SUB-COMPOUNDS ***





Transcriber’s Notes:

  Underscores “_” before and after a word or phrase indicate _italics_
    in the original text.
  Small capitals have been converted to SOLID capitals.
  Typographical and punctuation errors have been silently corrected.
  The symbol “ī” (small i with macron)was used in place of the numeral
    1 (one) with macron.
  The symbol “̅2” (overline + 2) was used in place of numeral 2 with
    macron.




         Determination of The Atomic Weight
          of Cadmium and The Preparation of
            Certain Of Its Sub-Compounds.

                    Dissertation,

    Presented to The Board of University Studies
                       of The
              Johns Hopkins University,

                  For The Degree of
                Doctor of Philosophy,

                         By
                   Harry C. Jones

                        1892.




_Contents._


                                                     Page
    Determination of the Atomic Weight of Cadmium      1
      Introduction and Historical Statement            2
    Preparation of Pure Cadmium                       22
      The Preparation of Pure Nitric Acid             28
      The Arrangement of Crucibles                    30
      The Mode of Procedure                           32
      The Weighing                                    37
      Taring the Crucibles                            40
      The Results                                     42
      Objections to the Method                        45
      Advantages of the Method                        48
    The Oxalate Method                                50
      Preparation of Pure Oxalic Acid                 51
      Preparation of Cadmium Oxalate                  52
      Mode of Procedure                               53
      The Drying and Weighing of the Oxalate          55
      The Results                                     58
      Advantages of the Method                        60
      Disadvantages of the Method                     61
    Preparation of Certain Sub-compounds of Cadmium   63
      Historical                                      64
      The Preparation of Cd₄Cl₇                       66
      The Preparation of Cd₄Br₇                       78
      The Preparation of Cd₁₂I₂₃                      82
      The Preparation of Cadmium Hydroxide and Oxide  82
    Notes on Crystals of Metallic Cadmium             97
    The Cohesion Phenomena of Cadmium                103
    Biographical Sketch                              106




Acknowledgment.


It affords me great pleasure to express my sincere thanks to Professor
Remsen for his instruction and personal supervision during my entire
connection with the University; to Dr. Morse, under whose immediate
guidance the work described in this dissertation was completed; to
Dr. Renouf for valuable assistance in qualitative chemistry and to
Dr. Williams, with whom the branches of mineralogy and geology were
followed as subordinate subjects.




Determination of the Atomic Weight of Cadmium.

Introduction and Historical statement.

A careful examination of the literature on the atomic weight of cadmium
will convince any one that considerable uncertainty yet remains in
reference to this constant. Six experimenters have worked on this
problem but the results of no one of them can be accepted as being more
accurate than those of all others. The value assigned to cadmium varies
from 111.48 to 112.32 on the basis of oxygen = 16. The best work has
apparently been done by von Hauer, Lenssen and Huntington. The results
of these three seem entitled to about equal confidence, yet the figure
obtained by von Hauer differs from that of Huntington by three tenths
of a unit.

The more prominent difficulties which have been encountered were:

    First. The preparation of cadmium compounds free from all
    impurities, and which at the same time were well adapted
    to weighing.

    Second. The lack of a thoroughly simple and exact method
    for the analysis of cadmium compounds.

    Third. Insufficient care in weighing in many cases whereby
    small errors were introduced into the results.

The methods which have been employed are:

    1 Conversion of the metal into the oxide. (Stromeyer).

    2 Conversion of the sulphate into the sulphide. (von Hauer
      and Partridge).

    3 Decomposition of the oxalate to the oxide. (Lenssen and
      Partridge).

    4 Determination of the chlorine in cadmium chloride, by
      which the relation between the chloride and metallic silver
      was established. (Dumas.)

    5 Precipitation of the bromine in cadmium bromide as silver
      bromide. (Huntington.)

    6 The conversion of the oxalate into the sulphide.
      (Partridge.)

The different pieces of work will be taken up in chronological order
and briefly considered.

Stromeyer, Schurigg Journ. 22, 366. 1818, determined the atomic weight
of cadmium a short time after the discovery of the element. He does
not describe his method in detail but established the relation between
cadmium and oxygen to be:

    Cd : O = 100 : 14.352.

    If the atomic weight of oxygen  =  16,
        ”    ”      ”    ”  cadmium = 111.483.

The very low result as compared with all subsequent work was probably
due to the presence of a small amount of zinc, since the cadmium used
was obtained from zinc ores and no adequate means of separation from
the zinc is described.

von Hauer, Journ. f. prakt. Chem. 72, 338. 1857. His method consisted
in reducing a weighed amount of cadmium sulphate to the sulphide
in a stream of hydrogen sulphide, under pressure, at an elevated
temperature, and weighing the sulphide. The reduction was shown to be
complete by proving the absence of sulphate in the sulphide.

         64.2051 grams of cadmium sulphate
    gave 44.4491   ”   ”     ”    sulphide.

    If the atomic weight of oxygen  =  16,
     ”  ”     ”     ”    ”  sulphur =  32.059,
        ”     ”     ”    ”  cadmium = 111.935.

The atomic weight of cadmium calculated as an average of the nine
determinations made using the above values for oxygen and sulphur =
111.94.

    Maximum, 112.121.
    Minimum, 111.796.
       Mean, 111.940.

The work of von Hauer is greatly to be preferred to that of Stromeyer.
The large amount of material used in each determination tended to
lessen any experimental error. A very considerable degree of care
seems to have been exercised in purifying the cadmium sulphate. In
determinations 1-5 a different specimen of sulphate was employed from
that in determinations 6-9. The average value found in the first
five determinations = 111.910, in the last four = 111.977. The close
agreement between the results obtained from the different preparations
of the sulphate argues in favor of a fair degree of purity for all the
material.

The method of weighing the more or less hygroscopic cadmium sulphate is
open to criticism when employed in accurate work. The cadmium sulphate
was placed in an open boat, dried, cooled over sulphuric acid, and
weighed. It was again dried, cooled as before, and weighed. The second
weighing could be quickly accomplished since the approximate weight
was known. The two weighings agreed to within less than a milligram
or a third drying and weighing were made. An error of a milligram
in the weight of the sulphate produced an average error in the atomic
weight of cadmium of about .06. That a discrepancy of greater or less
magnitude was introduced from this source will be readily seen.

Dumas Ann. Chim. Phys. 55, 158. 1859, determined the relation between
cadmium chloride and the metallic silver required to precipitate the
chlorine. Metallic cadmium was dissolved in boiling hydrochloric acid
and the solution evaporated. The cadmium chloride was fused for five or
six hours in a stream of hydrochloric acid gas. Six determinations were
made. 23.0645 grams of cadmium chloride were equivalent to 27.173 grams
of metallic silver.

    If the atomic weight of silver   = 107.93.
    ”   ”    ”       ”   ”  chlorine =  35.45.
        ”    ”       ”   ”  cadmium  = 112.322.

The atomic weight of cadmium calculated as the average of the six
determinations made, using the above values for silver and chlorine =
112.241.

    Maximum,   112.759.
    Minimum,   111.756.
    Mean,      112.241.

The large difference between the results would indicate some
considerable source of error in part or all of the determinations.
The first three determinations were made from a different specimen of
cadmium from the last three.

In the first three the cadmium used does not seem to have been
purified and the cadmium chloride prepared from it was more or less
tinted brown. In the last three a new specimen of metal was used
which in Dumas’ words could reasonably be considered to be absolutely
pure. The chloride prepared from it was colorless, well crystallized
and perfectly soluble in water. In order to show clearly the wide
discrepancy between the results obtained from the two specimens of
cadmium which were used, the separate determinations are given in
detail.

                                At. Wt.
       CdCl₂         Ag.        Cadmium.
    1  2.369        2.791       112.322
    2  4.540        5.348       112.347
    3  6.177        7.260       112.759
    4  2.404        2.841       111.756
    5  3.5325       4.166       112.135
    6  4.042        4.767       112.130

The average result of the first three determinations = 112.476. The
average result of the last three determinations = 112.007. From Dumas’
own statement concerning the purity of the cadmium chloride analyzed,
determinations 4-6 are much to be preferred to determinations 1-3 and
the most probable value from Dumas’ work would be very nearly 112.

Lenssen Journ. f. prakt. Chem. 79, 281. 1860, regarded the oxalate of
cadmium as well adapted to the determination of the atomic weight of
cadmium. A solution of cadmium chloride which had been purified by
repeated crystallization was treated with an excess of a solution of
pure oxalic acid. The cadmium oxalate formed was filtered off, washed,
and carefully dried in the air at 150° C. until the last trace of
water was removed. 1.5697 grams cadmium oxalate gave 1.0047 grams
cadmium oxide.

    If the atomic weight of oxygen =  16,
     ”  ”      ”      ”     carbon =  12.003,
        ”      ”      ”    cadmium = 112.043.

The average of the three determinations using the above values for
oxygen and carbon is 112.067.

    Maximum,  112.304.
    Minimum,  111.911.
    Mean,     112.067.

The small amount of material used in each determination, the small
number of determinations made, and the rather large difference between
the highest and lowest result are objectionable. There are certain weak
points in the method but to these reference will be made later.

Huntington, Proc. Amer. Acad. 17, 28. 1882, working with Cooke, made
two series of determinations of the atomic weight of cadmium. In the
first series the relation between cadmium bromide and the silver
bromide formed from it was determined. In the second, the relation
between cadmium bromide and the silver required to precipitate the
bromine.

The cadmium bromide was prepared by dissolving the carbonate in
hydrobromic acid and subliming the product in a stream of carbon
dioxide.

In the first series of eight determinations 23.3275 grams of cadmium
bromide were equivalent to 32.2098 grams of silver bromide.

    If the atomic weight of silver =  107.93.
     ”  ”    ”      ”    ”  bromine =  79.95.
        ”    ”      ”    ”  cadmium = 122.239.

    Maximum, 112.290.
    Minimum, 112.169.

Where the difference between the maximum and minimum value is slight,
the average of the separate determinations agrees closely with the
number found by comparing the total substance used with the total
product obtained. The latter method of calculation seems however to be
preferable.

In the second series of eight determinations 28.6668 grams of cadmium
bromide were equivalent to 22.7379 grams of silver.

Using the same values for silver and bromine, the atomic weight of
cadmium = 112.245.

    Maximum, 112.320.
    Minimum, 112.180.

The agreement of the separate determinations with each other is
fairly close and the average of the two series of determinations is
nearly the same. Huntington took great care in the purification of
his material and in the carrying out of his method, which are strong
arguments in favor of his work, yet his method is not as simple as
could be desired where the nature of the work demands the greatest
possible accuracy in all details and it also appears to be subject to
some of the errors common to ordinary analytical operations.

Partridge. Amer. Journ. Science XL, 377. 1890. Methods: 1ˢᵗ.
Decomposition of the oxalate to the oxide. 2ⁿᵈ. Reduction of the
sulphate to the sulphide. 3ʳᵈ. Conversion of the oxalate into the
sulphide. As an average of the determinations made by each method
Partridge gives:

    1ˢᵗ series, atomic weight of cadmium = 111.8027.
    2ⁿᵈ    ”       ”       ”   ”   ”     = 111.7969.
    3ʳᵈ    ”       ”       ”   ”   ”     = 111.8050.

An excellent agreement between results obtained by different methods[1].

That this very close agreement is only apparent has been shown by
Clarke. He has found that the above calculations are based on the
assumption that the atomic weight of carbon = 12, and that of sulphur
= 32 when oxygen = 16. There seems to be little justification for
this rather arbitrary selection by Partridge since the most refined
work shows that whole numbers do not express the most probable atomic
weights of carbon and sulphur in a system where oxygen = 16.

[1] Amer. Chem. Journ. 13, 34. 1891.

The atomic weight of cadmium calculated from the total material used
and the total product found in each of the three series is:

                          O = 16.  C = 12.     S = 32.     At.Wt.Cd.
    1ˢᵗ series, CdC₂O₄ :  CdO = 12.66368g. :   8.10031g.     111.805.
    2ⁿᵈ    ”    CdSO₄  :  CdS = 15.93505g. :  11.02691g.     111.786.
    3ʳᵈ    ”    CdC₂O₄ :  CdS = 16.85228g. :  12.12906g.     111.806.
                                               difference,    0.020.

                          O = 16.  C = 12.003  S = 32.059  At.Wt.Cd.
    1ˢᵗ series, CdC₂O₄ :  CdO = 12.66368g. :  8.10031g.     111.816.
    2ⁿᵈ    ”    CdSO₄  :  CdS = 15.93505g. : 11.02691g.     111.727.
    3ʳᵈ    ”    CdC₂O₄ :  CdS = 16.85228g. : 12.12906g.     111.610.
                                                difference,   0.206.

As Clarke has pointed out when those values are chosen for carbon
and sulphur which are founded on the best experimental evidence the
agreement between the different series of results as calculated by
Partridge is somewhat modified.

I have repeated the work on which series I is based and would call
attention to the following points in which it appears to have been
experimentally defective.

    1 The metal was only distilled twice in a vacuum. It has
      been found in this laboratory that perfectly pure
      cadmium or zinc can be prepared only by repeated
      distillations, each one being carried on slowly to allow
      the impurities to separate by means of their difference
      in volatility.

    2 The supposed mixture of metal and oxide resulting from
      the decomposition of the oxalate was only moistened with
      a few drops of nitric acid in order to reoxidize any
      reduced metal. Unless the entire mass of metal and oxide
      was dissolved there would be danger of the presence of
      free undissolved metal which would possess an appreciable
      vapor-tension below the temperature of decomposition of
      cadmium nitrate. An appreciable loss in weight resulting
      from a distillation of the metal out of the crucible might
      easily result.

    3 It seems very probable that the cadmium nitrate was
      not heated sufficiently to remove all traces of the
      oxides of nitrogen. I have found that this could only be
      accomplished by long continued heating. Constant weight
      was not sufficient to have decided this point since it was
      also found that this could be reached short of complete
      decomposition, if the temperature was too low to remove
      the last traces of these oxides. Some very delicate test
      for such oxides should have been applied at the end of
      each experiment.

The following table contains a summary of the results thus far obtained.

When two values are given for one series of determinations, the first
is calculated from the total material used and the total product found,
the second is an average of the results of the separate experiments.
Oxygen is taken as 16 throughout.

    Date.    Investigators.                 At.Wt.Cd.
    1818,    Stromeyer,                      111.483

    1857,    von Hauer,                      111.935 }
                                             111.940 }

    1859,    Dumas,                          112.322 }
                                             112.241 }

    1860,    Lenssen,                        112.043 }
                                             112.067 }

    1882,    Huntington, 1ˢᵗ series          112.239
                 ”       2ⁿᵈ   ”             112.245

    1890,    Partridge,  1ˢᵗ series          111.805
                 ”       2ⁿᵈ    ”            111.786
                 ”       3ʳᵈ    ”            111.806

In the above calculation of Partridge’s results C = 12. S = 32. In the
following carbon is taken as 12.003 and sulphur is 32.059.

    1890,    Partridge,  1ˢᵗ series          111.816
                ”        2ⁿᵈ    ”            111.727
                ”        3ʳᵈ    ”            111.610

After a careful examination of the methods available it becomes evident
that no one of them was _per se_ as accurate as the method employed
by Morse and Burton,[2] for the determination of the atomic weight of
zinc, and more recently by Burton and Morse,[3] for the determination
of the atomic weight of magnesium. The method of work was to prepare
pure metallic cadmium, to convert a weighed portion of the metal
into nitrate by means of pure nitric acid, to decompose the nitrate
completely to oxide and to weigh the oxide.

[2] Amer. Chem. Journ. X, 311.

[3] ib. XII, 219.




Preparation of Pure Cadmium.


The work of preparing pure cadmium was begun more than two years ago
by Mr. W. V. Metcalf with Dr. H. N. Morse. I wish to express here
my sincere thanks to him for the material with which the following
determinations were made. The cadmium used by him was obtained from
Schuchart and marked “Met. prss. (galv.) redus.”

The method of purification by fractional distillation in a vacuum, was
essentially that employed by Morse and Burton for the purification of
metallic zinc.

The distillation was carried out in hard glass tubes of the size of
ordinary combustion tubing.

[Illustration: FIG. 1.]

Fig. 1. represents such a tube. A hard glass tube, 600-700 mm.
in length, was closed at one end and about 130 grams of cadmium
introduced. The walls of the tube were heated and indented at the two
points a, and b, with a red-hot file, dividing the tube into three
sections marked A, B and C. The open end of the tube was drawn out,
bent, and attached to a Sprengel air-pump by means of a rubber tube.

The joint was tied tightly with waxed cord and surrounded by mercury.
When the manometer indicated that the tube was exhausted, it was
gradually heated by the combustion furnace in which it rested. The
metal in A melted and distilled slowly into the front portion of the
tube. Most of it condensed in B, while a small part, together with
any more volatile impurity, collected in C which was kept cooler than
the remainder of the tube. When about four-fifths of the metal placed
in A had distilled over, the tube was very slowly cooled. When cold,
the tube was broken open, the portions in A and C being rejected in
every case, while the metal was recovered from B in the form of a
bar resting on the bottom of the tube, together with some crystal
aggregates, suspended from the top and sides. A few crystal individuals
were secured but the measurement of these will be considered later. The
metal separated from the glass with a highly lustrous surface and did
not attack the glass in the least.

The first distillation was effected in a tube bridged as represented
in Fig. 1, but drawn out at each end. The original cadmium powder was
heated in the tube in a stream of pure hydrogen gas, for the purpose
of obtaining the metal in the form of bars, and to reduce any cadmium
oxide contained in the powder.

Six distillations were made in a vacuum. In the first, 630 grams of
metal were used being distilled in quantities of about 130 grams
each. At the end of the sixth distillation, there were about
100 grams of pure cadmium at disposal. In the fifth and sixth
distillations, the metal was heated just above the melting point for
from twenty to twenty-four hours, before being forced over into the
middle portion of the tube. By this means all the remaining traces of
the more volatile arsenic were driven into the front part of the tube
and separated from the cadmium.


The distillations.

The residue represents the undistilled portion remaining in A. The
distillate, the material obtained from B after the distillation was
completed. The coating, the substance which condensed in C.

                         Residue, Cd, Pt, Zn,? As?.
    Distillation I    Distillate, Cd,     Zn,? As?
                         Coating, Cd,     Zn,? As?.

                           Residue, Cd, Zn?, As?.
    Distillation II     Distillate, Cd, Zn?, As?.
                           Coating, Cd, Zn?, As?.

                           Residue, Cd, Zn?, As?.
    Distillation III    Distillate, Cd, Zn?, As?.
                           Coating, Cd, Zn?, As?.

                           Residue, Cd, Zn?, As?.
    Distillation IV     Distillate, Cd, Zn?, As?.
                           Coating, Cd, Zn?, As?.

                           Residue, Cd.
    Distillation V      Distillate, Cd.
                           Coating, Cd, As?.

                           Residue, Cd.
    Distillation VI     Distillate, Cd.
                           Coating, Cd.

The distillate from the last distillation was examined
spectroscopically by Professor Rowland and found to be free from all
traces of impurity which would be detected by that method. The chemical
test for arsenic was more delicate than the spectroscopic and this
failed to reveal a trace.


The preparation of pure nitric acid.

The method of preparing the pure acid and of preserving and
transferring it was the same as adopted by Morse and Burton in their
work on the atomic weight of zinc.

[Illustration: FIG. 2.]

The simple form of apparatus is represented in fig. 2. A large platinum
vessel containing fragments of ice was supported on a smaller platinum
dish, from which it was separated by hooks of large platinum wire. The
acid was distilled from a small flask as represented in the drawing.

The purest nitric acid which could be obtained was diluted with about
an equal volume of water. The vessel containing the acid was heated
very gently that the distillation might take place without boiling.
The dilute acid condensed on the cold surface of the larger dish and
collected in the smaller, in which it was preserved until used. This
acid gave no residue on evaporation.


The arrangement of crucibles.

[Illustration: FIG. 3.]

The arrangement of the crucibles in which the determinations were
made is represented in fig. 3. 1 is a small porcelain crucible, (00)
from the exterior and lid of which the glaze had been removed by
hydrofluoric acid. The lid was separated from the crucible by hooks
made from thick platinum wire, to allow free communication between the
contents of the crucible and the external air. This would facilitate
the outward diffusion of the oxides of nitrogen when liberated from
the nitrate. 2 is an uncovered porcelain crucible (no. II) in which 1
was placed. From the exterior the glaze had been removed to prevent
the crucible from adhering to the unglazed porcelain scorifier on
which it rested. The exterior was carefully brushed after treatment
with hydrofluoric acid to remove all loose particles adhering to its
surface. Crucibles 1 and 2 were not separated during a determination.

3 is a nickel crucible about two and a half inches in diameter. The
porcelain crucibles were not allowed to touch the nickel at any point.
The nickel crucible was covered by a lid of nickel.


The mode of procedure.

A piece of cadmium weighing from two to three grams was cut from the
bar of the metal by means of a steel chisel. This was seized with steel
forceps and filed with a hard steel file to about one half the original
weight. Care was taken to remove the entire exterior portion of the
metal which had come in contact with the chisel or had stood exposed to
the air. The plug of metal was then carefully brushed and examined with
a lens to insure the removal of all loose particles from the surface.

Crucibles 1 and 2 having been brought to constant weight against
their tare, were ready for use. The piece of cadmium was weighed and
placed in 1. An excess of pure nitric acid was added and a gentle heat
applied until all the metal had dissolved. This required from twenty
to forty hours.

A sand-bath was constructed by placing a large porcelain crucible in
an iron crucible and filling the intervening space with sand. The pair
of crucibles (1 and 2) was placed in the porcelain crucible and the
contents evaporated to dryness by warming very carefully at first and
gradually increasing the temperature. The pair of crucibles was then
transferred to a bath constructed as the above where iron filings took
the place of sand. This was heated by a single burner until the nitrate
was all decomposed when a triple burner was added and finally two
for six or eight hours. This was not sufficient to effect complete
decomposition. When cold, the pair of crucibles was placed in the
nickel crucible as represented in fig. 3 and sharply heated over a
blast-lamp for several hours. This completed the decomposition of the
nitrate and the removal of the last traces of oxides of nitrogen.

During the blasting the lid on crucible 3 was raised a little to one
side to allow free access of air. The nickel crucible was forced
tightly into a hole cut in the center of an asbestos board about
ten inches in diameter, to prevent any reducing gases from the lamp
entering the crucibles while hot. This was the same arrangement as was
used by Partridge[4].

[4] Amer. Journ. Science XL, 379.

It was found that the final decomposition of the nitrate could not
be effected in a muffle furnace as with zinc, since at very high
temperatures cadmium oxide attacked the porcelain with great energy and
injured the crucibles.

The decomposition of the nitrate was shown to be complete not by
constant weight alone, but by testing for oxides of nitrogen with
starch paste rendered extremely sensitive with potassium iodide. That
the test should be reliable, Morse and Burton have pointed out that all
the reagents used must be free from oxidizing agents. The presence of
iodate in the iodide is especially to be avoided. This was removed by
boiling the solution with zinc amalgam. Air was removed from all the
solutions by boiling.

When the starch-potassium-iodide solution had been prepared as
sensitive as possible, a portion of it was treated with a little
hydrochloric acid, to determine if any iodine was liberated. If no
coloration was observed the cadmium oxide was added. It dissolved in
the hydrochloric acid and if any oxides of nitrogen were present they
would have revealed themselves by the liberation of iodine and a blue
coloration of the starch paste.

In no one of the ten determinations was the slightest coloration
detected.

An equal volume of nitric acid was added to the pair of crucibles used
as a tare as to those containing the determination, and they were
heated in exactly the same manner and for the same length of time.

The crucibles containing the cadmium oxide were heated over the
blast-lamp for an hour, weighed against their tare, reheated, again
weighed, and this continued until there was no further change in
weight. Usually from two to four hours heating over the blast-lamp was
sufficient to completely decompose the nitrate. The test for oxides of
nitrogen was then applied.

I found that practically constant weight could be reached short of
compete decomposition, at a temperature below that necessary to
transform all the nitrate into the oxide. This necessitated the final
test for oxides of nitrogen.


The Weighing.

The balance used was a No. 8 long-armed one, made by Becker and Sons.
It was supported by iron brackets fastened to one of the foundation
walls of the laboratory.

Here it would be subjected to the least jar and was also well protected
from air currents. All weighings were made between the hours of one and
five in the morning when the surroundings were as quiet as could be
desired. A very slight disturbance was detected by the vibrations on
the surface of a cup of mercury placed conveniently between the pans.

That the presence of the operator might not produce any change in the
balance during the weighing, he closed the room, placed the light above
and behind his head and took his position in front of the balance at
least an hour before making a weighing. When his presence no longer
affected the balance (which was shown by the zero point remaining
constant in a series of determinations) the weighing was begun. The
method of weighing by vibrations and upon both pans was employed
throughout.

Each zero point was taken as the mean of three closely agreeing zero
determinations; each one of the three being the mean of seven readings.
The zero of the balance empty was determined just before and after
each weighing to detect any change in its position. Usually none was
observed. The sensibility of the balance was taken at each weighing
with the weights used at that weighing. A displacement of the zero
point about six divisions of the ivory scale was effected by the
addition of one milligram.

The weights had been especially adjusted and were carefully compared
with each other before using.

Weighing by tares was adopted as preferable to any other method. By
this means all errors resulting from changes in the moisture of the air
were avoided and any errors which might have been introduced by heating
or manipulating the crucibles would be counteracted by treating the
tare in exactly the same manner.


Taring The Crucibles.

A pair of crucibles (1 and 2 in the figure) was selected and treated as
described. Another pair about the same size but a little lighter was
prepared in exactly the same way. Each pair was placed in the nickel
crucible and heated by means of the blast-lamp for half an hour.

After cooling in desiccators, both pairs of crucibles where placed in
the closed balance until no longer affected by the moisture of the
air, which was also dried by calcium chloride. The tare was brought to
within one tenth of a milligram of the weight of the crucibles against
which it was being tared, by adding fragments of porcelain obtained
from another crucible of the same composition. The difference in weight
between the tare and its mate was then accurately ascertained.

Each pair of crucibles was again placed in the nickel crucible and
blasted for half an hour. They were then reweighed, to determine if the
difference in weight previously found had remained constant. In no case
was any change detected, yet this precaution was always taken.


The Results.

The following table contains the results of ten successive
determinations.

                                        At. Wt. Cd.   At. Wt. Cd.
             Wt. of Cd.   Wt. of CdO.   (O = 16)     (O = 15.96)
           I  1.77891      2.03288       112.070      111.790
          II  1.82492      2.08544       112.078      111.798
         III  1.74688      1.99626       112.078      111.798
          IV  1.57000      1.79418       112.053      111.773
           V  1.98481      2.26820       112.061      111.781
          VI  2.27297      2.59751       112.059      111.779
         VII  1.75695      2.00775       112.086      111.806
        VIII  1.70028      1.94305       112.059      111.779
          IX  1.92237      2.19679       112.083      111.803
           X  1.92081      2.19502       112.078      111.798
              -------      -------       -------      -------
                                 Mean,   112.0705.    111.7905.
                              Maximum,   112.086.     111.806.
                              Minimum,   112.053.     111.773.
                           Difference,      .033.        .033.

Calculating the atomic weight of cadmium from the total amount of metal
used and oxide found, we have:

    At. Wt. of Cd.    At. Wt. of Cd.
      (O = 16)         (O = 15.96)
      112.0706.          111.7904.

These results agree more closely with those of von Hauer and Lenssen
than with those of any other experimenter. The following table gives
a comparison of the work of these investigators with that herein
described:

           von Hauer.             Lenssen.        Work here described.
        9 determinations.    3 determinations.     10 determinations.
            (O = 16)              (O = 16)              (O = 16)
    Mean     111.940              112.067               112.0705
    Max.     112.121              112.304               112.086
    Min.     111.796              111.911               112.053
    Diff.       .325                 .393                  .033

A difference of three or four tenths of a unit between the different
results of a series leaves considerable doubt as to the accuracy of the
method employed and to the value obtained.

The figure selected by Ostwald,[5] as most probable for the atomic
weight of cadmium is 112.08. This is the mean of the results on von
Hauer and Huntington. My own work leads me to believe that this number
is very close to the true value when oxygen is taken as 16.

[5] Lehrb. d. Allg. Chem. I, 60.


Objections to the method.

Marignac[6] offered the objection to this method for determining the
atomic weight of zinc that the zinc oxide dissociated when heated in
platinum over the blast-lamp. The same objection might be urged against
this method for determining the atomic weight of cadmium, had it not
been shown that the objection does not hold for zinc[7]. What took
place was a reduction of the zinc oxide by the highly heated hydrogen
which passed through the hot platinum.

[6] Archives des Sciences Phys. et Nat. (3) 10, 193.

[7] Amer. Chem. Journ. X, 148.

It was shown that zinc oxide can be heated in a platinum vessel in a
muffle furnace, to the melting point of steel, without undergoing any
dissociation, or in any wise losing in weight. This source of error was
avoided by using porcelain vessels, which were not brought into contact
with the free flame.

The statement of Marignac that the oxide of zinc derived from the
nitrate retains oxides of nitrogen even when heated to the temperature
at which it begins to undergo dissociation, was shown by the same
authors to be without foundation. The basis of this objection is
doubtless to be found in the imperfect method of testing for such
oxides.

It might be urged as an objection to this method that the difference
in weight between the metal and oxide is not very great, therefore any
error in weighing would be multiplied in the result. At first sight
this objection may appear valid, but since the substances weighed were
so well adapted to that purpose and the weighings could be made with
such a high degree of accuracy no appreciable error could have resulted
from this source.

A crucible with its contents was repeatedly weighed against its tare
and weights to ascertain the difference between successive weighings
under the conditions employed. A number of weighings agreed to .00002
gr. and in some instances to half this amount.


Advantages of the Method.

    1 The great advantage of the method is its extreme
      simplicity. From the beginning of an experiment until
      the end the contents of the crucible are not brought
      into contact with any foreign substance. By this means
      small errors resulting from incomplete precipitation,
      and filtration and all other errors incident to ordinary
      processes of analysis were avoided.

    2 The nature of the metal and its oxide rendered them well
      adapted to weighing. The specific gravity of the metal and
      oxide approached so closely to that of the weights, that
      it was unnecessary to reduce the weighings to a vacuum
      standard.

    3 The advantages derived from weighing by tares have been
      pointed out.

    4 The closely agreeing results speak strongly in favor of
      the accuracy of the method.




The Oxalate Method.


The method consists in taking a weighed amount of cadmium oxalate,
decomposing it by heat, when a mixture of oxide and metal are said
to be formed, dissolving this mixture in nitric acid, converting the
nitrate into oxide and weighing the oxide.

Lenssen[8] obtained results by this method which agree very closely
with those recorded in the earlier part of this dissertation.

Working with the same method, Partridge[9] arrived at a value about one
fourth of a unit lower than that of Lenssen.

[8] Journ. f. prakt. Chem. 79, 281.

[9] Amer. Journ. Science XL, 377.

It appeared desirable that this method should be repeated with the
greatest care to ascertain what result it would give under the most
favorable conditions.

Having a supply of pure cadmium it was necessary to prepare pure oxalic
acid.


Preparation of Pure Oxalic Acid.

The commercial acid was crystallized three times from cold water to
separate it from acid oxalates. It was then boiled for two days with
a 15 per cent solution of hydrochloric acid, to remove any mineral
matter present. The acid which crystallized from the hydrochloric acid
solution was recrystallized twice from hot, redistilled alcohol and
twice from pure ether. It was finally boiled with water to decompose
any ethyl oxalate and twice crystallized from pure water. The acid was
dried in the air at ordinary temperatures. This acid left no residue on
ignition.


Preparation of Cadmium Oxalate.

A piece of cadmium was dissolved in pure nitric acid. On carefully
evaporating the solution cadmium nitrate was obtained. Twenty-five
grams of the nitrate were dissolved in 750 c.c. of redistilled water.
Somewhat less than an equivalent of the oxalic acid was dissolved in
an equal volume of water, and slowly added to the solution of the
nitrate with constant shaking. A little less than an equivalent of
oxalic acid was used to avoid any tendency to form acid oxalates.
Cadmium oxalate was precipitated on standing a few minutes as a white
crystalline compound, well adapted to washing. The oxalate was filtered
off and washed until the wash water was free from all traces of nitric
acid. It was then washed ten times with water which had been twice
redistilled and dried in an air-bath for twenty hours at 150°C.

The arrangement of the crucibles which were weighed was in all respects
like that in the preceding method.


Mode of Procedure.

The crucibles were heated, tared, and weighed exactly as in the
preceding method. The oxalate was weighed in ground-stoppered weighing
tubes from which it was transferred to the inner of the two porcelain
crucibles. The pair of crucibles, (1 and 2 fig. 3) was placed in a
third porcelain crucible and the whole system introduced into an
upright air-bath. The outer crucible was supported on a porcelain
triangle about an inch from the bottom of the bath and was not allowed
to touch its walls at any point. The top of the bath was covered with a
sheet of iron over which was placed an asbestos board. The exterior was
also covered with a lining of asbestos. A thermometer was introduced
well into the bath. The temperature was allowed to rise slowly until
the oxalate began to show a brown color around the edge. From this
stage the temperature was kept as low as possible in order to effect
the decomposition. When the oxalate was decomposed the bath was allowed
to cool and the contents of the crucible completely dissolved in nitric
acid. The nitrate was evaporated to dryness and decomposed as in the
method first described. The end of the decomposition was determined in
the same manner and the oxide, free from all impurities, weighed.


The Drying and Weighing of the Oxalate.

It was necessary to dry the oxalate before weighing from fifteen to
twenty hours at 150°C. in addition to the twenty hours drying of the
whole preparation. At this temperature the last traces of moisture were
removed by prolonged heating.

The weighing of the oxalate was made in the weighing glasses in which
it was dried. Two of these glasses had been previously tared against
each other, using the lighter as the tare and adding fragments of
glass to it until the difference in weight was a small fraction of
a milligram. The oxalate having been dried to constant weight, was
weighed. It was then poured as carefully and completely as possible
from the weighing glass into the crucible and the glass again weighed
against its tare. The difference in the two weights gave the amount of
oxalate. The glass and its tare were dried and reweighed to determine
if the few milligrams of oxalate adhering to the walls of the glass
had absorbed any moisture during the transfer of the oxalate. In one
experiment a slight difference was detected when a second drying and
weighing were made.

The weight of the cadmium oxalate as obtained from the balance was
corrected for the difference in specific gravity between the cadmium
oxalate and the weights.


The Results.

                            At. Wt.    At. Wt.      At. Wt.    At. Wt.
                              Cd.        Cd.          Cd.        Cd.
                            (O=16)     (O=16)    (O=15.96)   (O=15.96)
                          (C=12.001) (C=12.003)  (C=11.971)  (C=11.973)
          CdC₂O₄     CdO
    I    1.53937   .98526   112.026   112.033    111.746     111.753
    II   1.77483  1.13582   111.981   111.988    111.701     111.708
    III  1.70211  1.08949   112.049   112.056    111.769     111.776
    IV   1.70238  1.08967   112.051   112.058    111.771     111.778
    V    1.74447  1.11651   112.019   112.026    111.739     111.746
                            -------   -------    -------     -------
                  Mean,     112.025   112.032    111.745     111.752
               Maximum,     112.051   112.058    111.771     111.778
               Minimum,     111.981   111.988    111.701     111.708
            Difference,        .070      .070       .070        .070

The values assigned to carbon in the last two columns were found thus--

    When O = 16, C = 12.001, when O = 15.96, C = 11.971.
      ”  O = 16, C = 12.003,   ”  O = 15.96, C = 11.973.

Calculating the atomic weight directly from all the oxalate used and
oxide found it would give:

     At. Wt. Cd.    At. Wt. Cd.    At. Wt. Cd.    At. Wt. Cd.
    (O = 16)       (O = 16)        (O = 15.96)    (O = 15.96)
    (C = 12.001)   (C = 12.003)    (C = 11.971)   (C = 11.973)

      112.025.       112.032.        111.745.       111.752.

There seems about equal evidence for the two values assigned to carbon
when oxygen = 16. The value of cadmium as given by this method is
therefore 112.025 or 112.032.

As will be seen at a glance this figure agrees much more closely with
that of Lenssen than with that of Partridge.

    Lenssen     Partridge    My work
    112.043.     111.816.    112.025 or
                             112.032.

It also agrees fairly well with the figure 112.0706 which I obtained by
the first method described.


Advantages of the Method.

The method possesses no advantage whatever over the one which involves
starting with the element itself. The oxalate can however be obtained
pure having pure metal. The salt is of definite composition when
perfectly dry.

The method as carried out avoided the contact of any foreign material
with the salt after it was weighed.


Disadvantages of the Method.

    1 The avidity with which the dried oxalate takes up
      moisture from the air is an objection to its use for the
      determination of atomic weights. Even with the greatest
      care there is a slight element of uncertainty introduced
      from this source.

    2 The oxalate is stated to decompose into a mixture of
      the oxide and metal. The temperature required for this
      decomposition is somewhat higher than the melting point
      of cadmium. The metal heated above its melting point
      possesses a vapor-tension and loss in weight must result,
      whatever precaution is taken in heating. This is the
      probable explanation why the results obtained by this
      method are lower than those of the preceding.

A comparison of the two methods leads me to attach much more importance
to the results of that one which establishes the relation between
cadmium and cadmium oxide directly and I therefore regard the atomic
weight of cadmium as very closely expressed by the figure 112.07 when
oxygen = 16.




Preparation of Certain Sub-compounds of Cadmium.

Historical.

Cadmium acts so generally as a bivalent element that it is usually
regarded as entering into combination only where it can play this rôle.
The only compound described, in which it has apparently a lower valence
than two, was prepared by Marchand[10]. It was obtained by heating
cadmium oxalate to the melting point of lead when a green powder
remained behind which resembled chromium oxide. When heated on the air
it appeared to be decomposed into metal and oxide. When treated with
mercury the compound was not altered. An analysis showed it to have the
composition represented by the formula Cd₂O.

[10] Pogg. Ann. XXXVIII, 143.

A. Vogel[11] has shown that the green powder described by Marchand
consists of a mixture of the metal and oxide. When this mixture is
treated with dilute acetic acid the metal remains behind as microscopic
glistening globules. The lower the temperature at which the oxalate is
decomposed the more oxide and the less metal were found in the product.

There was then no compound known in which cadmium acted as if its
valence was less than two when this work was undertaken.

That it may act with a greater valence was shown by R. Haafs[12]. He
found that when zinc hydroxide was treated with hydrogen dioxide
certain compounds of zinc and oxygen were formed containing more oxygen
than the normal oxide ZnO. The close resemblance between zinc and
cadmium led him to try the same reaction with cadmium. Hydrogen dioxide
was accordingly allowed to act on cadmium hydroxide and the resulting
product analyzed. There were formed Cd₅O₈, Cd₃O₅ and Cd₄O₇. In no case
was the compound CdO₂ obtained. These compounds are described as fairly
stable even at a hundred degrees.

[11] Jahrb. 1855, 390.

[12] Ber. 1884, 2249.


The Preparation of Cd₄Cl₇.

When anhydrous cadmium chloride is heated with metallic cadmium in a
vacuum, or in an atmosphere of nitrogen, to the fusing point of the
chloride, the molten chloride quickly assumes a garnet red color.
In order to investigate this phenomenon a quantity of the chloride
was prepared by dissolving the redistilled metal in an excess of
hydrochloric acid, evaporating the chloride to dryness on a water
bath, and finally removing the water of crystallization by heating in
a current of dry hydrochloric acid gas. The heating was effected by
placing the chloride in a long platinum boat, which was shoved into a
large glass tube, through which was passed a current of the acid gas.
The tube was heated by means of a combustion furnace and the chloride
kept in the molten condition for two or three hours. By this means
a perfectly white crystalline chloride of the composition CdCl₂ was
obtained, free from water or oxychloride.

The chloride and an excess of metal were placed in a long-necked flask
of hard glass and after the displacement of the air by nitrogen, heated
to the melting point of the chloride. The liquid chloride attained its
maximum depth of color in a few minutes, nevertheless the heating was
continued for five hours. When the temperature was allowed to rise much
above the melting point of the chloride the red substance underwent
decomposition and globules of metal collected upon the walls of the
flask. For this reason no more heat was applied than was just necessary
to keep the contents of the flask in a liquid condition. During the
very gradual cooling of the flask it was shaken gently in order to
facilitate the sinking of any metal, which might be mechanically
retained by the chloride.

On cooling, the solidified mass possesses a slightly greenish tint
which disappeared when cold, the substance having then a grayish white
color and a cleavage resembling that of talc or brucite. When examined
under the microscope it was found to be perfectly homogeneous and
free from metal. It gave no metallic streak when rubbed between agate
surfaces.

An analysis of the first preparation showed the following composition;

    Amount of chloride used    .33541 gr.
       ”   ”  cadmium found    .21559  ”
       ”   ”  chlorine  ”      .11943  ”

       Cadmium.            Chlorine.
        64.27 per cent.     35.61 per cent.

These proportions are nearly those of a compound having the composition
Cd₄Cl₇, in which the calculated percentages are:

    Cadmium.           Chlorine.
     64.34              35.66

    (Foot note). In the paper in the American Chemical Journal
    XII, 488, which records this work the analyses and
    percentages were calculated on the basis of the atomic
    weight of cadmium = 111.7. Although my work since this date
    has shown that 112.07 is the true value, yet I think it
    preferable to use the old number here since the changes to
    be introduced would be very slight and the same results are
    thereby kept uniform in the two publications.

In order to determine whether the close approximation to definite
atomic proportions might not be accidental, the material was reheated
with an excess of the metal for twenty hours. The product was analyzed.

    Amount of chloride used      1.45970 gr.
       ”    ” cadmium found       .93904  ”
       ”    ” chlorine  ”         .52329  ”

       Cadmium.           Chlorine.
    64.33 per cent.    35.85 per cent.

A second preparation of the substance was made in all respects like the
first. Two analyses were made.

First Analysis:

    Amount of chloride used    .61010 gr.
       ”   ”  cadmium found    .39235 ”
       ”   ”  chlorine  ”      .21725 ”

       Cadmium.           Chlorine.
    64.31 per cent.    35.61 per cent.

Second Analysis:

    Amount of chloride used    .20616 gr.
       ”   ”  cadmium found    .13266 ”
       ”   ”  chlorine  ”      .07352 ”

       Cadmium.           Chlorine.
    64.35 per cent.    35.66 per cent.

A third preparation was made like the first and second and analyzed.

Analysis:

    Amount of chloride used      .2832  gr.
       ”   ”  cadmium found      .18244 ”
       ”   ”  chlorine  ”        .10123 ”

       Cadmium.           Chlorine.
    64.42 per cent.    35.74 per cent.

When the new substance is heated it fuses to a red liquid and then
breaks up into metal and the chloride of cadmium. Its reactions are in
general those of a strong reducing agent. Treated with nitric acid,
oxides of nitrogen are liberated. With dilute hydrochloric, sulphuric
and acetic acids it gives free hydrogen. In the presence of dilute
acids it reduces mercuric to mercurous chloride, or to metallic mercury.

Three determinations of the reducing power of the substance were made
with a freshly prepared specimen, by dissolving weighed portions in
hydrochloric acid and measuring the hydrogen liberated.

The following results were obtained:

                       Hydrogen found.    Hydrogen calculated
                                             for Cd₄Cl₇.
    1ˢᵗ  determination   15.67 c.c.          15.65 c.c.
    2ⁿᵈ      ”           11.80 c.c.          11.82 c.c.
    3ʳᵈ      ”           23.00 c.c.          23.03 c.c.

An examination of the analyses shows beyond question that the
substance formed by the action of metallic cadmium on the molten
anhydrous chloride is of definite composition. The proportion of
cadmium to chlorine could not be changed even when the substance was
heated with the metal for twenty hours, while a very short time was
sufficient for its formation when the metal and chloride were melted
together.

It may be possible that a substance possessing these properties is
not a definite chemical compound but a mixture of cadmous and cadmic
chlorides or a solution of one in the other.

If it were a solution it is difficult to see why the composition of the
solution should be so constant, since the solubility of a substance
is generally altered by a change in temperature. The different
preparations were not made at exactly the same temperature yet the
composition of the different preparations was the same.

If the substance was a mixture of the two chlorides, when treated with
water the cadmic chloride would most probably dissolve directly leaving
the cadmous chloride to be acted upon by the water. The decomposition
by water will however be seen not to be as simple as would be expected
under these conditions.

From the above considerations it appears highly probable that the
substance is a definite chemical compound of cadmic and cadmous
chlorides. If cadmic chloride can form a chemical compound with the
chloride of another element there appears to be no reason why it
should not form a compound with another chloride of cadmium, as with
cadmous chloride.


The preparation of Cd₄Br₇.

The anhydrous bromide of cadmium was prepared by dissolving the
carbonate in an aqueous solution of hydrobromic acid, evaporating
the bromide to dryness on the water bath and heating the residue in
a current of dry hydrobromic acid gas. When the bromide was heated
with an excess of the metal in an atmosphere of nitrogen it conducted
itself in general like the chloride. When the molten bromide and the
metal came in contact the salt quickly became deep red in color.
After heating for some time considerable dissociation was produced by
raising the temperature. This was more apparent in the preparation of
the bromide than with the chloride. On cooling, the mass possessed
a greenish tint which disappeared when cold, the bromide then being
very nearly the same color as the corresponding chloride. Also like
the chloride it appeared to be homogeneous and free from metal. Two
determinations of cadmium and two of bromine were made, using the
product as soon as prepared.

First determination of cadmium:

    Amount of substance used      .3736  gr.
      ”    ”  cadmium found       .16658  ”

    Cadmium.
    44.59 per cent.

Second determination of cadmium:

    Amount of substance used      .35930 gr.
        ”  ”  cadmium found       .16013 ”

    Cadmium.
    44.57 per cent.

First determination of bromine:

    Amount of substance used         .66640 gr.
      ”    ”  bromine found          .36953 ”

    Bromine.
    55.45 per cent.

Second determination of bromine:

    Amount of substance used         .56035 gr.
      ”    ”  bromine found          .31085 ”

    Bromine.
    55.47 per cent.

The percentage of cadmium and bromine found agrees very closely with
that of a compound of the formula Cd₄Br₇. The relation of cadmium to
bromine in this would be:

       Cadmium.             Bromine.
    44.44 per cent.      55.56 per cent.

When this compound was heated for a long time with an excess of the
metal its composition was not appreciably changed.

The compound Cd₄Br₇ is a strong reducing agent: giving with nitric
acid oxides of nitrogen, with dilute hydrochloric, sulphuric or acetic
acid, free hydrogen, and with mercuric chloride, mercurous chloride or
metallic mercury. The action of water on the bromide by means of which
cadmous hydroxide was formed, was not studied as carefully as with the
chloride but appeared to be essentially the same.


The Preparation of Cd₁₂I₂₃.

Cadmic iodide was prepared in the same manner as the bromide. It was
dried in a stream of hydriodic acid gas at as low temperature as
possible to lessen the decomposition of the hydriodic acid. When the
anhydrous iodide was heated with an excess of metal in an atmosphere of
nitrogen the red color of the iodide became intensified. Heating was
continued until there was evidence of dissociation, which, under the
same conditions, was less marked than with the chloride and much less
than with the bromide. Owing to the high specific gravity of the iodine
compound some difficulty was experienced in obtaining a preparation
free from metal. This difficulty was finally overcome by keeping
the material just above its melting temperature for a long time and
constantly jarring the flask. During the process of cooling a decidedly
greenish tint was observed which disappeared as the process was
continued. When cold the substance resembled the chloride and bromide.
Two determinations of cadmium were made in the first preparation.

First determination:

    Amount of substance used      .55540 gr.
      ”    ”  cadmium found       .17456 ”

    Cadmium.
    31.43 per cent.

Second determination:

    Amount of substance used     .47535 gr.
      ”    ”  cadmium found      .14980 ”

    Cadmium.
    31.51 per cent.

As these results did not correspond to the composition represented by
the formula Cd₄I₇, which our experience with the chloride and bromide
had led us to expect, we reheated the material for several hours with
an excess of the metal. Two analyses of the product gave:

       Cadmium.           Iodine.
    31.44 per cent.    68.65 per cent.
    31.39      68.68

showing that the iodide had taken up during the first heating all the
metal which it could retain. The analytical results suggest the formula
Cd₁₂I₂₃, in which the calculated percentages are:

    Cadmium.             Iodine.
    31.53 per cent.      68.47 per cent.

In its conduct towards dilute hydrochloric and acetic acids and water
the substance behaves like the corresponding chloride and bromide.


The Preparation of Cadmous Hydroxide and Oxide.

When the substance Cd₄I₇ is treated with water a complicated reaction
takes place. The general character of the reaction appears to be the
same with the chloride, bromide and iodide. The decomposition of the
chloride was studied more thoroughly than that of the other compounds.

When the finely powdered chloride is treated with water it yields
cadmic chloride which passes into solution, a small quantity of a white
flocculent material which may be cadmic hydroxide but which in no case
could be entirely freed from traces of chlorine, and a highly lustrous
crystalline substance which rapidly lost its crystalline appearance
and passed over into a grayish white amorphous compound, which when
freed from chlorine was found to be cadmous hydroxide, of the formula
Cd(OH). The separate products resulting from the treatment with water
were analyzed.

First Analysis:

    Amount of Cd₄Cl₇ treated with water           1.45970 gr.
    Cadmium found in flocculent precipitate        .02318  ”
       ”      ”   ”  crystalline substance         .09614  ”
       ”      ”   ”  solution in water             .81970  ”
        Total cadmium found                        .93902  ”

    Chlorine found in crystalline compound         .00371 gr.
       ”       ”   ”  solution in water            .51671  ”
        Total chlorine found                       .52042  ”

Approximately seven-eighths of the total cadmium dissolved as
cadmic chloride while the remainder was contained in the flocculent
precipitate and in the gray crystalline compound.

Second Analysis:

    Amount of Cd₄Cl₇ treated with water         1.0794  gr.
    Cadmium found in flocculent precipitate      .01469  ”
        ”     ”   ”  solution in water           .60795  ”

    Chlorine found in solution in water          .38491  ”

The percentage of cadmium in the white precipitate is less in this
analysis than in the former. The cadmium in solution is again about
seven-eighths of the total and the chlorine present in the same
solution shows that the cadmium was all combined as cadmic chloride.

All attempts to determine the composition of the gray crystalline
compound failed, owing to the rapidity with which it decomposed with
water. Even with the most rapid work it could not be isolated in the
undecomposed condition.

Analyses of the partially decomposed crystals gave variable proportions
of metal and halogen but never less than eight equivalents of the
former to one of the latter.

While the decomposition of Cd₄Cl₇ with water cannot at present be fully
explained, yet it is clear from the analyses that one eighth of the
total cadmium is thrown down as a white precipitate and a crystalline
compound which as will be seen passes over into cadmous hydroxide. One
half of the cadmous chloride is oxidized to cadmic chloride taking the
chlorine from the other half.

The compound Cd₄Cl₇ was treated directly with absolute alcohol with
the hope of obtaining the crystalline substance in an undecomposed
condition. Although a substance of the same general appearance as that
formed in the presence of water was obtained yet it decomposed so
readily that a satisfactory analysis could not be made.

Notwithstanding the rapidity with which the decomposition of the
crystalline compound begins, long continued washing was necessary in
order to completely remove the chlorine. The extraction of the last
traces of the halogen is hastened by the use of warm instead of cold
water. The temperature of the water must not exceed 50°C. In water
whose temperature approaches the boiling point the hydroxide is slowly
decomposed with liberation of metal.

The new hydroxide is a strong reducing agent. It dissolves in dilute
acids; yielding with nitric acid oxides of nitrogen, with hydrochloric
or sulphuric acid free hydrogen. After washing with warm water until
all the chlorine had disappeared, it was dried over phosphorus
pentoxide and analyzed.

First determination of cadmium.

    Amount of substance used     .0968 gr.
       ”   ”  cadmium found      .08415 ”

    Cadmium.
    86.93 per cent.

Second determination of cadmium.

    Amount of substance used     .09806 gr.
       ”   ”  cadmium found      .08522 ”

    Cadmium.
    86.91 per cent.

The calculated percentage of cadmium in Cd(OH) is:

    Cadmium.
    86.79 per cent.

The determination of water in cadmous hydroxide was made by placing a
small specimen tube containing the hydroxide in a Kjeldahl flask which
was heated in a bath of concentrated sulphuric acid. During the heating
a slow current of dry nitrogen was passed over the substance.

First determination of water.

    Amount of substance used      .08434 gr.
        ”   ” water found         .00609  ”

                      Water.     7.22 per cent.

Second determination of water.

    Amount of substance used      .08895 gr.
        ”  ”  water found         .00600  ”

                      Water.     6.74 per cent.

Third determination of water.

    Amount of substance used     .11766 gr.
       ”   ”  water found        .00856  ”

                      Water.     7.25 per cent.

    Average amount of water = 7.07 per cent.

The calculated percentage of water in Cd(OH) is, 6.99.

At the temperature at which concentrated sulphuric acid gives off
dense white fumes cadmous hydroxide gives off all its water and passes
over into a heavy yellow powder. At 150°C not a trace of water was
liberated. Under the microscope the yellow powder was found to consist
of minute translucent crystals.

First determination of cadmium.

    Amount of substance used      .08064 gr.
      ”    ”  cadmium found       .07511  ”

          Cadmium. 93.14 per cent.

Second determination of cadmium.

    Amount of substance used      .10846 gr.
      ”    ”  cadmium found       .10106  ”

          Cadmium.  93.17 per cent.

The calculated percentage of metal in Cd₂O is 93.32 per cent.

If water of too high temperature is employed in washing the
subhydroxide, the presence of free metal in it can be detected under
the microscope and by rubbing between agate surfaces. If the yellow
suboxide is strongly heated it breaks up into a mixture of oxide and
metal which possesses a distinctly green color. Towards acids the
suboxide conducts itself like the subhydroxide.

It is a fact of some interest in connection with the periodic
arrangement of the elements, that the tendency toward the formation
of a lower series of compounds which becomes so strongly developed in
mercury begins to exhibit itself in some slight degree in cadmium.




Notes on Crystals of Metallic Cadmium.


The measurements of the cadmium crystals were made by Dr. Williams who
has very kindly furnished me with his results.

No reliable crystallographic description of the element cadmium seems
thus far to have appeared--a fact due to the difficulty in obtaining
suitable material. The crystals examined, although not capable of
yielding entirely satisfactory results are nevertheless such as to make
them of interest.

In 1852 G. Rose noted the fact that distilled cadmium collected at the
neck of the retort in drops which solidified as complex polyhedral
aggregates[13] similar to those formed by zinc[14]. In 1874 Kammerer
encountered the same aggregates which he explained as complicated
isometric combinations[15]. This opinion was cited in 1881 by
Rammelsberg[16]. In 1884 Brögger and Flink stated that in their opinion
zinc, magnesium and probably cadmium were from analogy with beryllium
which they had studied, hexagonal and holohedral.[17]

[13] Pogg. Ann. 85, 293.

[14] Amer. Chem. Journ. 11, 219.

[15] Ber. d. deutch. Chem. Gesell. 1874, 1724.

[16] Handb. d. krystallographisch physicalischen Chemie. I, 184.

[17] Zeits &amp; Kryst. 9, 236.

This supposition has already been substantiated in the case of the two
former elements[18] while the present material leads to the same result
for the last named.

The cadmium crystals were produced in the same manner as were those of
zinc and magnesium measured before, viz; by distillation in a vacuum.
The appearance of the tubes thus obtained was closely like that in the
other cases.

[18] Amer. Chem. Journ. 11, 225 and Ibid. 12, 225.

The polyhedral aggregates were abundant and reached considerable
dimensions. The crystallizing power of the cadmium however, seems to be
less, so that the only crystals suitable for measurement were extremely
minute. The largest individuals were barrel-shaped, like those of zinc
and resembled little piles of basal plates. Their side planes are not
infrequently uneven and bent, probably as the result of the softness
and great ductility of the metal.

Only the most minute crystals show pyramidal planes of comparative
perfection. These are well suited for a microscopic examination, but
their small size renders their measurement on a reflecting goniometer
a matter of difficulty. After a careful search two crystals were
secured which, although they had a diameter of only one third of a
millimeter, from their microscopic appearances promised good results.
Their planes however were found to give compound reflections and a
somewhat disappointing variation in corresponding angles. On the best
crystal three zones were measured as follows: (normal angles)

        Zone I                  Zone II                Zone III
    0001 : 01ī1 =  62° 35′ |0001 : 10ī1 = 62° 4′  |0001 : 1ī01 = 62° 29′
    0001 : 01ī0 =  89° 50½′|                      |
    0001 : 01īī = 118° 57′ |0001 : 10īī = 118° 28′|

The second crystal was much less satisfactory, since values for the
angle between the base and pyramid (0001): (01ī1) were obtained which
varied all the way from 61° 2′ to 63° 43′. These measurements must
therefore be regarded as of little or no value. If we average the
readings for this angle on the first crystal we obtain 62° 23′, from
which

    ̲a : ̲c = 1 : 1.6554.

A comparison of the axial ratios of the four rhombohedral and four
holohedral hexagonal elements gives the following:


                { Bismuth     ̲a : ̲c = 1 : 1.3035 (G. Rose, 1849).
    Rhombo-     { Antimony    ̲a : ̲c = 1 : 1.3235 (Laspeyres, 1875).
        hedral. { Tellurium   ̲a : ̲c = 1 : 1.3298 (G. Rose, 1849).
                { Arsenic     ̲a : ̲c = 1 : 1.4025 (Zepharovich, 1875).

                { Zinc        ̲a : ̲c = 1 : 1.356425 (Williams and
                {                                      Burton, 1889).
    Holohedral. { Beryllium   ̲a : ̲c = 1 : 1.5802   (Brögger, 1884).
                { Magnesium   ̲a : ̲c = 1 : 1.6202   (Williams, 1890).
                { Cadmium     ̲a : ̲c = 1 : 1.6554   (Williams, 1891).

Zinc appears from its axial ratio to belong rather to the rhombohedral
group and this is the only one of the last four elements upon which the
faintest indication of any divergence from a holohedral development
of all of its forms has been observed. On crystals of this substance
there is an occasional rhombohedral alternative of the faces of
certain of the pyramids, although the crystals otherwise appear to be
holohedral.[19]

The crystals of cadmium like those of magnesium show only the three
forms OP (0001), P (10ī1)₂, and ∞P (10ī0). Brögger and Flink observed
on beryllium the additional forms ∞P₂ (2īī0) and ½P (20{̅2}1); while
upon zinc a large number of forms in the zone of the unit pyramid occur.

[19] Amer. Chem. Journ. 11, 224. pl. 2 fig. 8.

Not infrequently the cadmium crystals show a tendency toward a
hemimorphic development. This is plainly seen when a large number
of them are examined together under the microscope. The little
barrel-shaped crystals are mostly attached by their sides and yet one
of their ends is often broader than the other. Sometimes they taper
nearly to a point, quite like greenockite crystals.




The Cohesion Phenomena of Cadmium.


The cohesion phenomena of cadmium are similar to those of zinc but
are still more striking. When a crystal is sharply focused under the
microscope and then gently pressed on the side with the point of a
needle an unbroken pyramidal face is seen to suddenly become striated
parallel to the basal plane, as though a gliding in the basal section
took place. Some of these crystals were kindly examined by Prof. Otto
Mügge of Münster, Germany, who has added so much to our knowledge
of the cohesion phenomena in crystals. He has written in regard to
his observations as follows; “The cadmium crystals as far as their
gliding phenomena are concerned behave quite like zinc. If a crystal
is carefully loosened and then squeezed with a pair of pincers it is
easy to see that the smooth surface where it was attached to the glass
became striated parallel to OP (0001) and that at the same time two
other sets of striations are produced which meet at an angle of about
85° and intersect the trace of the basal plane at about 47½°. The plane
of attachment was selected for observation because it was smoother than
the pyramidal faces. In the above case this plane has the position of
a steep pyramid inclined to the base at an angle of about 100°. The
oblique sets of striations appear to represent gliding planes parallel
to the unit pyramid faces (2P (10ī2) of Rose) as in the case with zinc.
Whether the horizontal striations were due to gliding parallel to the
base I could not certainly decide. Many of the crystals appear when
pinched to be completely overturned, in which cases ordinary bending
accompanies gliding as in the case of gold set. This is shown by the
fact that both faces and striations become rounded.”




Biographical Sketch.


Harry Clary Jones was born near New London, Frederick County, Maryland,
Nov. 11ᵗʰ 1865.

After attending several schools in that state he entered the Johns
Hopkins University in the autumn of 1885 as a special student of
chemistry and physics. He matriculated in 1887 and received the degree
of Bachelor of Arts in 1889, having held an ordinary and an honorary
scholarship. For the last three years he has continued his studies
in the University following chemistry as a principal subject and
mineralogy and geology as subordinates. During this time he has been
appointed twice to a university scholarship, was lecture assistant to
professor Remsen,90-91, and Fellow in chemistry,91-92.