent events, there are numerous
excellent summaries of the progress of various branches of science,
especially up to 1881, which was the jubilee year of the British
Association.[D] And, with respect to the biological sciences, with
some parts of which my studies have familiarised me, my personal
experience nearly coincides with the preceding half-century. I may
hope, therefore, that my chance of escaping serious errors is as good
as that of anyone else, who might have been persuaded to undertake the
somewhat perilous enterprise in which I find myself engaged.

There is yet another prefatory remark which it seems desirable I
should make. It is that I think it proper to confine myself to the
work done, without saying anything about the doers of it. Meddling
with questions of merit and priority is a thorny business at the best
of times, and unless in case of necessity, altogether undesirable when
one is dealing with contemporaries. No such necessity lies upon me,
and I shall, therefore, mention no names of living men, lest,
perchance, I should incur the reproof which the Israelites, who
struggled with one another in the field, addressed to Moses--'Who made
thee a prince and a judge over us.'

[Sidenote: The aim of physical science]

Physical science is one and indivisible. Although, for practical
purposes, it is convenient to mark it out into the primary regions of
Physics, Chemistry, and Biology, and to subdivide these into
subordinate provinces, yet the method of investigation and the
ultimate object of the physical inquirer are everywhere the same.

[Sidenote: the discovery of the rational order of the universe]

The object is the discovery of the rational order which pervades the
universe, the method consists of observation and experiment (which is
observation under artificial conditions) for the determination of the
facts of nature, of inductive and deductive reasoning for the
discovery of their mutual relations and connection. The various
branches of physical science differ in the extent to which at any
given moment of their history, observation on the one hand, or
ratiocination on the other, is their more obvious feature, but in no
other way, and nothing can be more incorrect than the assumption one
sometimes meets with, that physics has one method, chemistry another,
and biology a third.

[Sidenote: It is based on postulates]

All physical science starts from certain postulates. One of them is
the objective existence of a material world. It is assumed that the
phenomena which are comprehended under this name have a 'substratum'
of extended, impenetrable, mobile substance, which exhibits the
quality known as inertia, and is termed matter.[E] Another postulate
is the universality of the law of causation; that nothing happens
without a cause (that is, a necessary precedent condition), and that
the state of the physical universe, at any given moment, is the
consequence of its state at any preceding moment. Another is that any
of the rules, or so-called 'laws of nature,' by which the relation of
phenomena is truly defined, is true for all time. The validity of
these postulates is a problem of metaphysics; they are neither
self-evident nor are they, strictly speaking, demonstrable. The
justification of their employment, as axioms of physical philosophy,
lies in the circumstance that expectations logically based upon them
are verified, or, at any rate, not contradicted, whenever they can be
tested by experience.

[Sidenote: and uses hypotheses.]

Physical science therefore rests on verified or uncontradicted
hypotheses; and, such being the case, it is not surprising that a
great condition of its progress has been the invention of verifiable
hypotheses. It is a favorite popular delusion that the scientific
inquirer is under a sort of moral obligation to abstain from going
beyond that generalisation of observed facts which is absurdly called
'Baconian' induction. But anyone who is practically acquainted with
scientific work is aware that those who refuse to go beyond fact,
rarely get as far as fact; and anyone who has studied the history of
science knows that almost every great step therein has been made by
the 'anticipation of Nature,' that is, by the invention of hypotheses,
which, though verifiable, often had very little foundation to start
with; and, not unfrequently, in spite of a long career of usefulness,
turned out to be wholly erroneous in the long run.

[Sidenote: Fruitful use of an hypothesis even when wrong.]

The geocentric system of astronomy, with its eccentrics and its
epicycles, was an hypothesis utterly at variance with fact, which
nevertheless did great things for the advancement of astronomical
knowledge. Kepler was the wildest of guessers. Newton's corpuscular
theory of light was of much temporary use in optics, though nobody now
believes in it; and the undulatory theory, which has superseded the
corpuscular theory and has proved one of the most fertile of
instruments of research, is based on the hypothesis of the existence
of an 'ether,' the properties of which are defined in propositions,
some of which, to ordinary apprehension, seem physical antinomies.

It sounds paradoxical to say that the attainment of scientific truth
has been effected, to a great extent, by the help of scientific
errors. But the subject-matter of physical science is furnished by
observation, which cannot extend beyond the limits of our faculties;
while, even within those limits, we cannot be certain that any
observation is absolutely exact and exhaustive. Hence it follows that
any given generalisation from observation may be true, within the
limits of our powers of observation at a given time, and yet turn out
to be untrue, when those powers of observation are directly or
indirectly enlarged. Or, to put the matter in another way, a doctrine
which is untrue absolutely, may, to a very great extent, be
susceptible of an interpretation in accordance with the truth. At a
certain period in the history of astronomical science, the assumption
that the planets move in circles was true enough to serve the purpose
of correlating such observations as were then possible; after Kepler,
the assumption that they move in ellipses became true enough in regard
to the state of observational astronomy at that time. We say still
that the orbits of the planets are ellipses, because, for all ordinary
purposes, that is a sufficiently near approximation to the truth; but,
as a matter of fact, the centre of gravity of a planet describes
neither an ellipse or any other simple curve, but an immensely
complicated undulating line. It may fairly be doubted whether any
generalisation, or hypothesis, based upon physical data is absolutely
true, in the sense that a mathematical proposition is so; but, if its
errors can become apparent only outside the limits of practicable
observation, it may be just as usefully adopted for one of the symbols
of that algebra by which we interpret nature, as if it were absolutely
true.

The development of every branch of physical knowledge presents three
stages which, in their logical relation, are successive. The first is
the determination of the sensible character and order of the
phenomena. This is _Natural History_, in the original sense of the
term, and here nothing but observation and experiment avail us. The
second is the determination of the constant relations of the phenomena
thus defined, and their expression in rules or laws. The third is the
explication of these particular laws by deduction from the most
general laws of matter and motion. The last two stages constitute
_Natural Philosophy_ in its original sense. In this region, the
invention of verifiable hypotheses is not only permissible, but is one
of the conditions of progress.

[Sidenote: and mutual assistance of observation, experiment, and
speculation.]

Historically, no branch of science has followed this order of growth;
but, from the dawn of exact knowledge to the present day, observation,
experiment, and speculation have gone hand in hand; and, whenever
science has halted or strayed from the right path, it has been, either
because its votaries have been content with mere unverified or
unverifiable speculation (and this is the commonest case, because
observation and experiment are hard work, while speculation is
amusing); or it has been, because the accumulation of details of
observation has for a time excluded speculation.

[Sidenote: Recognition of these truths in recent times, and consequent
progress.]

The progress of physical science, since the revival of learning, is
largely due to the fact that men have gradually learned to lay aside
the consideration of unverifiable hypotheses; to guide observation
and experiment by verifiable hypotheses; and to consider the latter,
not as ideal truths, the real entities of an intelligible world behind
phenomena, but as a symbolical language, by the aid of which nature
can be interpreted in terms apprehensible by our intellects. And if
physical science, during the last fifty years, has attained dimensions
beyond all former precedent, and can exhibit achievements of greater
importance than any former such period can show, it is because able
men, animated by the true scientific spirit, carefully trained in the
method of science, and having at their disposal immensely improved
appliances, have devoted themselves to the enlargement of the
boundaries of natural knowledge in greater number than during any
previous half-century of the world's history.

[Sidenote: The three great achievements. Doctrines of (1) molecular
constitution of matter, (2) conservation of energy, (3) evolution.]

I have said that our epoch can produce achievements in physical
science of greater moment than any other has to show, advisedly; and
I think that there are three great products of our time which justify
the assertion. One of these is that doctrine concerning the
constitution of matter which, for want of a better name, I will call
'molecular;' the second is the doctrine of conservation of energy; the
third is the doctrine of evolution. Each of these was foreshadowed,
more or less distinctly, in former periods of the history of science;
and, so far is either from being the outcome of purely inductive
reasoning, that it would be hard to overrate the influence of
metaphysical, and even of theological, considerations upon the
development of all three. The peculiar merit of our epoch is that it
has shown how these hypotheses connect a vast number of seemingly
independent partial generalisations; that it has given them that
precision of expression which is necessary for their exact
verification; and that it has practically proved their value as
guides to the discovery of new truth. All three doctrines are
intimately connected, and each is applicable to the whole physical
cosmos. But, as might have been expected from the nature of the case,
the first two grew, mainly, out of the consideration of
physico-chemical phenomena; while the third, in great measure, owes
its rehabilitation, if not its origin, to the study of biological
phenomena.

[Sidenote: (1) Molecular constitution of matter.]

In the early decades of this century, a number of important truths
applicable, in part, to matter in general, and, in part, to particular
forms of matter, had been ascertained by the physicists and chemists.

The laws of motion of visible and tangible, or _molar_, matter had
been worked out to a great degree of refinement and embodied in the
branches of science known as Mechanics, Hydrostatics, and Pneumatics.
These laws had been shown to hold good, so far as they could be
checked by observation and experiment, throughout the universe, on the
assumption that all such masses of matter possessed inertia and were
susceptible of acquiring motion, in two ways, firstly by impact, or
impulse from without; and, secondly, by the operation of certain
hypothetical causes of motion termed 'forces,' which were usually
supposed to be resident in the particles of the masses themselves, and
to operate at a distance, in such a way as to tend to draw any two
such masses together, or to separate them more widely.

[Sidenote: The two theories as to matter.]

With respect to the ultimate constitution of these masses, the same
two antagonistic opinions which had existed since the time of
Democritus and of Aristotle were still face to face. According to the
one, matter was discontinuous and consisted of minute indivisible
particles or atoms, separated by a universal vacuum; according to the
other, it was continuous, and the finest distinguishable, or
imaginable, particles were scattered through the attenuated general
substance of the plenum. A rough analogy to the latter case would be
afforded by granules of ice diffused through water; to the former,
such granules diffused through absolutely empty space.

[Sidenote: Reassertion by Dalton of atomic theory.]

In the latter part of the eighteenth century, the chemists had arrived
at several very important generalisations respecting those properties
of matter with which they were especially concerned. However plainly
ponderable matter seemed to be originated and destroyed in their
operations, they proved that, as mass or body, it remained
indestructible and ingenerable; and that, so far, it varied only in
its perceptibility by our senses. The course of investigation further
proved that a certain number of the chemically separable kinds of
matter were unalterable by any known means (except in so far as they
might be made to change their state from solid to fluid, or _vice
versâ_), unless they were brought into contact with other kinds of
matter, and that the properties of these several kinds of matter were
always the same, whatever their origin. All other bodies were found to
consist of two or more of these, which thus took the place of the four
'elements' of the ancient philosophers. Further, it was proved that,
in forming chemical compounds, bodies always unite in a definite
proportion by weight, or in simple multiples of that proportion, and
that, if any one body were taken as a standard, every other could have
a number assigned to it as its proportional combining weight. It was
on this foundation of fact that Dalton based his re-establishment of
the old atomic hypothesis on a new empirical foundation. It is
obvious, that if elementary matter consists of indestructible and
indivisible particles, each of which constantly preserves the same
weight relatively to all the others, compounds formed by the
aggregation of two, three, four, or more such particles must exemplify
the rule of combination in definite proportions deduced from
observation.

In the meanwhile, the gradual reception of the undulatory theory of
light necessitated the assumption of the existence of an 'ether'
filling all space. But whether this ether was to be regarded as a
strictly material and continuous substance was an undecided point, and
hence the revived atomism, escaped strangling in its birth. For it is
clear, that if the ether is admitted to be a continuous material
substance, Democritic atomism is at an end and Cartesian continuity
takes its place.

[Sidenote: The real value of hypothesis; it predicates the existence
of units of matter.]

The real value of the new atomic hypothesis, however, did not lie in
the two points which Democritus and his followers would have
considered essential--namely, the indivisibility of the 'atoms' and
the presence of an interatomic vacuum--but in the assumption that, to
the extent to which our means of analysis take us, material bodies
consist of definite minute masses, each of which, so far as physical
and chemical processes of division go, may be regarded as a
unit--having a practically permanent individuality. Just as a man is
the unit of sociology, without reference to the actual fact of his
divisibility, so such a minute mass is the unit of physico-chemical
science--that smallest material particle which under any given
circumstances acts as a whole.[F]

The doctrine of specific heat originated in the eighteenth century.
It means that the same mass of a body, under the same circumstances,
always requires the same quantity of heat to raise it to a given
temperature, but that equal masses of different bodies require
different quantities. Ultimately, it was found that the quantities of
heat required to raise equal masses of the more perfect gases, through
equal ranges of temperature, were inversely proportional to their
combining weights. Thus a definite relation was established between
the hypothetical units and heat. The phenomena of electrolytic
decomposition showed that there was a like close relation between
these units and electricity. The quantity of electricity generated by
the combination of any two units is sufficient to separate any other
two which are susceptible of such decomposition. The phenomena of
isomorphism showed a relation between the units and crystalline forms;
certain units are thus able to replace others in a crystalline body
without altering its form, and others are not.

Again, the laws of the effect of pressure and heat on gaseous bodies,
the fact that they combine in definite proportions by volume, and that
such proportion bears a simple relation to their combining weights,
all harmonised with the Daltonian hypothesis, and led to the bold
speculation known as the law of Avogadro--that all gaseous bodies,
under the same physical conditions, contain the same number of units.
In the form in which it was first enunciated, this hypothesis was
incorrect--perhaps it is not exactly true in any form; but it is
hardly too much to say that chemistry and molecular physics would
never have advanced to their present condition unless it had been
assumed to be true. Another immense service rendered by Dalton, as a
corollary of the new atomic doctrine, was the creation of a system of
symbolic notation, which not only made the nature of chemical
compounds and processes easily intelligible and easy of recollection,
but, by its very form, suggested new lines of inquiry. The atomic
notation was as serviceable to chemistry as the binomial nomenclature
and the classificatory schematism of Linnæus were to zoölogy and
botany.

[Sidenote: In biology a like theory of molecularstructure.]

Side by side with these advances arose in another, which also has a
close parallel in the history of biological science. If the unit of a
compound is made up by the aggregation of elementary units, the notion
that these must have some sort of definite arrangement inevitably
suggests itself; and such phenomena as double decomposition pointed,
not only to the existence of a molecular architecture, but to the
possibility of modifying a molecular fabric without destroying it, by
taking out some of the component units and replacing them by others.
The class of neutral salts, for example, includes a great number of
bodies in many ways similar, in which the basic molecules, or the acid
molecules, may be replaced by other basic and other acid molecules
without altering the neutrality of the salt; just as a cube of bricks
remains a cube, so long as any brick that is taken out is replaced by
another of the same shape and dimensions, whatever its weight or other
properties may be. Facts of this kind gave rise to the conception of
'types' of molecular structure, just as the recognition of the unity
in diversity of the structure of the species of plants and animals
gave rise to the notion of biological 'types.' The notation of
chemistry enabled these ideas to be represented with precision; and
they acquired an immense importance, when the improvement of methods
of analysis, which took place about the beginning of our period,
enabled the composition of the so-called 'organic' bodies to be
determined with, rapidity and precision.[G] A large proportion of
these compounds contain not more than three or four elements, of which
carbon is the chief; but their number is very great, and the diversity
of their physical and chemical properties is astonishing. The
ascertainment of the proportion of each element in these compounds
affords little or no help towards accounting for their diversities;
widely different bodies being often very similar, or even identical,
in that respect. And, in the last case, that of _isomeric_ compounds,
the appeal to diversity of arrangement of the identical component
units was the only obvious way out of the difficulty. Here, again,
hypothesis proved to be of great value; not only was the search for
evidence of diversity of molecular structure successful, but the study
of the process of taking to pieces led to the discovery of the way to
put together; and vast numbers of compounds, some of them previously
known only as products of the living economy, have thus been
artificially constructed. Chemical work, at the present day, is, to a
large extent, synthetic or creative--that is to say, the chemist
determines, theoretically, that certain non-existent compounds ought
to be producible, and he proceeds to produce them.

It is largely because the chemical theory and practice of our epoch
have passed into this deductive and synthetic stage, that they are
entitled to the name of the 'New Chemistry' which they commonly
receive. But this new chemistry has grown up by the help of
hypotheses, such as those of Dalton and of Avogadro, and that
singular conception of 'bonds' invented to colligate the facts of
'valency' or 'atomicity,' the first of which took some time to make
its way; while the second fell into oblivion, for many years after it
was propounded, for lack of empirical justification. As for the third,
it may be doubted if anyone regards it as more than a temporary
contrivance.

But some of these hypotheses have done yet further service. Combining
them with the mechanical theory of heat and the doctrine of the
conservation of energy, which are also products of our time,
physicists have arrived at an entirely new conception of the nature of
gaseous bodies and of the relation of the physico-chemical units of
matter to the different forms of energy. The conduct of gases under
varying pressure and temperature, their diffusibility, their relation
to radiant heat and to light, the evolution of heat when bodies
combine, the absorption of heat when they are dissociated, and a host
of other molecular phenomena, have been shown to be deducible from the
dynamical and statical principles which apply to molar motion and
rest; and the tendency of physico-chemical science is clearly towards
the reduction of the problems of the world of the infinitely little,
as it already has reduced those of the infinitely great world, to
questions of mechanics.[H]

In the meanwhile, the primitive atomic theory, which has served as the
scaffolding for the edifice of modern physics and chemistry, has been
quietly dismissed. I cannot discover that any contemporary physicist
or chemist believes in the real indivisibility of atoms, or in an
interatomic matterless vacuum. 'Atoms' appear to be used as mere
names for physico-chemical units which have not yet been subdivided,
and 'molecules' for physico-chemical units which are aggregates of the
former. And these individualised particles are supposed to move in an
endless ocean of a vastly more subtle matter--the ether. If this ether
is a continuous substance, therefore, we have got back from the
hypothesis of Dalton to that of Descartes. But there is much reason to
believe that science is going to make a still further journey, and, in
form, if not altogether in substance, to return to the point of view
of Aristotle.

[Sidenote: Elementary bodies]

The greater number of the so-called 'elementary' bodies, now known,
had been discovered before the commencement of our epoch; and it had
become apparent that they were by no means equally similar or
dissimilar, but that some of them, at any rate, constituted groups,
the several members of which were as much like one another as they
were unlike the rest. Chlorine, iodine, bromine, and fluorine thus
formed a very distinct group; sulphur and selenium another; boron and
silicon another; potassium, sodium, and lithium another; and so on. In
some cases, the atomic weights of such allied bodies were nearly the
same, or could be arranged in series, with like differences between
the several terms. In fact, the elements afforded indications that
they were susceptible of a classification in natural groups, such as
those into which animals and plants fall.

[Sidenote: fall into different series.]

Recently this subject has been taken up afresh, with a result which
may be stated roughly in the following terms: If the sixty-five or
sixty-eight recognised 'elements' are arranged in the order of their
atomic weights--from hydrogen, the lightest, as unity, to uranium, the
heaviest, as 240--the series does not exhibit one continuous
progressive modification in the physical and chemical characters of
its several terms, but breaks up into a number of sections, in each of
which the several terms present analogies with the corresponding terms
of the other series.

Thus the whole series does not run:

_a, b, d, e, f, g, h, i, k,_ &c.,

but

_a, b, c, d_, A, B, C, D, alpha, beta, gamma, delta, &c.;

so that it is said to express a _periodic law_ of recurrent
similarities. Or the relation may be expressed in another way. In each
section of the series, the atomic weight is greater than in the
preceding section, so that if _w_ is the atomic weight of any element
in the first segment, _w+x_ will represent the atomic weight of any
element in the next, and _w+x+y_ the atomic weight of any element in
the next, and so on. Therefore the sections may be represented as
parallel series, the corresponding terms of which have analogous
properties; each successive series starting with a body the atomic
weight of which is greater than that of any in the preceding series,
in the following fashion:

_d_       D         delta
_c_       C         gamma
_b_       B          beta
_a_       A         alpha
 -      -----     ---------
_w_    _w + x_   _w + x + y_

[Sidenote: The possibility of a primary form of matter.]

This is a conception with, which biologists are very familiar, animal
and plant groups constantly appearing as series of parallel
modifications of similar and yet different primary forms. In the
living world, facts of this kind are now understood to mean evolution
from a common prototype. It is difficult to imagine that in the
not-living world they are devoid of significance. Is it not possible,
nay probable that they may mean the evolution of our 'elements' from a
primary undifferentiated form of matter? Fifty years ago, such a
suggestion would have been scouted as a revival of the dreams of the
alchemists. At present, it may be said to be the burning question of
physico-chemical science.

In fact, the so-called 'vortex-ring' hypothesis is a very serious and
remarkable attempt to deal with material units from a point of view
which is consistent with the doctrine of evolution. It supposes the
ether to be a uniform substance, and that the 'elementary' units are,
broadly speaking, permanent whirlpools, or vortices, of this ether,
the properties of which depend on their actual and potential modes of
motion. It is curious and highly interesting to remark that this
hypothesis reminds us not only of the speculations of Descartes, but
of those of Aristotle. The resemblance of the 'vortex-rings' to the
'tourbillons' of Descartes is little more than nominal; but the
correspondence between the modern and the ancient notion of a
distinction between primary and derivative matter is, to a certain
extent, real. For this ethereal 'Urstoff' of the modern corresponds
very closely with the prhôtê hylê of Aristotle, the _materia prima_ of
his mediæval followers; while matter, differentiated into our
elements, is the equivalent of the first stage of progress towards the
heschhatê hylê, or finished matter, of the ancient philosophy.

If the material units of the existing order of nature are specialised
portions of a relatively homogeneous _materia prima_--which were
originated under conditions that have long ceased to exist and which
remain unchanged and unchangeable under all conditions, whether
natural or artificial, hitherto known to us--it follows that the
speculation that they may be indefinitely altered, or that new units
may be generated under conditions yet to be discovered, is perfectly
legitimate. Theoretically, at any rate, the transmutability of the
elements is a verifiable scientific hypothesis; and such inquiries as
those which have been set afoot, into the possible dissociative action
of the great heat of the sun upon our elements, are not only
legitimate, but are likely to yield results which, whether affirmative
or negative, will be of great importance. The idea that atoms are
absolutely ingenerable and immutable 'manufactured articles' stands on
the same sort of foundation as the idea that biological species are
'manufactured articles' stood thirty years ago; and the supposed
constancy of the elementary atoms, during the enormous lapse of time
measured by the existence of our universe, is of no more weight
against the possibility of change in them, in the infinity of
antecedent time, than the constancy of species in Egypt, since the
days of Rameses or Cheops, is evidence of their immutability during
all past epochs of the earth's history. It seems safe to prophesy
that the hypothesis of the evolution of the elements from a primitive
matter will, in future, play no less a part in the history of science
than the atomic hypothesis, which, to begin with, had no greater, if
so great, an empirical foundation.

[Sidenote: The old and the new atomic theory.]

It may perhaps occur to the reader that the boasted progress of
physical science does not come to much, if our present conceptions of
the fundamental nature of matter are expressible in terms employed,
more than two thousand years ago, by the old 'master of those that
know.' Such a criticism, however, would involve forgetfulness of the
fact, that the connotation of these terms, in the mind of the modern,
is almost infinitely different from that which they possessed in the
mind of the ancient, philosopher. In antiquity, they meant little more
than vague speculation; at the present day, they indicate definite
physical conceptions, susceptible of mathematical treatment, and
giving rise to innumerable deductions, the value of which can be
experimentally tested. The old notions produced little more than
floods of dialectics; the new are powerful aids towards the increase
of solid knowledge.

[Sidenote: (2) Conservation of energy.]

Everyday observation shows that, of the bodies which compose the
material world, some are in motion and some are, or appear to be, at
rest. Of the bodies in motion, some, like the sun and stars, exhibit a
constant movement, regular in amount and direction, for which no
external cause appears. Others, as stones and smoke, seem also to move
of themselves when external impediments are taken away. But these
appear to tend to move in opposite directions: the bodies we call
heavy, such as stones, downwards, and the bodies we call light, at
least such as smoke and steam, upwards. And, as we further notice
that the earth, below our feet, is made up of heavy matter, while the
air, above our heads, is extremely light matter, it is easy to regard
this fact as evidence that the lower region is the place to which
heavy things tend--their proper place, in short--while the upper
region is the proper place of light things; and to generalise the
facts observed by saying that bodies, which are free to move, tend
towards their proper places. All these seem to be natural motions,
dependent on the inherent faculties, or tendencies, of bodies
themselves. But there are other motions which are artificial or
violent, as when a stone is thrown from the hand, or is knocked by
another stone in motion. In such cases as these, for example, when a
stone is cast from the hand, the distance travelled by the stone
appears to depend partly on its weight and partly upon the exertion
of the thrower. So that, the weight of the stone remaining the same,
it looks as if the motive power communicated to it were measured by
the distance to which the stone travels--as if, in other words, the
power needed to send it a hundred yards was twice as great as that
needed to send it fifty yards. These, apparently obvious, conclusions
from the everyday appearances of rest and motion fairly represent the
state of opinion upon the subject which prevailed among the ancient
Greeks, and remained dominant until the age of Galileo. The
publication of the 'Principia' of Newton, in 1686-7, marks the epoch
at which the progress of mechanical physics had effected a complete
revolution of thought on these subjects. By this time, it had been
made clear that the old generalisations were either incomplete or
totally erroneous; that a body, once set in motion, will continue to
move in a straight line for any conceivable time or distance, unless
it is interfered with; that any change of motion is proportional to
the 'force' which causes it, and takes place in the direction in which
that 'force' is exerted; and that, when a body in motion acts as a
cause of motion on another, the latter gains as much as the former
loses, and _vice versâ_. It is to be noted, however, that while, in
contradistinction to the ancient idea of the inherent tendency to
motion of bodies, the absence of any such spontaneous power of motion
was accepted as a physical axiom by the moderns, the old conception
virtually maintained itself is a new shape. For, in spite of Newton's
well-known warning against the 'absurdity' of supposing that one body
can act on another at a distance through a vacuum, the ultimate
particles of matter were generally assumed to be the seats of
perennial causes of motion termed 'attractive and repulsive forces,'
in virtue of which, any two such particles, without any external
impression of motion, or intermediate material agent, were supposed to
tend to approach or remove from one another; and this view of the
duality of the causes of motion is very widely held at the present
day.

Another important result of investigation, attained in the seventeenth
century, was the proof and quantitative estimation of physical
inertia. In the old philosophy, a curious conjunction of ethical and
physical prejudices had led to the notion that there was something
ethically bad and physically obstructive about matter. Aristotle
attributes all irregularities and apparent dysteleologies in nature to
the disobedience, or sluggish yielding, of matter to the shaping and
guiding influence of those reasons and causes which were hypostatised
in his ideal 'Forms.' In modern science, the conception of the
inertia, or resistance to change, of matter is complex. In part, it
contains a corollary from the law of causation: A body cannot change
its state in respect of rest or motion without a sufficient cause.
But, in part, it contains generalisations from experience. One of
these is that there is no such sufficient cause resident in any body,
and that therefore it will rest, or continue in motion, so long as no
external cause of change acts upon it. The other is that the effect
which the impact of a body in motion produces upon the body on which
it impinges depends, other things being alike, on the relation of a
certain quality of each which is called 'mass.' Given a cause of
motion of a certain value, the amount of motion, measured by distance
travelled in a certain time, which it will produce in a given quantity
of matter, say a cubic inch, is not always the same, but depends on
what that matter is--a cubic inch of iron will go faster than a cubic
inch of gold. Hence, it appears, that since equal amounts of motion
have, _ex hypothesi_, been produced, the amount of motion in a body
does not depend on its speed alone, but on some property of the body.
To this the name of 'mass' has been given. And since it seems
reasonable to suppose that a large quantity of matter, moving slowly,
possesses as much motion as a small quantity moving faster, 'mass' has
been held to express 'quantity of matter.' It is further demonstrable
that, at any given time and place, the relative mass of any two bodies
is expressed by the ratio of their weights.

[Sidenote: Mechanical theory of heat.]

When all these great truths respecting molar motion, or the movements
of visible and tangible masses, had been shown to hold good not only
of terrestrial bodies, but of all those which constitute the visible
universe, and the movements of the macrocosm had thus been expressed
by a general mechanical theory, there remained a vast number of
phenomena, such as those of light, heat, electricity, magnetism, and
those of the physical and chemical changes, which do not involve molar
motion. Newton's corpuscular theory of light was an attempt to deal
with one great series of these phenomena on mechanical principles, and
it maintained its ground until, at the beginning of the nineteenth
century, the undulatory theory proved itself to be a much better
working hypothesis. Heat, up to that time, and indeed much later, was
regarded as an imponderable substance, _caloric_; as a thing which was
absorbed by bodies when they were wanned, and was given out as they
cooled; and which, moreover, was capable of entering into a sort of
chemical combination with them, and so becoming latent. Rumford and
Davy had given a great blow to this view of heat by proving that the
quantity of heat which two portions of the same body could be made to
give out, by rubbing them together, was practically illimitable. This
result brought philosophers face to face with the contradiction of
supposing that a finite body could contain an infinite quantity of
another body; but it was not until 1843, that clear and unquestionable
experimental proof was given of the fact that there is a definite
relation between mechanical work and heat; that so much work always
gives rise, under the same conditions, to so much heat, and so much
heat to so much mechanical work. Thus originated the mechanical theory
of heat, which became the starting-point of the modern doctrine of the
conservation of energy. Molar motion had appeared to be destroyed by
friction. It was proved that no destruction took place, but that an
exact equivalent of the energy of the lost molar motion appears as
that of the _molecular_ motion, or motion of the smallest particles of
a body, which constitutes heat. The loss of the masses is the gain of
their particles.

[Sidenote: Earlier approaches towards doctrine of conservation.]

Before 1843, however, the doctrine of conservation of energy had been
approached Bacon's chief contribution to positive science is the happy
guess (for the context shows that it was little more) that heat may be
a mode of motion; Descartes affirmed the quantity of motion in the
world to be constant; Newton nearly gave expression to the complete
theorem; while Rumford's and Davy's experiments suggested, though they
did not prove, the equivalency of mechanical and thermal energy.
Again, the discovery of voltaic electricity, and the marvellous
development of knowledge, in that field, effected by such men as Davy,
Faraday, Oersted, Ampère, and Melloni, had brought to light a number
of facts which tended to show that the so-called 'forces' at work in
light, heat, electricity, and magnetism, in chemical and in mechanical
operations, were intimately, and, in various cases, quantitatively
related. It was demonstrated that any one could be obtained at the
expense of any other; and apparatus was devised which exhibited the
evolution of all these kinds of action from one source of energy.
Hence the idea of the 'correlation of forces' which was the immediate
forerunner of the doctrine of the conservation of energy.

It is a remarkable evidence of the greatness of the progress in this
direction which has been effected in our time, that even the second
edition of the 'History of the Inductive Sciences,' which was
published in 1846, contains no allusion either to the general view of
the 'Correlation of Forces' published in England in 1842, or to the
publication in 1843 of the first of the series of experiments by which
the mechanical equivalent of heat was correctly ascertained.[I] Such a
failure on the part of a contemporary, of great acquirements and
remarkable intellectual powers, to read the signs of the times, is a
lesson and a warning worthy of being deeply pondered by anyone who
attempts to prognosticate the course of scientific progress.

[Sidenote: What this doctrine is.]

I have pointed out that the growth of clear and definite views
respecting the constitution of matter has led to the conclusion that,
so far as natural agencies are concerned, it is ingenerable and
indestructible. In so far as matter may be conceived to exist in a
purely passive state, it is, imaginably, older than motion. But, as it
must be assumed to be susceptible of motion, a particle of bare matter
at rest must be endowed with the potentiality of motion. Such a
particle, however, by the supposition, can have no energy, for there
is no cause why it should move. Suppose now that it receives an
impulse, it will begin to move with a velocity inversely proportional
to its mass, on the one hand, and directly proportional to the
strength of the impulse, on the other, and will possess _kinetic
energy_, in virtue of which it will not only continue to move for ever
if unimpeded, but if it impinges on another such particle, it will
impart more or less of its motion, to the latter. Let it be conceived
that the particle acquires a tendency to move, and that nevertheless
it does not move. It is then in a condition totally different from
that in which it was at first. A cause competent to produce motion is
operating upon it, but, for some reason or other, is unable to give
rise to motion. If the obstacle is removed, the energy which was
there, but could not manifest itself, at once gives rise to motion.
While the restraint lasts, the energy of the particle is merely
potential; and the case supposed illustrates what is meant by
_potential energy_. In this contrast of the potential with the actual,
modern physics is turning to account the most familiar of Aristotelian
distinctions--that between dunamis and energeia.

That kinetic energy appears to be imparted by impact is a fact of
daily and hourly experience: we see bodies set in motion by bodies,
already in motion, which seem to come in contact with them. It is a
truth which could have been learned by nothing but experience, and
which cannot be explained, but must be taken as an ultimate fact
about which, explicable or inexplicable, there can be no doubt.
Strictly speaking, we have no direct apprehension of any other cause
of motion. But experience furnishes innumerable examples of the
production of kinetic energy in a body previously at rest, when no
impact is discernible as the cause of that energy. In all such cases,
the presence of a second body is a necessary condition; and the amount
of kinetic energy, which its presence enables the first to gain, is
strictly dependent on the relative positions of the two. Hence the
phrase _energy of position_, which is frequently used as equivalent to
potential energy. If a stone is picked up and held, say, six feet
above the ground, it has _potential energy_, because, if let go, it
will immediately begin to move towards the earth; and this energy may
be said to be _energy of position_, because it depends upon the
relative position of the earth and the stone. The stone is solicited
to move but cannot, so long as the muscular strength of the holder
prevents the solicitation from taking effect. The stone, therefore,
has potential energy, which becomes kinetic if it is let go, and the
amount of that kinetic energy which will be developed before it
strikes the earth depends on its position--on the fact that it is,
say, six feet off the earth, neither more nor less. Moreover, it can
be proved that the raiser of the stone had to exert as much energy in
order to place it in its position, as it will develop in falling.
Hence the energy which was exerted, and apparently exhausted, in
raising the stone, is potentially in the stone, in its raised
position, and will manifest itself when the stone is set free. Thus
the energy, withdrawn from the general stock to raise the stone, is
returned when it falls, and there is no change in the total amount.
Energy, as a whole, is conserved.

Taking this as a very broad and general statement of the essential
facts of the case, the raising of the stone is intelligible enough, as
a case of the communication of motion from one body to another. But
the potential energy of the raised stone is not so easily
intelligible. To all appearance, there is nothing either pushing or
pulling it towards the earth, or the earth towards it; and yet it is
quite certain that the stone tends to move towards the earth and the
earth towards the stone, in the way defined by the law of gravitation.

In the currently accepted language of science, the cause of motion, in
all such cases as this, when bodies tend to move towards or away from
one or another, without any discernible impact of other bodies, is
termed a 'force,' which is called 'attractive' in the one case, and
'repulsive' in the other. And such attractive or repulsive forces are
often spoken of as if they were real things, capable of exerting a
pull, or a push, upon the particles of matter concerned. Thus the
potential energy of the stone is commonly said to be due to the
'force' of gravity which is continually operating upon it.

Another illustration may make the case plainer. The bob of a pendulum
swings first to one side and then to the other of the centre of the
arc which it describes. Suppose it to have just reached the summit of
its right-hand half-swing. It is said that the 'attractive forces' of
the bob for the earth, and of the earth for the bob, set the former in
motion; and as these 'forces' are continually in operation, they
confer an accelerated velocity on the bob; until, when it reaches the
centre of its swing, it is, so to speak, fully charged with kinetic
energy. If, at this moment, the whole material universe, except the
bob, were abolished, it would move for ever in the direction of a
tangent to the middle of the arc described. As a matter of fact, it
is compelled to travel through its left-hand half-swing, and thus
virtually to go up hill. Consequently, the 'attractive forces' of the
bob and the earth are now acting against it, and constitute a
resistance which the charge of kinetic energy has to overcome. But, as
this charge represents the operation of the attractive forces during
the passage of the bob through the right-hand half-swing down to the
centre of the arc, so it must needs be used up by the passage of the
bob upwards from the centre of the arc to the summit of the left-hand
half-swing. Hence, at this point, the bob comes to a momentary rest.
The last fraction of kinetic energy is just neutralised by the action
of the attractive forces, and the bob has only potential energy equal
to that with which it started. So that the sum of the phenomena may be
stated thus: At the summit of either half-arc of its swing, the bob
has a certain amount of potential energy; as it descends it gradually
exchanges this for kinetic energy, until at the centre it possesses an
equivalent amount of kinetic energy; from this point onwards, it
gradually loses kinetic energy as it ascends, until, at the summit of
the other half-arc, it has acquired an exactly similar amount of
potential energy. Thus, on the whole transaction, nothing is either
lost or gained; the quantity of energy is always the same, but it
passes from one form into the other.

To all appearance, the phenomena exhibited by the pendulum are not to
be accounted for by impact: in fact, it is usually assumed that
corresponding phenomena would take place if the earth and the pendulum
were situated in an absolute vacuum, and at any conceivable distance
from, one another. If this be so, it follows that there must be two
totally different kinds of causes of motion: the one impact--a _vera
causa_, of which, to all appearance, we have constant experience; the
other, attractive or repulsive 'force'--a metaphysical entity which is
physically inconceivable. Newton expressly repudiated the notion of
the existence of attractive forces, in the sense in which that term is
ordinarily understood; and he refused to put forward any hypothesis as
to the physical cause of the so-called 'attraction of gravitation.' As
a general rule, his successors have been content to accept the
doctrine of attractive and repulsive forces, without troubling
themselves about the philosophical difficulties which it involves. But
this has not always been the case; and the attempt of Le Sage, in the
last century, to show that the phenomena of attraction and repulsion
are susceptible of explanation by his hypothesis of bombardment by
ultra-mundane particles, whether tenable or not, has the great merit
of being an attempt to get rid of the dual conception of the causes
of motion which has hitherto prevailed. On this hypothesis, the
hammering of the ultra-mundane corpuscles on the bob confers its
kinetic energy, on the one hand, and takes it away on the other; and
the state of potential energy means the condition of the bob during
the instant at which the energy, conferred by the hammering during the
one half-arc, has just been exhausted by the hammering during the
other half-arc. It seems safe to look forward to the time when the
conception of attractive and repulsive forces, having served its
purpose as a useful piece of scientific scaffolding, will be replaced
by the deduction of the phenomena known as attraction and repulsion,
from the general laws of motion.

The doctrine of the conservation of energy which I have endeavored to
illustrate is thus defined by the late Clerk Maxwell:

'The total energy of any body or system of bodies is a quantity which
can neither be increased nor diminished by any mutual action of such
bodies, though it may be transformed into any one of the forms of
which energy is susceptible.' It follows that energy, like matter, is
indestructible and ingenerable in nature. The phenomenal world, so far
as it is material, expresses the evolution and involution of energy,
its passage from the kinetic to the potential condition and back
again. Wherever motion of matter takes place, that motion is effected
at the expense of part of the total store of energy.

Hence, as the phenomena exhibited by living beings, in so far as they
are material, are all molar or molecular motions, these are included
under the general law. A living body is a machine by which energy is
transformed in the same sense as a steam-engine is so, and all its
movements, molar and molecular, are to be accounted for by the energy
which is supplied to it. The phenomena of consciousness which arise,
along with certain transformations of energy, cannot be interpolated
in the series of these transformations, inasmuch as they are not
motions to which the doctrine of the conservation of energy applies.
And, for the same reason, they do not necessitate the using up of
energy; a sensation has no mass and cannot be conceived to be
susceptible of movement. That a particular molecular motion does give
rise to a state of consciousness is experimentally certain; but the
how and why of the process are just as inexplicable as in the case of
the communication of kinetic energy by impact.

When dealing with the doctrine of the ultimate constitution of matter,
we found a certain resemblance between the oldest speculations and the
newest doctrines of physical philosophers. But there is no such
resemblance between the ancient and modern views of motion and its
causes, except in so far as the conception of attractive and repulsive
forces may be regarded as the modified descendant of the Aristotelian
conception of forms. In fact, it is hardly too much to say that the
essential and fundamental difference between ancient and modern
physical science lies in the ascertainment of the true laws of statics
and dynamics in the course of the last three centuries; and in the
invention of mathematical methods of dealing with all the consequences
of these laws. The ultimate aim of modern physical science is the
deduction of the phenomena exhibited by material bodies from
physico-mathematical first principles. Whether the human intellect is
strong enough to attain the goal set before it may be a question, but
thither will it surely strive.

[Sidenote: (3) Evolution.]

The third great scientific event of our time, the rehabilitation of
the doctrine of evolution, is part of the same tendency of increasing
knowledge to unify itself, which has led to the doctrine of the
conservation of energy. And this tendency, again, is mainly a product
of the increasing strength conferred by physical investigation on the
belief in the universal validity of that orderly relation of facts,
which we express by the so-called 'Laws of Nature.'

[Sidenote: Early stages of this theory]

The growth of a plant from its seed, of an animal from its egg, the
apparent origin of innumerable living things from mud, or from the
putrefying remains of former organisms, had furnished the earlier
scientific thinkers with abundant analogies suggestive of the
conception of a corresponding method of cosmic evolution from a
formless 'chaos' to an ordered world which might either continue for
ever or undergo dissolution into its elements before starting on a new
course of evolution. It is therefore no wonder that, from the days of
the Ionian school onwards, the view that the universe was the result
of such a process should have maintained itself as a leading dogma of
philosophy. The emanistic theories which played so great a part in
Neoplatonic philosophy and Gnostic theology are forms of evolution. In
the seventeenth century, Descartes propounded a scheme of evolution,
as an hypothesis of what might have been the mode of origin of the
world, while professing to accept the ecclesiastical scheme of
creation, as an account of that which actually was its manner of
coming into existence. In the eighteenth century, Kant put forth a
remarkable speculation as to the origin of the solar system, closely
similar to that subsequently adopted by Laplace and destined to become
famous under the title of the 'nebular hypothesis.'

The careful observations and the acute reasonings of the Italian
geologists of the seventeenth and eighteenth centuries; the
speculations of Leibnitz in the 'Protogaea' and of Buffon in his
'Théorie de la Terre;' the sober and profound reasonings of Hutton, in
the latter part of the eighteenth century; all these tended to show
that the fabric of the earth itself implied the continuance of
processes of natural causation for a period of time as great, in
relation to human history, as the distances of the heavenly bodies
from us are, in relation to terrestrial standards of measurement. The
abyss of time began to loom as large as the abyss of space. And this
revelation to sight and touch, of a link here and a link there of a
practically infinite chain of natural causes and effects, prepared the
way, as perhaps nothing else has done, for the modern form of the
ancient theory of evolution.

In the beginning of the eighteenth century, De Maillet made the first
serious attempt to apply the doctrine to the living world. In the
latter part of it, Erasmus Darwin, Goethe, Treviranus, and Lamarck
took up the work more vigorously and with better qualifications. The
question of special creation, or evolution, lay at the bottom of the
fierce disputes which broke out in the French Academy between Cuvier
and St.-Hilaire; and, for a time, the supporters of biological
evolution were silenced, if not answered, by the alliance of the
greatest naturalist of the age with their ecclesiastical opponents.
Catastrophism, a short-sighted teleology, and a still more
short-sighted orthodoxy, joined forces to crush evolution.

Lyell and Poulett Scrope, in this country, resumed the work of the
Italians and of Hutton; and the former, aided by a marvellous power of
clear exposition, placed upon an irrefragable basis the truth that
natural causes are competent to account for all events, which can be
proved to have occurred, in the course of the secular changes which
have taken place during the deposition of the stratified rocks. The
publication of 'The Principles of Geology,' in 1830, constituted an
epoch in geological science. But it also constituted an epoch in the
modern history of the doctrines of evolution, by raising in the mind
of every intelligent reader this question: If natural causation is
competent to account for the not-living part of our globe, why should
it not account for the living part?

By keeping this question before the public for some thirty years,
Lyell, though the keenest and most formidable of the opponents of the
transmutation theory, as it was formulated by Lamarck, was of the
greatest possible service in facilitating the reception of the sounder
doctrines of a later day. And, in like fashion, another vehement
opponent of the transmutation of species, the elder Agassiz, was
doomed to help the cause he hated. Agassiz not only maintained the
fact of the progressive advance in organisation of the inhabitants of
the earth at each successive geological epoch, but he insisted upon
the analogy of the steps of this progression with those by which the
embryo advances to the adult condition, among the highest forms of
each group. In fact, in endeavoring to support these views he went a
good way beyond the limits of any cautious interpretation of the facts
then known.

[Sidenote: Darwin]

Although little acquainted with biological science, Whewell seems to
have taken particular pains with that part of his work which deals
with the history of geological and biological speculation; and several
chapters of his seventeenth and eighteenth books, which comprise the
history of physiology, of comparative anatomy and of the
palætiological sciences, vividly reproduce the controversies of the
early days of the Victorian epoch. But here, as in the case of the
doctrine of the conservation of energy, the historian of the inductive
sciences has no prophetic insight; not even a suspicion of that which
the near future was to bring forth. And those who still repeat the
once favorite objection that Darwin's 'Origin of Species' is nothing
but a new version of the 'Philosophie zoologique' will find that, so
late as 1844, Whewell had not the slightest suspicion of Darwin's main
theorem, even as a logical possibility. In fact, the publication of
that theorem by Darwin and Wallace, in 1859, took all the biological
world by surprise. Neither those who were inclined towards the
'progressive transmutation' or 'development' doctrine, as it was then
called, nor those who were opposed to it, had the slightest suspicion
that the tendency to variation in living beings, which all admitted as
a matter of fact; the selective influence of conditions, which no one
could deny to be a matter of fact, when his attention was drawn to the
evidence; and the occurrence of great geological changes which also
was matter of fact; could be used as the only necessary postulates of
a theory of the evolution of plants and animals which, even if not at
once, competent to explain all the known facts of biological science,
could not be shown to be inconsistent with any. So far as biology is
concerned, the publication of the 'Origin of Species,' for the first
time, put the doctrine of evolution, in its application to living
things, upon a sound scientific foundation. It became an instrument of
investigation, and in no hands did it prove more brilliantly
profitable than in those of Darwin himself. His publications on the
effects of domestication in plants and animals, on the influence of
cross-fertilisation, on flowers as organs for effecting such
fertilisation, on insectivorous plants, on the motions of plants,
pointed out the routes of exploration which have since been followed
by hosts of inquirers, to the great profit of science.

Darwin found the biological world a more than sufficient field for
even his great powers, and left the cosmical part of the doctrine to
others. Not much has been added to the nebular hypothesis, since the
time of Laplace, except that the attempt to show (against that
hypothesis) that all nebulæ are star clusters, has been met by the
spectroscopic proof of the gaseous condition of some of them.
Moreover, physicists of the present generation appear now to accept
the secular cooling of the earth, which is one of the corollaries of
that hypothesis. In fact, attempts have been made, by the help of
deductions from the data of physics, to lay down an approximate limit
to the number of millions of years which have elapsed since the earth
was habitable by living beings. If the conclusions thus reached should
stand the test of further investigation, they will undoubtedly be very
valuable. But, whether true or false, they can have no influence upon
the doctrine of evolution in its application to living organisms. The
occurrence of successive forms of life upon our globe is an historical
fact, which cannot be disputed; and the relation of these successive
forms, as stages of evolution of the same type, is established in
various cases. The biologist has no means of determining the time over
which the process of evolution has extended, but accepts the
computation of the physical geologist and the physicist, whatever that
may be.

[Sidenote: and philosophy]

Evolution as a philosophical doctrine applicable to all phenomena,
whether physical or mental, whether manifested by material atoms or by
men in society, has been dealt with systematically in the 'Synthetic
Philosophy' of Mr. Herbert Spencer. Comment on that great undertaking
would not be in place here. I mention it because, so far as I know, it
is the first attempt to deal, on scientific principles, with modern
scientific facts and speculations. For the 'Philosophic positive' of
M. Comte, with which Mr. Spencer's system of philosophy is sometimes
compared, though it professes a similar object, is unfortunately
permeated by a thoroughly unscientific spirit, and its author had no
adequate acquaintance with the physical sciences even of his own time.

       *       *       *       *       *

The doctrine of evolution, so far as the present physical cosmos is
concerned, postulates the fixity of the rules of operation of the
causes of motion in the material universe. If all kinds of matter are
modifications of one kind, and if all modes of motion are derived from
the same energy, the orderly evolution of physical nature out of one
substratum and one energy implies that the rules of action of that
energy should be fixed and definite. In the past history of the
universe, back to that point, there can be no room for chance or
disorder. But it is possible to raise the question whether this
universe of simplest matter and definitely operating energy, which
forms our hypothetical starting point, may not itself be a product of
evolution from a universe of such matter, in which the manifestations
of energy were not definite--in which, for example, our laws of motion
held good for some units and not for others, or for the same units at
one time and not at another--and which would therefore be a real
epicurean chance-world?

For myself, I must confess that I find the air of this region of
speculation too rarefied for my constitution, and I am disposed to
take refuge in 'ignoramus et ignorabimus.'

[Sidenote: Other achievements in physical science.]

The execution of my further task, the indication of the most important
achievements in the several branches of physical science during the
last fifty years, is embarrassed by the abundance of the objects of
choice; and by the difficulty which everyone, but a specialist in each
department, must find in drawing a due distinction between discoveries
which strike the imagination by their novelty, or by their practical
influence, and those unobtrusive but pregnant observations and
experiments in which the germs of the great things of the future
really lie. Moreover, my limits restrict me to little more than a bare
chronicle of the events which I have to notice.

[Sidenote: Physics and chemistry.]

In physics and chemistry, the old boundaries of which sciences are
rapidly becoming effaced, one can hardly go wrong in ascribing a
primary value to the investigations into the relation between the
solid, liquid, and gaseous states of matter on the one hand, and
degrees of pressure and of heat on the other. Almost all, even the
most refractory, solids have been vaporised by the intense heat of the
electric arc; and the most refractory gases have been forced to assume
the liquid, and even the solid, forms by the combination of high
pressure with intense cold. It has further been shown that there is no
discontinuity between these states--that a gas passes into the liquid
state through a condition which is neither one nor the other, and that
a liquid body becomes solid, or a solid liquid, by the intermediation
of a condition in which it is neither truly solid nor truly liquid.

Theoretical and experimental investigations have concurred in the
establishment of the view that a gas is a body, the particles of which
are in incessant rectilinear motion at high velocities, colliding with
one another and bounding back when they strike the walls of the
containing vessel; and, on this theory, the already ascertained
relations of gaseous bodies to heat and pressure have been shown to be
deducible from mechanical principles. Immense improvements have been
effected, in the means of exhausting a given space of its gaseous
contents; and experimentation on the phenomena which attend the
electric discharge and the action of radiant heat, within the
extremely rarefied media thus produced, has yielded a great number of
remarkable results, some of which have been made familiar to the
public by the Gieseler tubes and the radiometer. Already, these
investigations have afforded an unexpected insight into the
constitution of matter and its relations with thermal and electric
energy, and they open up a vast field for future inquiry into some of
the deepest problems of physics. Other important steps, in the same
direction, have been effected by investigations into the absorption of
radiant heat proceeding from different sources by solid, fluid, and
gaseous bodies. And it is a curious example of the interconnection of
the various branches of physical science, that some of the results
thus obtained have proved of great importance in meteorology.

[Sidenote: The spectroscope.]

The existence of numerous dark lines, constant in their number and
position in the various regions of the solar spectrum, was made out by
Fraunhofer in the early part of the present century, but more than
forty years elapsed before their causes were ascertained and their
importance recognised. Spectroscopy, which then took its rise, is
probably that employment of physical knowledge, already won, as a
means of further acquisition, which most impresses the imagination.
For it has suddenly and immensely enlarged our power of overcoming the
obstacles which almost infinite minuteness on the one hand, and almost
infinite distance on the other, have hitherto opposed to the
recognition of the presence and the condition of matter. One
eighteen-millionth of a grain of sodium in the flame of a spirit-lamp
may be detected by this instrument; and, at the same time, it gives
trust-worthy indications of the material constitution not only of the
sun, but of the farthest of those fixed stars and nebulæ which afford
sufficient light to affect the eye, or the photographic plate, of the
inquirer.

[Sidenote: Electricity.]

The mathematical and experimental elucidation of the phenomena of
electricity, and the study of the relations of this form of energy
with chemical and thermal action, had made extensive progress before
1837. But the determination of the influence of magnetism on light,
the discovery of diamagnetism, of the influence of crystalline
structure on magnetism, and the completion of the mathematical theory
of electricity, all belong to the present epoch. To it also appertain
the practical execution and the working out of the results of the
great international system of observations on terrestrial magnetism,
suggested by Humboldt in 1836; and the invention of instruments of
infinite delicacy and precision for the quantitative determination of
electrical phenomena. The voltaic battery has received vast
improvements; while the invention of magneto-electric engines and of
improved means of producing ordinary electricity has provided sources
of electrical energy vastly superior to any before extant in power,
and far more convenient for use.

It is perhaps this branch of physical science which may claim the palm
for its practical fruits, no less than for the aid which it has
furnished to the investigation of other parts of the field of physical
science. The idea of the practicability of establishing a
communication between distant points, by means of electricity, could
hardly fail to have simmered in the minds of ingenious men since,
well nigh a century ago, experimental proof was given that electric
disturbances could be propagated through a wire twelve thousand feet
long. Various methods of carrying the suggestion into practice had
been carried out with some degree of success; but the system of
electric telegraphy, which, at the present time, brings all parts of
the civilised world within a few minutes of one another, originated
only about the commencement of the epoch under consideration. In its
influence on the course of human affairs, this invention takes its
place beside that of gunpowder, which tended to abolish the physical
inequalities of fighting men; of printing, which tended to destroy the
effect of inequalities in wealth among learning men; of steam
transport, which has done the like for travelling men. All these gifts
of science are aids in the process of levelling up; of removing the
ignorant and baneful prejudices of nation against nation, province
against province, and class against class; of assuring that social
order which is the foundation of progress, which has redeemed Europe
from barbarism, and against which one is glad to think that those who,
in our time, are employing themselves in fanning the embers of ancient
wrong, in setting class against class, and in trying to tear asunder
the existing bonds of unity, are undertaking a futile struggle. The
telephone is only second in practical importance to the electric
telegraph. Invented, as it were, only the other day, it has already
taken its place as an appliance of daily life. Sixty years ago, the
extraction of metals from their solutions, by the electric current,
was simply a highly interesting scientific fact. At the present day,
the galvano-plastic art is a great industry; and, in combination with
photography, promises to be of endless service in the arts. Electric
lighting is another great gift of science to civilisation, the
practical effects of which have not yet been fully developed, largely
on account of its cost. But those whose memories go back to the
tinder-box period, and recollect the cost of the first lucifer
matches, will not despair of the results of the application of science
and ingenuity to the cheap production of anything for which there is a
large demand.

The influence of the progress of electrical knowledge and invention
upon that of investigation in other fields of science is highly
remarkable. The combination of electrical with mechanical contrivances
has produced instruments by which, not only may extremely small
intervals of time be exactly measured, but the varying rapidity of
movements, which take place in such intervals and appear to the
ordinary sense instantaneous, is recorded. The duration of the winking
of an eye is a proverbial expression for an instantaneous action;
but, by the help of the revolving cylinder and the electrical
marking-apparatus, it is possible to obtain a graphic record of such
an action, in which, if it endures a second, that second shall be
subdivided into a hundred, or a thousand, equal parts, and the state
of the action at each hundredth, or thousandth, of a second exhibited.
In fact, these instruments may be said to be time-microscopes. Such
appliances have not only effected a revolution in physiology, by the
power of analysing the phenomena of muscular and nervous activity
which they have conferred, but they have furnished new methods of
measuring the rate of movement of projectiles to the artillerist.
Again, the microphone, which renders the minutest movements audible,
and which enables a listener to hear the footfall of a fly, has
equipped the sense of hearing with the means of entering almost as
deeply into the penetralia of nature, as does the sense of sight.

[Sidenote: Photography as an instrument of science.]

That light exerts a remarkable influence in bringing about certain
chemical combinations and decompositions was well known fifty years
ago, and various more or less successful attempts to produce permanent
pictures, by the help of that knowledge, had already been made. It was
not till 1839, however, that practical success was obtained; but the
'daguerreotypes' were both cumbrous and costly, and photography would
never have attained its present important development had not the
progress of invention substituted paper and glass for the silvered
plates then in use. It is not my affair to dwell upon the practical
application of the photography of the present day, but it is germane
to my purpose to remark that it has furnished a most valuable
accessory to the methods of recording motions and lapse of time
already in existence. In the hands of the astronomer and the
meteorologist, it has yielded means of registering terrestrial, solar,
planetary, and stellar phenomena, independent of the sources of error
attendant on ordinary observation; in the hands of the physicist, not
only does it record spectroscopic phenomena with unsurpassable ease
and precision, but it has revealed the existence of rays having
powerful chemical energy, or beyond the visible limits of either end
of the spectrum; while, to the naturalist, it furnishes the means by
which the forms of many highly complicated objects may be represented,
without that possibility of error which is inherent in the work of the
draughtsman. In fact, in many cases, the stern impartiality of
photography is an objection to its employment: it makes no distinction
between the important and the unimportant; and hence photographs of
dissections, for example, are rarely so useful as the work of a
draughtsman who is at once accurate and intelligent.

[Sidenote: Astronomy,]

The determination of the existence of a new planet, Neptune, far
beyond the previously known bounds of the solar system, by
mathematical deduction from the facts of perturbation; and the
immediate confirmation of that determination, in the year 1846, by
observers who turned their telescopes into the part of the heavens
indicated as its place, constitute a remarkable testimony of nature to
the validity of the principles of the astronomy of our time. In
addition, so many new asteroids have been added to those which were
already known to circulate in the place which theoretically should be
occupied by a planet, between Mars and Jupiter, that their number now
amounts to between two and three hundred. I have already alluded to
the extension of our knowledge of the nature of the heavenly bodies by
the employment of spectroscopy. It has not only thrown wonderful
light upon the physical and chemical constitution of the sun, fixed
stars, and nebulæ, and comets, but it holds out a prospect of
obtaining definite evidence as to the nature of our so-called
elementary bodies.

[Sidenote: its relation to geology.]

The application of the generalisations of thermotics to the problem of
the duration of the earth, and of deductions from tidal phenomena to
the determination of the length of the day and of the time of
revolution of the moon, in past epochs of the history of the universe;
and the demonstration of the competency of the great secular changes,
known under the general name of the precession of the equinoxes, to
cause corresponding modifications in the climate of the two
hemispheres of our globe, have brought astronomy into intimate
relation with geology. Geology, in fact, proves that, in the course of
the past history of the earth, the climatic conditions of the same
region have been widely different, and seeks the explanation of this
important truth from the sister sciences. The facts that, in the
middle of the Tertiary epoch, evergreen trees abounded within the
arctic circle; and that, in the long subsequent Quaternary epoch, an
arctic climate, with its accompaniment of gigantic glaciers, obtained
in the northern hemisphere, as far south as Switzerland and Central
France, are as well established as any truths of science. But, whether
the explanation of these extreme variations in the mean temperature of
a great part of the northern hemisphere is to be sought in the
concomitant changes in the distribution of land and water surfaces of
which geology affords evidence, or in astronomical conditions, such as
those to which I have referred, is a question which must await its
answer from the science of the future.

[Sidenote: Biological sciences.]

[Sidenote: The 'cell theory.']

Turning now to the great steps in that progress which the biological
sciences have made since 1837, we are met, on the threshold of our
epoch, with perhaps the greatest of all--namely, the promulgation by
Schwann, in 1839, of the generalisation known as the 'cell theory,'
the application and extension of which by a host of subsequent
investigators has revolutionised morphology, development, and
physiology. Thanks to the immense series of labors thus inaugurated,
the following fundamental truths have been established.

[Sidenote: Fundamental truths established.]

All living bodies contain substances of closely similar physical and
chemical composition, which constitute the physical basis of life,
known as protoplasm. So far as our present knowledge goes, this takes
its origin only from pre-existing protoplasm.

All complex living bodies consist, at one period of their existence,
of an aggregate of minute portions of such substance, of similar
structure, called cells, each cell having its own life independent of
the others, though influenced by them.

All the morphological characters of animals and plants are the results
of the mode of multiplication, growth, and structural metamorphosis of
these cells, considered as morphological units.

All the physiological activities of animals and plants--assimilation,
secretion, excretion, motion, generation--are the expression of the
activities of the cells considered as physiological units. Each
individual, among the higher animals and plants, is a synthesis of
millions of subordinate individualities. Its individuality, therefore,
is that of a 'civitas' in the ancient sense, or that of the Leviathan
of Hobbes.

There is no absolute line of demarcation between animals and plants.
The intimate structure, and the modes of change, in the cells of the
two are fundamentally the same. Moreover, the higher forms are
evolved from lower, in the course of their development, by analogous
processes of differentiation, coalescence, and reduction in both the
vegetable and the animal worlds.

At the present time, the cell theory, in consequence of recent
investigations into the structure and metamorphosis of the 'nucleus,'
is undergoing a new development of great significance, which, among
other things, foreshadows the possibility of the establishment of a
physical theory of heredity, on a safer foundation than those which
Buffon and Darwin have devised.

[Sidenote: Spontaneous generation disproved.]

The popular belief in abiogenesis, or the so-called 'spontaneous'
generation of the lower forms of life, which was accepted by all the
philosophers of antiquity, held its ground down to the middle of the
seventeenth century. Notwithstanding the frequent citation of the
phrase, wrongfully attributed to Harvey, 'Omne vivum ex ovo,' that
great physiologist believed in spontaneous generation as firmly as
Aristotle did. And it was only in the latter part of the seventeenth
century, that Redi, by simple and well-devised experiments,
demonstrated that, in a great number of cases of supposed spontaneous
generation, the animals which made their appearance owed their origin
to the ordinary process of reproduction, and thus shook the ancient
doctrine to its foundations. In the middle of the eighteenth century,
it was revived, in a new form, by Needham and Buffon; but the
experiments of Spallanzani enforced the conclusions of Redi, and
compelled the advocates of the occurrence of spontaneous generation to
seek evidence for their hypothesis only among the parasites and the
lowest and minutest organisms. It is just fifty years since Schwann
and others proved that, even with respect to them, the supposed
evidence of abiogenesis was untrustworthy.

During the present epoch, the question, whether living matter can be
produced in any other way than by the physiological activity of other
living matter, has been discussed afresh with great vigor; and the
problem has been investigated by experimental methods of a precision
and refinement unknown to previous investigators. The result is that
the evidence in favor of abiogenesis has utterly broken down, in every
case which has been properly tested. So far as the lowest and minutest
organisms are concerned, it has been proved that they never make their
appearance, if those precautions by which their germs are certainly
excluded are taken. And, in regard to parasites, every case which
seemed to make for their generation from the substance of the animal,
or plant, which they infest has been proved to have a totally
different significance. Whether not-living matter may pass, or ever
has, under any conditions, passed into living matter, without the
agency of pre-existing living matter, necessarily remains an open
question; all that can be said is that it does not undergo this
metamorphosis under any known conditions. Those who take a monistic
view of the physical world may fairly hold abiogenesis as a pious
opinion, supported by analogy and defended by our ignorance. But, as
matters stand, it is equally justifiable to regard the physical world
as a sort of dual monarchy. The kingdoms of living matter and of
not-living matter are under one system of laws, and there is a perfect
freedom of exchange and transit from one to the other. But no claim to
biological nationality is valid except birth.

[Sidenote: Morphology.]

In the department of anatomy and development, a host of accurate and
patient inquirers, aided by novel methods of preparation, which
enable the anatomist to exhaust the details of visible structure and
to reproduce them with geometrical precision, have investigated every
important group of living animals and plants, no less than the fossil
relics of former faunæ and floræ. An enormous addition has thus been
made to our knowledge, especially of the lower forms of life, and it
may be said that morphology, however inexhaustible in detail, is
complete in its broad features. Classification, which is merely a
convenient summary expression of morphological facts, has undergone a
corresponding improvement. The breaks which formerly separated our
groups from one another, as animals from plants, vertebrates from
invertebrates, cryptogams from phanerogams, have either been filled
up, or shown to have no theoretical significance. The question of the
position of man, as an animal, has given rise to much disputation,
with the result of proving that there is no anatomical or
developmental character by which he is more widely distinguished from
the group of animals most nearly allied to him, than they are from one
another. In fact, in this particular, the classification of Linnæus
has been proved to be more in accordance with the facts than those of
most of his successors.

[Sidenote: Anthropology.]

The study of man, as a genus and species of the animal world,
conducted with reference to no other considerations than those which
would be admitted by the investigator of any other form of animal
life, has given rise to a special branch of biology, known, as
Anthropology, which has grown with great rapidity. Numerous societies
devoted to this portion of science have sprung up, and the energy of
its devotees has produced a copious literature. The physical
characters of the various races of men have been studied with a
minuteness and accuracy heretofore unknown; and demonstrative
evidence of the existence of human contemporaries of the extinct
animals of the latest geological epoch has been obtained, physical
science has thus been brought into the closest relation with history
and with archæology; and the striking investigations which, during our
time, have put beyond doubt the vast antiquity of Babylonian and
Egyptian civilisation, are in perfect harmony with the conclusions of
anthropology as to the antiquity of the human species.

Classification is a logical process which consists in putting together
those things which are like and keeping asunder those which are
unlike; and a morphological classification, of course, takes notes
only of morphological likeness and unlikeness. So long, therefore, as
our morphological knowledge was almost wholly confined to anatomy, the
characters of groups were solely anatomical; but as the phenomena of
embryology were explored, the likeness and unlikeness of individual
development had to be taken into account; and, at present, the study
of ancestral evolution introduces a new element of likeness and
unlikeness which is not only eminently deserving of recognition, but
must ultimately predominate over all others. A classification which
shall represent the process of ancestral evolution is, in fact, the
end which the labors of the philosophical taxonomist must keep in
view. But it is an end which cannot be attained until the progress of
palæontology has given us far more insight than we yet possess, into
the historical facts of the case. Much of the speculative 'phylogeny,'
which abounds among my present contemporaries, reminds me very
forcibly of the speculative morphology, unchecked by a knowledge of
development, which was rife in my youth. As hypothesis, suggesting
inquiry in this or that direction, it is often extremely useful; but,
when the product of such speculation is placed on a level with those
generalisations of morphological truths which are represented by the
definitions of natural groups, it tends to confuse fancy with fact and
to create mere confusion. We are in danger of drifting into a new
'Natur-Philosophie' worse than the old, because there is less excuse
for it. Boyle did great service to science by his 'Sceptical Chemist,'
and I am inclined to think that, at the present day, a 'Sceptical
Biologist' might exert an equally beneficent influence.

[Sidenote: Physiology.]

Whoso wishes to gain a clear conception of the progress of physiology,
since 1837, will do well to compare Müller's 'Physiology,' which
appeared in 1835, and Drapiez's edition of Richard's 'Nouveaux
Eléments de Botanique,' published in 1837, with any of the present
handbooks of animals and vegetable physiology. Müller's work was a
masterpiece, unsurpassed since the time of Haller, and Richard's book
enjoyed a great reputation at the time; but their successors transport
one into a new world. That which characterises the new physiology is
that it is permeated by, and indeed based upon, conceptions which,
though not wholly absent, are but dawning on the minds of the older
writers.

Modern physiology sets forth as its chief ends: Firstly, the
ascertainment of the facts and conditions of cell-life in general.
Secondly, in composite organisms, the analysis of the functions of
organs into those of the cells of which they are composed. Thirdly,
the explication of the processes by which this local cell-life is
directly, or indirectly, controlled and brought into relation with the
life of the rest of the cells which compose the organism. Fourthly,
the investigation of the phenomena of life in general, on the
assumption that the physical and chemical processes which take place
in the living body are of the same order as those which take place out
of it; and that whatever energy is exerted in producing such phenomena
is derived from the common stock of energy in the universe. In the
fifth place, modern physiology investigates the relation between
physical and psychical phenomena, on the assumption that molecular
changes in definite portions of nervous matter stand in the relation
of necessary antecedents to definite mental states and operations. The
work which has been done in each of the directions here indicated is
vast, and the accumulation of solid knowledge, which has been
effected, is correspondingly great. For the first time in the history
of science, physiologists are now in the position to say that they
have arrived at clear and distinct, though by no means complete,
conceptions of the manner in which the great functions of
assimilation, respiration, secretion, distribution of nutriment,
removal of waste products, motion, sensation, and reproduction are
performed; while the operation of the nervous system, as a regulative
apparatus, which influences the origination and the transmission of
manifestations of activity, either within itself or in other organs,
has been largely elucidated.

[Sidenote: Practical value of physiological discovery.]

I have pointed out, in an earlier part of this chapter, that the
history of all branches of science proves that they must attain a
considerable stage of development before they yield practical
'fruits;' and this is eminently true of physiology. It is only within
the present epoch, that physiology and chemistry have reached the
point at which they could offer a scientific foundation to
agriculture; and it is only within the present epoch, that zoology and
physiology have yielded any very great aid to pathology and hygiene.
But within that time, they have already rendered highly important
services by the exploration of the phenomena of parasitism. Not only
have the history of the animal parasites, such as the tapeworms and
the trichina, which infest men and animals, with deadly results, been
cleared up by means of experimental investigations, and efficient
modes of prevention deduced from the data so obtained; but the
terrible agency of the parasitic fungi and of the infinitesimally
minute microbes, which work far greater havoc among plants and
animals, has been brought to light. The 'particulate' or 'germ' theory
of disease, as it is called, long since suggested, has obtained a firm
foundation, in so far as it has been proved to be true in respect of
sundry epidemic disorders. Moreover, it has theoretically justified
prophylactic measures, such as vaccination, which formerly rested on a
merely empirical basis; and it has been extended to other diseases
with excellent results. Further, just as the discovery of the cause of
scabies proved the absurdity of many of the old prescriptions for the
prevention and treatment of that disease; so the discovery of the
cause of splenic fever, and other such maladies, has given a new
direction to prophylactic and curative measures against the worst
scourges of humanity. Unless the fanaticism of philozoic sentiment
overpowers the voice of philanthropy, and the love of dogs and cats
supersedes that of one's neighbor, the progress of experimental
physiology and pathology will, indubitably, in course of time, place
medicine and hygiene upon a rational basis. Two centuries ago England
was devastated by the plague; cleanliness and common sense were enough
to free us from its ravages. One century since, small-pox was almost
as great a scourge; science, though working empirically, and almost in
the dark, has reduced that evil to relative insignificance. At the
present time, science, working in the light of clear knowledge, has
attacked splenic fever and has beaten it; it is attacking hydrophobia
with no mean promise of success; sooner or later it will deal, in the
same way, with diphtheria, typhoid and scarlet fever. To one who has
seen half a street swept clear of its children, or has lost his own by
these horrible pestilences, passing one's offspring through the fire
to Moloch seems humanity, compared with the proposal to deprive them
of half their chances of health and life because of the discomfort to
dogs and cats, rabbits and frogs, which may be involved in the search
for means of guarding them.

[Sidenote: Scientific exploration.]

An immense extension has been effected in our knowledge of the
distribution of plants and animals; and the elucidation of the causes
which have brought about that distribution has been greatly advanced.
The establishment of meteorological observations by all civilised
nations, has furnished a solid foundation to climatology; while a
growing sense of the importance of the influence of the 'struggle for
existence' affords a wholesome check to the tendency to overrate the
influence of climate on distribution. Expeditions, such as that of the
Challenger,' equipped, not for geographical exploration and discovery,
but for the purpose of throwing light on problems of physical and
biological science, have been sent out by our own and other
Governments, and have obtained stores of information of the greatest
value. For the first time, we are in possession of something like
precise knowledge of the physical features of the deep seas, and of
the living population of the floor of the ocean. The careful and
exhaustive study of the phenomena presented by the accumulations of
snow and ice, in polar and mountainous regions, which has taken place
in our time, has not only revealed to the geologist an agent of
denudation and transport, which has slowly and quietly produced
effects, formerly confidently referred to diluvial catastrophes, but
it has suggested new methods of accounting for various puzzling facts
of distribution.

[Sidenote: Palæontology.]

Palæontology, which treats of the extinct forms of life and their
succession and distribution upon our globe, a branch of science which
could hardly be said to exist a century ago, has undergone a wonderful
development in our epoch. In some groups of animals and plants, the
extinct representatives, already known, are more numerous and
important than the living. There can be no doubt that the existing
Fauna and Flora is but the last term of a long series of equally
numerous contemporary species, which have succeeded one another, by
the slow and gradual substitution of species for species, in the vast
interval of time which has elapsed between the deposition of the
earliest fossiliferous strata and the present day. There is no
reasonable ground for believing that the oldest remains yet obtained
carry us even near the beginnings of life. The impressive warnings of
Lyell against hasty speculations, based upon negative evidence, have
been fully justified; time after time, highly organised types have
been discovered in formations of an age in which the existence of such
forms of life had been confidently declared to be impossible. The
western territories of the United States alone have yielded a world of
extinct animal forms, undreamed of fifty years ago. And, wherever
sufficiently numerous series of the remains of any given group, which
has endured for a long space of time, are carefully examined, their
morphological relations are never in discordance with the requirements
of the doctrine of evolution, and often afford convincing evidence of
it. At the same time, it has been shown that certain forms persist
with very little change, from the oldest to the newest fossiliferous
formations; and thus show that progressive development is a
contingent, and not a necessary result, of the nature of living
matter.

[Sidenote: Geology.]

Geology is, as it were, the biology of our planet as a whole. In so
far as it comprises the surface configuration and the inner structure
of the earth, it answers to morphology; in so far as it studies
changes of condition and their causes, it corresponds with physiology;
in so far as it deals with the causes which have effected the progress
of the earth from its earliest to its present state, it forms part of
the general doctrine of evolution. An interesting contrast between the
geology of the present day and that of half a century ago, is
presented by the complete emancipation of the modern geologist from
the controlling and perverting influence of theology, all-powerful at
the earlier date. As the geologist of my young days wrote, he had one
eye upon fact, and the other on Genesis; at present, he wisely keeps
both eyes on fact, and ignores the pentateuchal mythology altogether.
The publication of the 'Principles of Geology' brought upon its
illustrious author a period of social ostracism; the instruction given
to our children is based upon those principles. Whewell had the
courage to attack Lyell's fundamental assumption (which surely is a
dictate of common sense) that we ought to exhaust known causes before
seeking for the explanation of geological phenomena in causes of which
we have no experience. But geology has advanced to its present state
by working from Lyell's[J] axiom; and, to this day, the record of the
stratified rocks affords no proof that the intensity or the rapidity
of the causes of change has ever varied, between wider limits, than
those between which the operations of nature have taken place in the
youngest geological epochs.

An incalculable benefit has accrued to geological science from the
accurate and detailed surveys, which have now been executed by skilled
geologists employed by the Governments of all parts of the civilised
world. In geology, the study of large maps is as important as it is
said to be in politics; and sections, on a true scale, are even more
important, in so far as they are essential to the apprehension of the
extraordinary insignificance of geological perturbations in relation
to the whole mass of our planet. It should never be forgotten that
what we call 'catastrophes,' are, in relation to the earth, changes,
the equivalents of which would be well represented by the development
of a few pimples, or the scratch of a pin, on a man's head. Vast
regions of the earth's surface remain geologically unknown; but the
area already fairly explored is many times greater than it was in
1837; and, in many parts of Europe and the United States, the
structure of the superficial crust of the earth has been investigated
with great minuteness.

The parallel between Biology and Geology, which I have drawn, is
further illustrated by the modern growth of that branch of the science
known as Petrology, which answers to Histology, and has made the
microscope as essential an instrument to the geological as to the
biological investigator.

The evidence of the importance of causes now in operation has been
wonderfully enlarged by the study of glacial phenomena; by that of
earthquakes and volcanoes; and by that of the efficacy of heat and
cold, wind, rain, and rivers as agents of denudation and transport. On
the other hand, the exploration of coral reefs and of the deposits now
taking place at the bottom of the great oceans, has proved that, in
animal and plant life, we have agents of reconstruction of a potency
hitherto unsuspected.

There is no study better fitted than that of geology to impress upon
men of general culture that conviction of the unbroken sequence of the
order of natural phenomena, throughout the duration of the universe,
which is the great, and perhaps the most important, effect of the
increase of natural knowledge.


THE END.

FOOTNOTES:

     [A] There are excellent remarks to the same effect in
     Zeller's _Philosophie der Griechen_, Theil II. Abth. ii p.
     407, and in Eucken's _Die Methode der Aristotelischen,
     Forschung_, pp. 136 _et seq_.

     [B] Fresnel, after a brilliant career of discovery in some
     of the most difficult regions of physico-mathematical
     science, died at thirty-nine years of age. The following
     passage of a letter from him to Young (written in November
     1824), quoted by Whewell, so aptly illustrates the spirit
     which animates the scientific inquirer that I may cite it:

          'For a long time that sensibility, or that vanity,
          which people call love of glory is munch blunted
          in me. I labor much less to catch the suffrages of
          the public than to obtain an inward approval which
          has always been the mental reward of my efforts.
          Without doubt I have often wanted the spur of
          vanity to excite me to pursue my researches in
          moments of disgust and discouragement. But all the
          compliments which I have received from M.M. Arago,
          De Laplace, or Biot, never gave me so much
          pleasure as the discovery of a theoretical truth
          or the confirmation of a calculation by
          experiment.'

     [C] 'Mémorable exemple de l'impuissance des recherches
     collectives appliquées à la découverte des vérités
     nouvelles!' says one of the most distinguished of living
     French _savants_ of the corporate chemical work of the old
     Académie des Sciences. (See Berthelot, _Science et
     Philosophie_, p. 201.)

     [D] I am particularly indebted to my friend and colleague
     Professor Rücker, F.R.S., for the many acute criticisms and
     suggestions on my remarks respecting the ultimate problems
     of physics, with which he has favored me, and by which I
     have greatly profited.

     [E] I am aware that this proposition may be challenged. It
     may be said, for example, that, on the hypothesis of
     Boscovich, matter has no extension, being reduced to
     mathematical points serving as centres of 'forces.' But as
     the 'forces' of the various centres are conceived to limit
     one another's action in such a manner that an area around
     each centre has an individuality of its own extension comes
     back in the form of that area. Again, a very eminent
     mathematician and physicist--the late Clerk Maxwell--has
     declared that impenetrability is not essential to our
     notions of matter, and that two atoms may conceivably occupy
     the same space. I am loth to dispute any dictum of a
     philosopher as remarkable for the subtlety of his intellect
     as for his vast knowledge; but the assertion that one and
     the same point or area of space can have different
     (conceivably opposite) attributes appears to me to violate
     the principle of contradiction, which is the foundation not
     only of physical science, but of logic in general. It means
     that A can be not-A.

     [F] 'Molecule' would be the more appropriate name for such a
     particle. Unfortunately, chemists employ this term in a
     special sense, as a name for an aggregation of their
     smallest particles, for which they retain the designation of
     'atoms.'

     [G] 'At present more organic analyses are made in a single
     day than were accomplished before Liebig's time in a whole
     year.'--Hofmann, _Faraday Lecture_, p. 46.

     [H] In the preface to his _Mécanique Chimique_ M. Berthelot
     declares his object to be 'ramener la chimie tout entirère
     ... aux mêmes principes mécaniques qui régissent déjà les
     diverses branches de la physique.'

     [I] This is the more curious, as Ampère's hypothesis that
     vibrations of molecules, causing and caused by vibrations of
     the ether, constitute heat, is discussed. See vol. ii. p.
     587, 2nd ed. In the _Philosophy of the Inductive Sciences_,
     2nd ed., 1847, p. 239, Whewell remarks, _à propos_ of
     Bacon's definition of heat, 'that it is an expansive,
     restrained motion, modified in certain ways, and exerted in
     the smaller particles of the body;' that 'although the exact
     nature of heat is still an obscure and controverted matter,
     the science of heat now consists of many important truths;
     and that to none of these truths is there any approximation
     in Bacon's essay.' In point of fact, Bacon's statement,
     however much open to criticism, does contain a distinct
     approximation to the most important of all the truths
     respecting heat which had been discovered when Whewell
     wrote.

     [J] Perhaps I ought rather to say Button's axiom. For that
     great naturalist and writer embodied the principles of sound
     geology in a pithy phrase of the _Théoris de la Terre_:
     'Pour juger de ce qui est arrivé, et même de ce qui
     arrivera, nous n'avons qu'à examiner ce qui arrive.'




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     senses and the intellect. The value of the third edition of the
     work is greatly enhanced by an account of the psychology of
     Aristotle, which has been contributed by Mr. Grote.

THE EMOTIONS AND THE WILL. By ALEXANDER BAIN, LL.D.
     8vo. Cloth, $5.00.
     The present publication la a sequel to the former one on "The
     Senses and the Intellect," and completes a systematic exposition
     of the human mind.

MENTAL SCIENCE. A Compendium of Psychology and the History of
Philosophy. Designed as a Text-book for High-Schools and Colleges. By
ALEXANDER BAIN, LL.D.
     12mo. Cloth, leather back, $1.50.
     This present volume is an abstract of two voluminous works, "The
     Senses and the intellect" and "The Emotions and the Will," and
     presents in a compressed and lucid form the views which are there
     more extensively elaborated.

MORAL SCIENCE. A Compendium of Ethics. By ALEXANDER BAIN, LL.D.
     12mo. Cloth, leather back, $1.50.
     The present dissertation falls under two divisions. The first
     division entitled The Theory of Ethics, gives an account of the
     questions or points brought into discussion, and handles at
     length the two of greatest prominence, the Ethical Standard and
     the Moral Faculty. The second division--on the Ethical
     Systems--is a full detail of all the systems, ancient and modern.

MIND AND BODY. Theories of their Relations. By ALEXANDER BAIN, LL.D.
     12mo. Cloth, $1.50.
     "A forcible statement of the connection between mind and body,
     studying their subtile interworkings by the light of the most
     recent physiological investigations."--_Christian Register_.

LOGIC, DEDUCTIVE AND INDUCTIVE. By ALEXANDER BAIN, LL.D.
     Revised edition, 12mo. Cloth, leather back, $2.00.

EDUCATION AS A SCIENCE. By ALEXANDER BAIN, LL.D.
     12mo. Cloth, $1.75.

ENGLISH COMPOSITION AND RHETORIC. Enlarged edition. Part I.
Intellectual Elements of Style. By ALEXANDER BAIN, LL.D., Emeritus
Professor of Logic in the University of Aberdeen.
     12mo. Cloth, leather back, $1.50.

ON TEACHING ENGLISH. With Detailed Examples and an Inquiry into the
Definition of Poetry By ALEXANDER BAIN, LL.D.
     12mo. Cloth, $1.25.

PRACTICAL ESSAYS. By ALEXANDER BAIN, LL.D.
     12mo. Cloth, $1.50.




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