THE WORK OF EILHARDT MITSCHERLICH AND HIS DISCOVERY OF ISOMORPHISM.
During the height of the French Revolution, which caused the work of the
Abbe Haüy to be suspended for a time, although he was fortunately not
one of the many scientific victims of that terrible period, there was
born, on the 7th of January 1794, in the village of Neuende, near Jever,
in Oldenburg, the man who was destined to continue that work on its
chemical side. Eilhardt Mitscherlich was the son of the village pastor,
and nephew of the celebrated philologer, Prof. Mitscherlich of
Göttingen. His uncle’s influence appears to have given young
Mitscherlich a leaning towards philological studies, for during his
later terms at the Gymnasium at Jever, where he received his early
education, he devoted himself with great energy to the study of history
and languages, for which he had a marked talent, under the able
direction and kind solicitude of the head of the Gymnasium at that time,
the historian Schlosser. He eventually specialised on the Persian
language, and when Schlosser was promoted to Frankfort young
Mitscherlich accompanied him, and there prosecuted these favourite
studies until the year 1811, when he went to the university of
Heidelberg.
For some time now he had cherished the hope of proceeding to Persia and
conducting philological investigations on the spot, and in 1813, an
opportunity presenting itself in the prospect of an embassy being
despatched to Persia by Napoleon, he transferred himself to the
university of Paris, with the object of obtaining permission from
Napoleon to accompany the embassy. This visit to Paris must have been
one of Mitscherlich’s most exciting and interesting experiences. For
Napoleon had just returned from the disastrous Russian campaign of 1812,
and was feverishly engaged in raising a new army wherewith to stem the
great rise of the people which was now re-awakening patriotic spirit
throughout the whole of Germany, and which threatened to sweep away, as
it eventually did, the huge fabric of his central European Empire.
Indeed Mitscherlich appears to have been detained in Paris during the
exciting years 1813 and 1814, and with the abdication of Napoleon on
April 4th of that year, he was obliged to give up all idea of proceeding
to Persia. He decided that the only way of accomplishing his purpose was
to attempt to travel thither as a doctor of medicine. He therefore
returned to his native Germany during the summer of 1814, and proceeded
to Göttingen, which was then famous for its medical school. Here he
worked hard at the preliminary science subjects necessary for the
medical degree, while still continuing his philology to such serious
purpose as to enable him to publish, in 1815, the first volume of a
history of the Ghurides and Kara-Chitayens, entitled “Mirchondi historia
Thaheridarum.” It is obvious from the sequel, however, that he very soon
began to take much more than a merely passing interest in his scientific
studies, and he eventually became so fascinated by them, and
particularly chemistry, as to abandon altogether his cherished idea of a
visit to Persia. Europe was now settling down after the stormy period of
the hundred days which succeeded Napoleon’s escape from Elba,
terminating in his final overthrow on June 18th, 1815, at Waterloo, and
Mitscherlich was able to devote himself to the uninterrupted prosecution
of the scientific work now opening before him. He had the inestimable
advantage of bringing to it a culture and a literary mind of quite an
unusually broad and original character; and if the fall of Napoleon
brought with it the loss to the world of an accomplished philologist, it
brought also an ample compensation in conferring upon it one of the most
erudite and broad-minded of scientists.
In 1818 Mitscherlich went to Berlin, and worked hard at chemistry in the
university laboratory under Link. It was about the close of this year or
the beginning of 1819 that he commenced his first research, and it
proved to be one which will ever be memorable in the annals both of
chemistry and of crystallography. He had undertaken the investigation of
the phosphates and arsenates, and his results confirmed the conclusions
which had just been published by Berzelius, then the greatest chemist of
the day, namely, that the anhydrides of phosphoric and arsenic acids
each contain five equivalents of oxygen, while those of the lower
phosphorous and arsenious acids contain only three. But while making
preparations of the salts of these acids, which they form when combined
with potash and ammonia, he observed a fact which had escaped Berzelius,
namely, that the phosphates and arsenates of potassium and ammonium
_crystallise in similar forms_, the crystals being so like each other,
in fact, as to be indistinguishable on a merely cursory inspection.
Being unacquainted with crystallography, and perceiving the importance
of the subject to the chemist, he acted in a very practical and sensible
manner, which it is more than singular has not been universally imitated
by all chemists since his time. He at once commenced the study of
crystallography, seeing the impossibility of further real progress
without a working knowledge of that subject. He was fortunate in finding
in Gustav Rose, the Professor of Geology and Mineralogy at Berlin, not
merely a teacher close at hand, but also eventually a life-long intimate
friend. Mitscherlich worked so hard under Rose that he was very soon
able to carry out the necessary crystal measurements with his newly
prepared phosphates and arsenates. He first established the complete
morphological similarity of the acid phosphates and arsenates of
ammonium, those which have the composition NH_{4}H_{2}PO_{4} and
NH_{4}H_{2}AsO_{4} and crystallise in primary tetragonal prisms
terminated by the primary pyramid faces; and then he endeavoured to
produce other salts of ammonia with other acids which should likewise
give crystals of similar form. But he found this to be impossible, and
that only the phosphates and arsenates of ammonia exhibited the same
crystalline forms, composed of faces inclined at similar angles, which
to Mitscherlich at this time appeared to be identical. He next tried the
effect of combining phosphoric and arsenic acids with other bases, and
he found that potassium gave salts which crystallised apparently exactly
like the ammonium salts.
He then discovered that not only do the acid phosphates and arsenates of
potassium and ammonium, H_{2}KPO_{4}, H_{2}(NH_{4})PO_{4},
H_{2}KAsO_{4}, and H_{2}(NH_{4})AsO_{4} crystallise in similar
tetragonal forms, but also that the four neutral di-metallic salts of
the type HK_{2}PO_{4} crystallise similarly to each other.
He came, therefore, to the conclusion that there do exist bodies of
dissimilar chemical composition having the same crystalline form, but
that these substances are of similar constitution, in which one element,
or group of elements, may be exchanged for another which appears to act
analogously, such as arsenic for phosphorus and the ammonium group
(although its true nature was not then determined) for potassium. He
observed that certain minerals also appeared to conform to this rule,
such as the rhombohedral carbonates of the alkaline earths, calcite
CaCO_{3}, dolomite CaMg(CO_{3})_{2}, chalybite FeCO_{3}, and dialogite
MnCO_{3}; and the orthorhombic sulphates of barium (barytes, BaSO_{4}),
strontium (celestite, SrSO_{4}), and lead (anglesite, PbSO_{4}).
Wollaston, who, in the year 1809, had invented the reflecting
goniometer, and thereby placed a much more powerful weapon of research
in the hands of crystallographers, had already, in 1812, shown this to
be a fact as regards the orthorhombic carbonates (witherite,
strontianite, and cerussite) and sulphates (barytes, celestite, and
anglesite) of barium, strontium, and lead, as the result of the first
exact angular measurements made with his new instrument; but his
observations had been almost ignored until Mitscherlich reinstated them
by his confirmatory results.
While working under the direction of Rose, Mitscherlich had become
acquainted with the work of Haüy, whose ideas were being very much
discussed about this time, Haüy himself taking a very strong part in the
discussion, being particularly firm on the principle that every
substance of definite chemical composition is characterised by its own
specific crystalline form. Such a principle appeared to be flatly
contradicted by these first surprising results of Mitscherlich, and it
naturally appeared desirable to the latter largely to extend his
observations to other salts of different groups. It was for this reason
that he had examined the orthorhombic sulphates of barium, strontium,
and lead, and the rhombohedral carbonates of calcium, magnesium, iron,
and manganese, with the result already stated that the members of each
of these groups of salts were found to exhibit the same crystalline
form, a fact as regards the former group of sulphates which had already
been pointed out not only by Wollaston but by von Fuchs (who appears to
have ignored the work of Wollaston) in 1815, but had been explained by
him in a totally unsatisfactory manner. Moreover, about the same time
the vitriols, the sulphates of zinc, iron, and copper, had been
investigated by Beudant, who had shown that under certain conditions
mixed crystals of these salts could be obtained; but Beudant omitted to
analyse his salts, and thus missed discovering the all-important fact
that the vitriols contain water of crystallisation, and in different
amounts under normal conditions. Green vitriol, the sulphate of ferrous
iron, crystallises usually with seven molecules of water of
crystallisation, as does also white vitriol, zinc sulphate; but blue
vitriol, copper sulphate, crystallises with only five molecules of water
under ordinary atmospheric conditions of temperature and pressure.
Moreover, copper sulphate forms crystals which belong to the triclinic
system, while the sulphates of zinc and iron are dimorphous, the common
form of zinc sulphate, ZnSO_{4}.7H_{2}O, being rhombic, like Epsom
salts, the sulphate of magnesia which also crystallises with seven
molecules of water, MgSO_{4}.7H_{2}O, while that of ferrous sulphate,
FeSO_{4}.7H_{2}O, is monoclinic, facts which still further complicate
the crystallography of this group and which were quite unknown to
Beudant and were unobserved by him. But Beudant showed that the addition
of fifteen per cent. of ferrous sulphate to zinc sulphate, or nine per
cent. to copper sulphate, caused either zinc or copper sulphate to
crystallise in the same monoclinic form as ferrous sulphate. He also
showed that all three vitriols will crystallise in mixed crystals with
magnesium or nickel sulphates, the ordinary form of the latter salt,
NiSO_{4}.7H_{2}O, being rhombic like that of Epsom salts.
The idea that two chemically distinct substances not crystallising in
the cubic system, where the high symmetry determines identity of form,
can occur in crystals of the same form, was most determinedly combated
by Haüy, and the lack of chemical analyses in Beudant’s work, and the
altogether incorrect “vicarious” explanation given by von Fuchs, gave
Haüy very grave cause for suspicion of the new ideas. The previous
observations of Rome de l’Isle in 1772, Le Blanc in 1784, Vauquelin in
1797, and of Gay-Lussac in 1816, that the various alums, potash alum,
ammonia alum, and iron alum, will grow together in mixed crystals or in
overgrowths of one crystal on another, when a crystal of any one of them
is hung up in the solution of any other, does not affect the question,
as the alums crystallise in the cubic system, the angles of the highly
symmetric forms of which are absolutely identical by virtue of the
symmetry itself.
It was while this interesting discussion was proceeding that
Mitscherlich was at work in Berlin, extending his first researches on
the phosphates and arsenates to the mineral sulphates and carbonates.
But he recognised, even thus early, what has since become very clear,
namely, that owing to the possibility of the enclosure of impurities and
of admixture with isomorphous analogues, minerals are not so suitable
for investigation in this regard as the crystals of artificially
prepared chemical salts. For the latter can be prepared in the
laboratory in a state of definitely ascertained purity, and there is no
chance of that happening which Haüy was inclined to think was the
explanation of Mitscherlich’s results, namely, that certain salts have
such an immense power of crystallisation that a small proportion of them
in a solution of another salt may coerce the latter into crystallisation
in the form of that more powerfully crystallising salt. Mitscherlich
made a special study, therefore, of the work of Beudant, and repeated
the latter observer’s experiments, bringing to the research both his
crystallographic experience and that of a skilful analyst. He prepared
the pure sulphates of ferrous iron, copper, zinc, magnesium, nickel and
cobalt, all of which form excellent crystals. He soon cleared up the
mystery in which Beudant’s work had left the subject, by showing that
the crystals contained water of crystallisation, and in different
amounts. He found what has since been abundantly verified, that the
sulphates of copper and manganese crystallise in the triclinic system
with five molecules of water, CuSO_{4}.5H_{2}O and MnSO_{4}.5H_{2}O; in
the case of manganese sulphate, however, this is only true when the
temperature is between 7° and 20°, for if lower than 7° rhombic crystals
of MnSO_{4}.7H_{2}O similar to those of the magnesium sulphate group are
deposited, and if higher than 20° the crystals are tetragonal and
possess the composition MnSO_{4}.4H_{2}O. The Epsom salts group
crystallising in the rhombic system with seven molecules of water
consists of magnesium sulphate itself, MgSO_{4}.7H_{2}O, zinc sulphate
ZnSO_{4}.7H_{2}O, and nickel sulphate NiSO_{4}.7H_{2}O. The third group
of Mitscherlich consists of sulphate of ferrous iron FeSO_{4}.7H_{2}O
and cobalt sulphate CoSO_{4}.7H_{2}O, and both crystallise at ordinary
temperatures with seven molecules of water as indicated by the formulæ,
but in the monoclinic system. Thus two of the groups contain the same
number of molecules of water, yet crystallise differently. But
Mitscherlich next noticed a very singular fact, namely, that if a
crystal of a member of either of these two groups be dropped into a
saturated solution of a salt of the other group, this latter salt will
crystallise out in the form of the group to which the stranger crystal
belongs. Hence he concluded that both groups are capable of
crystallising in two different systems, rhombic and monoclinic, and that
under the ordinary circumstances of temperature and pressure three of
the salts form most readily the rhombic crystals, while the other two
take up most easily the monoclinic form. Mitscherlich then mixed the
solutions of the different salts, and found that the mixed crystals
obtained presented the form of some one of the salts employed. Thus even
so early in his work Mitscherlich indicated the possibility of
dimorphism. Moreover, before the close of the year 1819 he had satisfied
himself that aragonite is a second distinct form of carbonate of lime,
crystallising in the rhombic system and quite different from the
ordinary rhombohedral form calcite. Hence this was another undoubted
case of dimorphism.
During this same investigation in 1819, Mitscherlich studied the effect
produced by mixing the solution of each one of the above-mentioned seven
sulphates of dyad-acting metals with the solution of sulphate of potash,
and made the very important discovery that a double salt of definite
composition was produced, containing one equivalent of potassium
sulphate, one equivalent of the dyad sulphate (that of magnesium, zinc,
iron, manganese, nickel, cobalt, or copper), and six equivalents of
water of crystallisation, and that they all crystallised well in similar
forms belonging to the monoclinic system. Some typical crystals of one
of these salts, ammonium magnesium sulphate, are illustrated in Fig. 30
(Plate VII., facing page 44). This is probably the most important series
of double salts known to us, and is the series which has formed the
subject of prolonged investigation on the part of the author, no less
than thirty-four different members of the series having been studied
crystallographically and physically since the year 1893, and many other
members still remain to be studied. An account of this work is given in
a Monograph published in the year 1910 by Messrs Macmillan & Co., and
entitled, “Crystalline Structure and Chemical Constitution.”
This remarkable record for a first research was presented by
Mitscherlich to the Berlin Academy on the 9th December 1819. During the
summer of the same year Berzelius visited Berlin, and was so struck with
the abilities of Mitscherlich, then twenty-five years old, that he
persuaded him to accompany him on his return to Stockholm, and
Mitscherlich continued his investigations there under the eye of the
great chemist. His first work at Stockholm consisted of a more complete
study of the acid and neutral phosphates and arsenates of potash, soda,
ammonia, and lead. He showed that in every case an arsenate crystallises
in the same form as the corresponding phosphate. Moreover, in 1821 he
demonstrated that sodium dihydrogen phosphate, NaH_{2}PO_{4},
crystallises with a molecule of water of crystallisation in two
different forms, both belonging to the rhombic system but with quite
different axial ratios; this was consequently a similar occurrence to
that which he had observed with the sulphates of the iron and zinc
groups.
It was while Mitscherlich was in Stockholm that Berzelius suggested to
him that a name should be given to the new discovery that analogous
elements can replace each other in their crystallised compounds without
any apparent change of crystalline form. Mitscherlich, therefore, termed
the phenomenon “isomorphism,” from ἰσός, equal to, and μορφή, shape. The
term “isomorphous” thus strictly means “equal shaped,” implying not only
similarity in the faces displayed, but also absolute equality of the
crystal angles. The fact that the crystals of isomorphous substances are
not absolutely identical in form, but only very similar, was not likely
to be appreciated by Mitscherlich at this time. For the reflecting
goniometer had only been invented by Wollaston in 1809, and accurate
instruments reading to minutes of arc were mechanical rarities. It will
be shown in the sequel, as the result of the author’s investigations,
that there _are_ angular differences, none the less real because
relatively very small, between the members of such series. But
Mitscherlich was not in the position to observe them. It must be
remembered, moreover, that he was primarily a chemist, and that he had
only acquired sufficient crystallographic knowledge to enable him to
detect the system of symmetry, and the principal forms (groups of faces
having equal value as regards the symmetry) developed on the crystals
which he prepared. His doctrine of isomorphism, accepted in this broad
sense, proved of immediate and important use in chemistry. For there
were uncertainties as to the equivalents of some of the chemical
elements, as tabulated by Berzelius, then the greatest authority on the
subject, and these were at once cleared up by the application of the
principle of isomorphism.
The essence of Mitscherlich’s discovery was, that the chemical nature of
the elements present in a compound influences the crystalline form by
determining the number and the arrangement of the atoms in the molecule
of the compound; so that elements having similar properties, such for
instance as barium, strontium, and calcium, or phosphorus and arsenic,
combine with other elements to form similarly constituted compounds,
both as regards number of atoms and their arrangement in the molecule.
Number of atoms alone, however, is no criterion, for the five atoms of
the ammonium group NH_{4} replace the one atom of potassium without
change of form.
This case of the base ammonia had been one of Mitscherlich’s greatest
difficulties during the earlier part of his work, and remained a
complete puzzle until about this time, when its true chemical character
was revealed. For until the year 1820 Berzelius believed that it
contained oxygen. Seebeck and Berzelius had independently discovered
ammonium amalgam in 1808, and Davy found, on repeating the experiment,
that a piece of sal-ammoniac moistened with water produced the amalgam
with mercury just as well as strong aqueous ammonia. Both Berzelius and
Davy came to the conclusion that ammonia contains oxygen, like potash
and soda, and that a metallic kind of substance resembling the alkali
metals, potassium and sodium, was isolated from this oxide or hydrate by
the action of the electric current, which Seebeck had shown facilitated
the formation of the so-called ammonium amalgam. Davy, however, accepted
in part the views of Gay-Lussac and Thénard, who, in 1809, concluded
from their experiments that ammonium consisted of ammonia gas NH_{3}
with an additional atom of hydrogen, the group NH_{4} then acting like
an alkali metal, views which time has substantiated. But their further
erroneous conclusion that sodium and potassium also contained hydrogen
was rejected by him. Berzelius, however, set his face both against this
latter fallacy and the really correct NH_{4} theory, and it was not
until four years after Ampère, in 1816, had shown that sal-ammoniac was,
in fact, the compound of the group NH_{4} with chlorine, that Berzelius,
about the year 1820, after thoroughly sifting the work of Ampère,
accepted the view of the latter that in the ammonium salts it is the
group NH_{4}, acting as a radicle capable of replacing the alkali
metals, which is present.
The fact that this occurred at this precise moment, four years after the
publication of Ampère’s results, leads to the conclusion that the
observation of Mitscherlich, that the ammonium compounds are isomorphous
with the potassium compounds, was the compelling argument which caused
Berzelius finally to admit what has since proved to be the truth.
While still at Stockholm Mitscherlich showed that the chromates and
manganates are isomorphous with the sulphates, and also that the
perchlorates and permanganates are isomorphous with each other. Although
these facts could not be properly explained at the time, owing to the
inadequate progress of the chemistry of manganese, it was seen that
potassium chromate, K_{2}CrO_{4}, contained the same number of atoms as
potassium sulphate, K_{2}SO_{4}, and that potassium permanganate
KMnO_{4} and perchlorate KClO_{4} likewise resembled each other in
regard to the number of atoms contained in the molecule.
As a good instance of the use of the principle of isomorphism, we may
recall that when Marignac, in 1864, found himself in great difficulty
about the atomic weights of the little known metals tantalum and niobium
which he was investigating, he discovered that their compounds are
isomorphous; the pentoxides of the two metals occur together in
isomorphous mixture in several minerals, and the double fluorides with
potassium fluoride, K_{2}TaF_{7} and K_{2}NbF_{7} are readily obtained
in crystals of the same form. The specific heat of tantalum was then
unknown, so that the law of Dulong and Petit connecting specific heat
with atomic weight could not be applied, and the vapour density of
tantalum chloride, as first determined by Deville and Troost with impure
material, did not indicate an atomic weight for tantalum which would
give it the position among the elements that the chemical reactions of
the metal indicated. Yet Marignac was able definitely to decide, some
time before the final vapour density determinations of Deville and
Troost with pure salts, from the fact of the isomorphism of their
compounds, that the only possible positions for tantalum and niobium
were such as corresponded with the atomic weights 180 and 93
respectively. Time has only confirmed this decision, and we now know
that niobium and tantalum belong to the same family group of elements as
that to which vanadium belongs, and the only difference which modern
research has introduced has been to correct the decimal places of the
atomic weights, that of niobium (now also called columbium, the name
given to it by its discoverer, Hatchett, in 1801) being now accepted as
92.8 and that of tantalum 179.6, when that of hydrogen = 1.
Applying the law of isomorphism in a similar manner, Berzelius was
enabled to fix the atomic weights of copper, cadmium, zinc, nickel,
cobalt, iron, manganese, chromium, sulphur, selenium, and chlorine, the
numbers accepted to-day differing only in the decimal places, in
accordance with the more accurate results acquired by the advance of
experimental and quantitative analytical methods. But with regard to
several other elements, owing to inadequate data, Berzelius made serious
mistakes, showing how very great is the necessity for care and for ample
experimental data and accurate measurements, before the principle of
isomorphism can be applied with safety. Given these, and we have one of
the most valuable of all the aids known to us in choosing the correct
atomic weight of an element from among two or three possible
alternatives. We are only on absolutely sure ground when we are dealing
not only with a series of compounds consisting of the same number of
atoms, but when also the interchangeable elements are the intimately
related members of a family group, such as we have since become familiar
with in the vertical groups of elements in the periodic table of
Mendeléeff.
Before leaving Stockholm Mitscherlich showed, from experiments on the
crystallisation of mixtures of the different sulphates with which he had
been working, that isomorphous substances intermix in crystals in all
proportions, and that they also replace one another in minerals in
indefinite proportions, a fact which has of recent years been
wonderfully exemplified in the cases of the hornblende (amphibole) and
augite (pyroxene) groups.
In November 1821 Mitscherlich closed these memorable labours at
Stockholm and returned to Berlin, where he acted as extraordinary
professor of the university until 1825, when he was elected professor in
ordinary. His investigations for a time were largely connected with
minerals, but on July 6th, 1826, he presented a further most important
crystallographic paper to the Berlin Academy, in which he announced his
discovery of the fact that one of the best known chemical elements,
sulphur, is capable of crystallising in two distinct forms. The ordinary
crystals found about Etna and Vesuvius and in other volcanic regions
agree with those deposited from solution in carbon bisulphide in
exhibiting rhombic symmetry. But Mitscherlich found that when sulphur is
fused and allowed to cool until partially solidified, and the still
liquid portion is then poured out of the crucible, the walls of the
latter are found to be lined with long monoclinic prisms. These have
already been illustrated in Fig. 2, Plate I., in Chapter I.
Here was a perfectly clear case of an element—not liable to any charge
of difference of chemical composition such as might have applied to the
cases of sodium dihydrogen phosphate, carbonate of lime, and iron
vitriol and its analogues, which he had previously described as cases of
the same substance crystallising in two different forms—which could be
made to crystallise in two different systems of symmetry at will, by
merely changing the circumstances under which the crystallisation
occurred. His explanation being thus proved absolutely, he no longer
hesitated, but at once applied the term “dimorphous” to these substances
exhibiting two different forms, and referred to the phenomenon itself as
“dimorphism.” The case of carbonate of lime had given rise to prolonged
discussion, for the second variety, the rhombic aragonite, had been
erroneously explained by Stromeyer, after Mitscherlich’s first
announcement in 1819, as being due to its containing strontia as well as
lime, and the controversy raged until Buchholz discovered a specimen of
aragonite which was absolutely pure calcium carbonate, so that
Mitscherlich’s dimorphous explanation was fully substantiated.
Dimorphism is very beautifully illustrated by the case of the trioxide
of antimony, Sb_{2}O_{3}, a slide of which, obtained by sublimation of
the oxide from a heated tube on to the cool surface of a glass
microscope slip, is seen reproduced in Fig. 50, Plate XI. The two forms
are respectively rhombic and cubic. The rhombic variety usually takes
the form of long needle-shaped crystals, which are shown in Fig. 50
radiating across the field and interlacing with one another; the cubic
variety crystallises in octahedra, of which several are shown in the
illustration, perched on the needles, one interesting individual being
poised on the end of one of the needles. The two forms occur also in
nature as the rhombic mineral valentinite and the cubic mineral
senarmontite, which latter crystallises in excellent regular octahedra.
Antimonious oxide, moreover, is not only isomorphous, but isodimorphous
with arsenious oxide, a slide of octahedra of which has already been
reproduced in Fig. 3, Plate I., in Chapter I. For besides this common
octahedral form of As_{2}O_{3} artificial crystals of arsenious oxide
have been prepared of rhombic symmetry, resembling valentinite. Hence
the two lower oxides of arsenic and antimony afford us a striking case
of the simultaneous display of Mitscherlich’s two principles of
isomorphism and dimorphism.
Thus the position in 1826 was that Mitscherlich had discovered the
principle of isomorphism, and had also shown the occurrence of
dimorphism in several well-proved specific cases, and that he regarded
at this time isomorphism as being a literal reality, absolute identity
of form.
[Illustration:
_PLATE XI._
FIG. 50.—Rhombic Needles and Cubic Octahedra of Antimony Trioxide
obtained by Sublimation. An interesting Example of Dimorphism.
]
[Illustration:
FIG. 101.—Ammonium Chloride crystallising· from a Labile
Supersaturated Solution (see p. 248).
REPRODUCTIONS OF PHOTOMICROGRAPHS.
]
These striking results appeared at once to demolish the theory that any
one substance of definite chemical composition is characterised by a
specific crystalline form, which was Haüy’s most important
generalisation. Mitscherlich, however, soon expressed doubts as to the
absolute identity of form of his isomorphous crystals, and saw that it
was quite possible that in the systems other than the cubic (in which
latter system the highly perfect symmetry itself determines the form,
and that the angles shall be identically constant), there might be
slight distinctive differences in the crystal angles. For he caused to
be constructed, by the celebrated optician and mechanician, Pistor, the
most accurate goniometer which had up till then been seen, provided with
four verniers, each reading to ten seconds of arc, and with a telescope
magnifying twenty times, for viewing the reflections of a signal,
carried by a collimator, from the crystal faces. Unfortunately in one
respect, he was almost at once diverted, by the very excess of
refinement of this instrument, to the question of the alteration of the
crystal angles by change of temperature, and lost the opportunity, never
to recur, of doing that which would at once have reconciled his views
with those of Haüy in regard to this important matter, namely, the
determination of these small but real differences in the crystal angles
of the different members of isomorphous series, and the discovery of the
interesting law which governs them, a task which in these later days has
fallen to the lot of the author.
Another remarkable piece of crystallographic work, this time in the
optical domain, which has rendered the name of Mitscherlich familiar,
was his discovery of the phenomenon of crossed-axial-plane dispersion of
the optic axes in gypsum. (The nature and meaning of “optic axes” will
be explained in Chapter XIII., page 185.) During the course of a lecture
to the Berlin Academy in the year 1826 Mitscherlich, always a brilliant
lecturer and experimenter at the lecture table, exhibited an experiment
with a crystal of gypsum (selenite) which has ever since been referred
to as the “Mitscherlich experiment.” He had been investigating the
double refraction of a number of crystalline substances at different
temperatures, and had observed that gypsum, hydrated calcium sulphate,
CaSO_{4}.2H_{2}O, was highly sensitive in this respect, especially as
regards the position of its optic axes. At the ordinary temperature it
is biaxial, with an optic axial angle of about 60°, but on heating the
crystal the angle diminishes, until just above the temperature of
boiling water the axes become identical, as if the crystal were
uniaxial, and then they again separate as the temperature rises further,
but in the plane at right angles to that which formerly contained them;
hence the phenomenon is spoken of as “crossed-axial-plane dispersion.”
Mitscherlich employed a plate of the crystal cut perpendicularly to the
bisectrix of the optic axial angle, and showed to the Academy the
interference figures (see Plate XII.) which it afforded in convergent
polarised light with rising temperature. At first, for the ordinary
temperature, the usual rings and lemniscates surrounding the two optic
axes were apparent at the right and left margins of the field; as the
crystal was gently heated (its supporting metallic frame being heated
with a spirit lamp) the axes approached each other, with ever changing
play of colour and alteration of shape of the rings and lemniscates,
until eventually the dark hyperbolic brushes, marking by their well
defined vertices the positions of the two optic axes within the
innermost rings, united in the centre of the field to produce the
uniaxial dark rectangular cross; the rings around the centre had now
become circles, the lemniscates having first become ellipses which more
and more approximated, as the temperature rose, to circles. Then the
dark cross opened out again, and the axial brushes separated once more,
but in the vertical direction, and the circles became again first
ellipses and then lemniscates, eventually developing inner rings around
the optic axes; if the source of heat were not removed at this stage the
crystal would suddenly decompose, becoming dehydrated, and the field on
the screen would become dark. If, however, the spirit lamp were removed
before this occurred, the phenomena were repeated in the reverse order
as the crystal cooled.
This beautiful experiment is now frequently performed, as gypsum is
perhaps the best example yet known which exhibits the phenomenon of
crossed-axial-plane dispersion by change of temperature alone. A
considerable number of other cases are known, such as brookite, the
rhombic form of titanium dioxide TiO_{2}, and the triple tartrate of
potassium, sodium, and ammonium, but these are more sensitive to change
of wave-length in the illuminating light than to change of temperature.
[Illustration:
FIG. 51.—The Mitscherlich Experiment with Gypsum.
]
[Illustration:
_PLATE XII._
FIG. 52.—Appearance of the Interference Figure half a Minute after
commencing the Experiment. Temperature of Crystal about 40° C.
]
[Illustration:
FIG. 53.—Appearance a Minute or so later, the Axes approaching the
Centre. Temperature of Crystal about 85° C.
]
[Illustration:
FIG. 54.—The Two Optic Axes coincident in the Centre of the Figure,
two or three Minutes from the commencement. Temperature of Crystal
106° C.
]
[Illustration:
FIG. 55.—The Axes re-separated in the Vertical Plane a Minute or two
later. Temperature of Crystal about 125° C.
THE MITSCHERLICH EXPERIMENT WITH GYPSUM.
FOUR STAGES IN THE TRANSFORMATION OF THE INTERFERENCE FIGURE IN
CONVERGENT POLARISED LIGHT, FROM HORIZONTALLY BIAXIAL THROUGH
UNIAXIAL TO VERTICALLY BIAXIAL, ON RAISING THE TEMPERATURE TO 125°
C.
(From Photographs by the author.)
]
The author has recently exhibited the “Mitscherlich experiment” to the
Royal Society,[2] and also in his Evening Discourse to the British
Association at their 1909 meeting in Winnipeg, in a new and more elegant
manner, employing the large Nicolprism projection polariscope shown in
Fig. 51, and a special arrangement of lenses for the convergence of the
light, which is so effective that no extraneous heating of the crystal
is required. The convergence of the rays is so true on a single spot in
the centre of the crystal plate about two millimetres diameter, that a
crystal plate not exceeding 6 mm. is of adequate size, mounted in a
miniature holder-frame of platinum or brass with an aperture not more
than 3 mm; the thickness of the crystal should remain about 2 mm., in
order that the rings round the axes may not be too large and diffuse,
the crystal being endowed with very feeble double refraction, which is
one of the causes of the phenomenon. Such a small crystal heats up so
rapidly in the heat rays accompanying the converging light rays—even
with the essential cold water cell two inches thick between the lantern
condenser and the polarising Nicol, for the protection of the balsam of
the latter—that any extraneous heating by a spirit or other lamp is
entirely unnecessary. The moment the electric arc of the lantern is
switched on, the optic axial rings appear at the right and left margins
of the screen, when the crystal is properly adjusted and the arc
correctly centred, and they march rapidly to the crossing point in the
centre, where the dark hyperbolæ unite to produce the rectangular St
Andrew’s cross, the rings, figure-eight curves, and other lemniscates
passing through the most exquisite evolutions and colour changes all the
time until they form the circular Newton’s rings, around the centre of
the cross; after this the cross and circles again open out, but along
the vertical diameter of the screen, into hyperbolæ and rings and
loop-like lemniscates surrounding two axes once more. It is wise as soon
as the separation in this plane is complete and the first or second
separate rings have appeared round the axes, to arrest the heating by
merely interposing intermittently a hand screen between the lantern and
polariser, or by blowing a current of cool air past the crystal, which
will cause the axes to recede again, and the phenomena to be reversed,
the crossing point being repassed, and the axes brought into the
original horizontal plane again. By manipulation of the screen, or
air-current, the axes can thus be caused to approach or to recede from
the centre at will, along either the horizontal or vertical diameter.
Four characteristic stages of the experiment are shown in Figs. 52 to
55, Plate XII. Fig. 52 exhibits the appearance just after commencing the
experiment, the optic axes being well in the field of view. Fig. 53
shows the axes horizontally approaching the centre. Fig. 54 shows the
actual crossing, which occurs for different crystals at temperatures
varying from 105°.5 to 111°.5 C.; and Fig. 55 represents the axes again
separated, but vertically.
The experiment as thus performed is one of the most beautiful
imaginable, and it can readily be understood how delighted were
Mitscherlich’s audience on the occasion of its first performance by him.
The author has since discovered no less than six other cases of
substances which exhibit crossed-axial-plane dispersion of the optic
axes, in the course of his investigations, one of which is illustrated
in Plate XIII., facing page 108; and, moreover, has arrived at a general
explanation of the whole phenomenon, the main points of which are that
such substances, besides showing very feeble double refraction (the two
extreme of the three refractive indices being very close together), also
exhibit very close approximation of the intermediate refractive index β
to either the minimum index α or the maximum index γ. Also, change of
temperature, or of wave-length, or most usually both, must so operate as
to bring the two indices closest together into actual identity and then
to pass beyond each other, these two indices thus exchanging positions,
the extreme one becoming the intermediate index. In other words, the
uniaxial cross and circular rings are produced owing to two of the three
refractive indices (corresponding to the directions of the three
rectangular axes of the ellipsoid which, in general, expresses the
optical properties of a crystal) becoming equal at the particular
temperature at which the phenomenon is observed to occur, and for light
of the specific wave-length in question. The ellipsoid of general form
which represents the optical properties of a biaxial crystal thus
becomes converted into a rotation ellipsoid corresponding to a uniaxial
crystal. Brookite and the triple tartrate are excellent examples of the
predominance of the effect of change of wave-length, for the optic axes
are separated in both cases widely in one plane for red light and almost
equally widely in the perpendicular plane for blue light. The new cases
observed by the author are sensitive both to change of wave-length and
to change of temperature, and so fall midway between the cases just
quoted and the case of gypsum. The cause of it, in four of these new
instances, is a very interesting one, connected with the regular change
of the refractive indices in accordance with the law of progression in
an isomorphous series according to the atomic weight of the alkali metal
present, which will be discussed in Chapter X.
A further most important discovery was made by Mitscherlich in the year
1827, which also profoundly concerns the work of the author, namely,
that of selenic acid, H_{2}SeO_{4}, analogous to sulphuric acid, and of
the large group of salts derived from it, the selenates, analogous to
the sulphates. He showed first that potassium selenate, K_{2}SeO_{4}, is
isomorphous with potassium sulphate, K_{2}SO_{4}, and subsequently that
the selenates in general are isomorphous with the corresponding
sulphates; consequently it followed that selenium is a member of the
sulphur family of elements. This element selenium had only been
discovered ten years previously by his friend Berzelius, and doubtless
Mitscherlich had seen a great deal of the work in connection with it
during the two years which he spent in the laboratory of Berzelius at
Stockholm, and was deeply interested in it.
The discovery has proved a most fruitful one, for the selenates are
beautifully crystalline salts, particularly suitable for
crystallographic researches, and their detailed investigation has
afforded a most valuable independent confirmation of the important
results obtained for the sulphates.
Again in 1830 Mitscherlich, following up the preliminary work already
referred to, definitely established another fact bearing on the same
series, namely, the isomorphism of potassium manganate K_{2}MnO_{4} with
the sulphate and selenate of potash; moreover, on continuing his study
of the manganese salts he further substantiated the isomorphism of the
permanganates with the perchlorates, and isolated permanganic acid. This
also proved a most important step forward, as these salts likewise
afford admirable material for crystallographic investigation, and such
an examination, carried out by Muthmann and Barker, has yielded most
valuable results.
Much later in his career Mitscherlich also described the dimorphous
iodide of mercury, HgI_{2}, one of the most remarkable and interesting
salts known to us, the unstable yellow rhombic modification being
converted into the more stable red tetragonal form by merely touching
with a hard substance. Also we are indebted to him at the same later
period for our knowledge of the crystalline forms of the elements
phosphorus, iodine, and selenium, when crystallised from solution in
bisulphide of carbon.
From the record of achievements which has now been given in this chapter
it will be obvious how much chemical crystallography owes to
Mitscherlich. The description of his work has taken us into almost every
branch of the subject, morphological, optical, and thermal, and although
it has consequently been necessary to refer to phenomena which have not
yet been explained in this book, it has doubtless proved on the whole
most advantageous thus to present the life work of this great master as
a complete connected story.