MORPHOTROPY AS DISTINCT FROM ISOMORPHISM.
It has been shown in the last chapter how Mitscherlich discovered the
principle of isomorphism, as applying to the cases of substances so
closely related that their interchangeable chemical elements are members
of the same family group; and also how the principle enabled him to
determine the chemical constitution of two hitherto unknown acids which
he isolated, selenic H_{2}SeO_{4} and permanganic HMnO_{4}. For he
observed that the selenates were isomorphous with the sulphates, and the
permanganates with the perchlorates. It was further made clear that the
principle as bequeathed to us by Mitscherlich was only defined in very
general terms, and its details have only recently been precisely
decided.
Before proceeding further (in Chapter X.) with the elucidation of the
true nature of isomorphism, however, some important crystallographic
relationships between substances less closely related than family
analogues must be referred to, as the outcome of a series of
investigations by von Groth, chiefly between the derivatives of the
hydrocarbon benzene. Also, some suggestive results obtained by the
author from an investigation of an organic homologous series, that is,
one the members of which differ by the regular addition of a CH_{3}
group, may be briefly referred to.
The interval between the work of Mitscherlich and that of von Groth was
one of doubt, discouragement, and somewhat of discredit for chemical
crystallography. The chemists Laurent[3] and Nicklès[4] carried out
during the years from 1842 to 1849 measurements of numerous organic
substances and of some inorganic compounds, the former chiefly halogen
or other derivatives of particular hydrocarbons or salts of homologous
fatty acids. Laurent, for instance, found that naphthalene
tetrachloride, C_{10}H_{8}.Cl_{4}, and chloronaphthalene tetrachloride,
C_{10}H_{7}Cl.Cl_{4}, crystallise in different systems, the former in
the monoclinic and the latter in the rhombic system. Yet the primary
prism angles of the two are less than a degree different, namely, 109°
0′ and 109° 45′. Laurent named this kind of similarity “hemimorphism,” a
most unfortunate term as it was already employed in crystallography in
its other well-known geometrical significance, that is, to denote a
crystal differently terminated at the two ends of an axis. Many other
like similarities were discovered by Laurent, and he again coined an
objectionable term, now discarded, to represent the cases of similarity
extending over more than the same system, namely, “isomeromorphism.”
Nicklès observed similar facts in connection with the barium salts of
the fatty acids, which crystallise in different systems with different
amounts of water of crystallisation. But their prism angles are all
within a couple of degrees of each other, varying from 98° to 100°. Thus
the phenomenon of “isogonism,” a term much less objectionable than those
invented by Laurent, appears to be a common observance not only for
different kinds of derivatives of the same original hydrocarbon or other
organic nucleus, but also for the case of homologous series. But Nicklès
missed the real point by including salts with different amounts of
water, which, it will be shown later, entirely upset the crystalline
structure. When this is eliminated the resemblance between true
similarly constituted homologues, differing by regular increments of
CH_{3}, is very much closer than would appear from Nicklès’ results.
Unfortunately, some of the work of Laurent and Nicklès was not carried
out with the care and accuracy which is indispensable for researches
which are to retain permanent value, and critics were not slow to arise.
Kopp,[5] in 1849, unmercifully exposed these failings, so that the real
kernel of the work, which was of considerable value, came into
discredit.
Pasteur,[6] however, in 1848, besides the important observations
regarding enantiomorphism, to be described in Chapter XI., had noticed
similar zonal likenesses between related tartrates, amounting only
therefore to isogonism and not to isomorphism; for here again the system
often differed, particularly when the members of a series compared
differed in their water of crystallisation. Thus there was ample
evidence of a really significant series of facts in the work of these
authors, but they were not properly arranged and explained.
So high was the feeling against the whole subject carried, however,
after Kopp’s memoir, that had it not been for the steadying influence of
Rammelsberg and Marignac, who themselves carried out many
crystallographic measurements as new substances continued to be
discovered with great rapidity, the science would have suffered a
serious set-back. Moreover, even Rammelsberg was led astray in the
direction of the views of the chemists of the time, that isomorphism
could be extended over the crystal system. Frankenheim, whose discovery
of the space-lattice, to be referred to in the next chapter, will ever
render his name famous, strongly opposed this view. Delafosse, on the
other hand, recognised some truth in both views, and assumed that there
were two kinds of isomorphism, that of Mitscherlich on the one hand, and
the broader one of Laurent on the other hand, and that in the case of
the latter kind the overstepping of the system is no bar.
Hjortdahl,[7] in the year 1865, supported the views of Delafosse more or
less, at any rate so far as to assume the possibility of the existence
of partial isomorphism, that is, of isogonism. He was very definite,
however, against accepting the proposition that any general law could be
applied. He himself discovered a partial similarity of angles in several
homologous series of organic compounds.
About this time Sella[8] uttered a warning which is one worthy of being
prominently posted in every research laboratory, namely, that _It is
unwise to make hasty generalisations from the results of a small number
of observations_. Were this principle more generally followed, much
greater progress would in the end be achieved, and without the
discouragement and discredit which inevitably follows the detection of
errors due to lack of broad experimental foundation. It is certainly an
incontrovertible fact that only such generalisations as find themselves
in accordance with all new but well-verified experimental facts as they
are revealed can stand the test of time and become accepted universally
as true laws of nature. And it is unreasonable to expect any
generalisation to be of such a character unless it is already based on
so large a number of facts that there is little fear of other new ones
upsetting them.
Some order was, however, introduced into this chaotic state of chemical
crystallography in the year 1870 by P. von Groth.[9] He investigated
systematically the derivatives of the hydrocarbon benzene, C_{6}H_{6},
many of which are excellently crystallising solids suitable for
goniometrical measurement. He showed that although the crystal system
may be and often is altered, yet there is a striking similarity in the
angles between the faces of certain zones, which for the purposes of
comparison he arranged to be parallel to each other in his descriptions
of the crystals, so that the relationship would then consist in an
elongation or a shortening of this particular zone axis, which was
usually a crystallographic axis. He recognised that this was a totally
different phenomenon from isomorphism, and called it “morphotropy.”
Although it may possibly be permissible from one point of view to regard
isomorphism as a particular case of complete morphotropy along all
zones, such a course is not advisable, as morphotropic similarities are
frequently of a comparatively loose and often indeed of a somewhat vague
character, while isomorphous relationships are governed by very precise
laws.
Thus von Groth showed first that benzene, C_{6}H_{6}, crystallises in
the rhombic system with axial ratios _a_ : _b_ : _c_ = 0.891 : 1 :
0.977. Next, that when one or two of the hydrogen atoms are replaced by
hydroxyl OH groups the substances produced, phenol C_{6}H_{5}.OH and
resorcinol C_{6}H_{4}(OH)_{2}, are found also to crystallise in the
rhombic system, and in the second case, for which alone the axial ratios
could be determined, the ratio _a_ : _b_ proved to be very similar, but
the ratio _c_ : _b_ was different, the actual values being _a_ : _b_ :
_c_ = 0.910 : 1 : 0.540. Pyrocatechol, the isomer of resorcinol, also
crystallises in the rhombic system, but the crystals have not been
obtained sufficiently well formed to enable any deductions to be made
from any measurements carried out with them.
Similarly, the nitro-derivatives of phenol, orthonitrophenol
C_{6}H_{4}.OH.NO_{2}, dinitrophenol C_{6}H_{3}.OH.(NO_{2})_{2}, and
trinitrophenol C_{6}H_{2}.OH.(NO_{2})_{3}, also crystallise in the
rhombic system, and with the following respective axial ratios: 0.873 :
1 : 0.60; 10.933 : 1 : 0.753; 0.937 : 1 : 0.974. Again, the value for
the ratio _a_ : _b_ is not very different from that of benzene itself,
while the ratio _c_ : _b_ differs considerably in the first two cases.
Similar relations were also found to hold good in the cases of
meta-dinitrobenzene, C_{6}H_{4}(NO_{2})_{2}, axial ratios 0.943 : 1 :
0.538, and trinitrobenzene, C_{6}H_{3}(NO_{2})_{3}, which possesses the
axial ratios 0.954 : 1 : 0.733.
The introduction of a chlorine or bromine atom or a CH_{3} group in
place of hydrogen was found by von Groth to produce more than the above
effect, the symmetry being often lowered to monoclinic, a fact which had
also been observed to occur in the cases of certain isomers of the
substances quoted above, ortho-dinitrobenzene for instance. But it was
nevertheless observed that the angles between the faces in the prism
zone remained very similar, the angles between the faces of the primary
prism (110) and (1̄10), for instance, only varying in eight such
derivatives of all three types, whether rhombic or monoclinic, from 93°
45′ to 98° 51′.
The crystallographic relationships of organic substances, however, are
very much complicated by the possibilities of isomerism, the ortho,
meta, and para compounds—corresponding to the replacement of the two
hydrogen atoms attached to two adjacent, alternate, or opposite carbon
atoms respectively, of the six forming the benzene ring—generally
differing extensively and sometimes completely in crystalline form.
Consequently, the phenomenon of morphotropy is best considered quite
independently of isomorphism.
An interesting intermediate case between morphotropy and true
isomorphism was investigated by the author in the year 1890, namely, a
series of homologous organic compounds differing by regular increments
of the organic radicle CH_{3}. They were prepared by Prof. Japp and Dr
Klingemann, and consisted of the methyl, CH_{3}, ethyl, C_{2}H_{5}, and
propyl, C_{3}H_{7}, derivatives of the substance triphenyl pyrrholone,
all of them being solids crystallising well. The problem was somewhat
complicated by the development of polymorphism, the methyl, ethyl, and
propyl compounds having each been found to be dimorphous, and not
improbably trimorphous, but only two varieties of each salt were
obtained in crystals adequately perfect for measurement. That the
production of these different forms was due to polymorphism and not to
chemical isomerism (different arrangement of the chemical atoms in the
molecule) was shown by the fact that one variety could be obtained from
the other by simply altering the conditions of crystallisation from the
same solvent. Their identical chemical composition was established by
direct analysis.
The methyl (CH_{3}) compound crystallised in rhombohedra and in
triclinic prisms. The ethyl (C_{2}H_{5}) derivative was deposited in
triclinic prisms exactly resembling those of the methyl compound in
habit and disposition of faces. A crystal of the triclinic methyl
derivative which would represent equally well the ethyl compound is
shown in Fig. 56. The angles also of the crystals of the two substances
are so similar that one might infer the existence of true and complete
isomorphism. The actual angular differences rarely exceeded three
degrees.
[Illustration:
FIG. 56.—Crystal of Methyl Triphenyl Pyrrholone.
]
Besides the triclinic form the ethyl derivative was also obtained in
monoclinic crystals, one of which is represented in Fig. 57. This
illustration might serve equally well, however, for a corresponding
monoclinic form of the propyl (C_{3}H_{7}) derivative, and the angles of
these two monoclinic ethyl and propyl compounds are even closer than
those of the triclinic methyl and ethyl derivatives, the closeness
increasing with the advent of symmetry.
[Illustration:
FIG. 57.—Crystal of Ethyl Triphenyl Pyrrholone.
]
This similarity of angles in the cases of the two pairs of triclinic and
monoclinic compounds is not only true about particular zones, but about
all the zones, so that it is a case isomorphism rather than of isogonism
(morphotropy). The similarity of optical properties is also very close,
and so much so in the cases of the monoclinic crystals of ethyl and
propyl triphenyl pyrrholone that both exhibit very high dispersion of
the optic axes. In the case of the propyl derivative the difference
between the apparent angle of the optic axes for red lithium light and
for green thallium light amounts to 11°. In the case of the ethyl
compound this difference is enhanced so considerably that the crystals
afford a remarkable instance of dispersion of the optic axes in crossed
axial planes, resembling the case of gypsum discovered by Mitscherlich
and described in the last chapter, except that the sensitiveness is to
change of wave-length in the illuminating light rather than to change of
temperature. The optic axial plane is perpendicular to the symmetry
plane for lithium and sodium light, as it is also in the case of the
propyl compound; but in the ethyl derivative it crosses over for
thallium light and rays beyond that towards the violet, into a plane at
right angles to the former plane, namely, the symmetry plane itself. The
total dispersion between the two axes as separated in the one plane for
red light, and as separated in the other perpendicular plane for blue
light, is more than 70°. Fig. 58, Plate XIII., shows the nature of the
interference figures afforded in convergent polarised light of different
wave-lengths by a section-plate perpendicular to the first median line.
The figure at _f_ represents what is observed in white light, as far as
is possible by a drawing in black and white. It consists of a series of
concave coloured curves, falling in between the arms of the cross, and
looping round the axes, a figure very much like that afforded by
brookite and triple tartrate of ammonium, potassium, and sodium, the
substances already mentioned in Chapter VII. as being similarly very
sensitive to change of wave-length. The figure in red monochromatic
lithium light is shown at _a_ in Fig. 58, and that for yellow sodium
light at _b_, the axes being now much closer together. On changing to
green thallium light the line joining the optic axes becomes vertical
instead of horizontal, as shown at _d_.
When, instead of employing monochromatic flames, the spectroscopic
monochromatic illuminator (Fig. 75, page 193), described by the author
some years ago to the Royal Society, is employed to illuminate the
polariscope, the source of light being the electric arc, the change of
the figure from that given by the extreme red of the spectrum to that
afforded by the violet may be beautifully followed, and the exact
wave-length in the greenish yellow determined for which the crossing
occurs and an apparently uniaxial figure of circular rings and
rectangular cross is produced. For it is possible with the aid of this
illuminator directly to observe the production of the uniaxial figure.
The wave-length is either directly afforded by the graduation of the
fine-adjustment micrometric drum or is obtained from a curve of
wave-lengths, constructed to correspond to the circle readings of the
illuminator. The appearance of the interference figure for this critical
wave-length is shown at _c_ in Fig. 58. The remaining figure at _e_
represents the appearance when a mixture of sodium and thallium light is
employed, which clearly indicates the four extreme axial positions, and
assists in elucidating the nature of the figure _f_ exhibited in white
light.
The second form of the propyl derivative belongs to the rhombic system,
and a similar rhombic form of the ethyl compound was once obtained, but
lost again on attempting to recrystallise.
These interesting relationships of the homologous methyl, ethyl, and
propyl derivatives of triphenyl pyrrholone thus appear to form a
connecting link between cases of isogonism or morphotropy and of true
isomorphism.
[Illustration:
_PLATE XIII._
FIG. 58.—Interference Figures in Convergent Polarised Light of
different Wave-lengths afforded by the Monoclinic Variety of Ethyl
Triphenyl Pyrrholone; _a_, in Red Lithium Light; _b_, in Yellow
Sodium Light; _c_, in Greenish-Yellow Light of the Critical
Wave-length for Production of the Uniaxial Figure; _d_, in Green
Thallium Light; _e_, in mixed Sodium and Thallium Light; and _f_, in
White Light.
(Reproductions of Drawings by the author.)
]
We are now, therefore, in a position to approach the question of true
isomorphism, and as leading up to the fuller treatment of the subject in
Chapter X. we may conclude this chapter by referring first to one
important investigation in which the necessity for extreme accuracy of
measurement and perfection of material was fully appreciated. This was
an admirable research carried out in the years 1887 and 1888 by H. A.
Miers[10] on the red silver minerals, proustite, sulpharsenite of
silver, Ag_{3}AsS_{3}, and pyrargyrite, the analogous sulphantimonite of
silver, Ag_{3}SbS_{3}, which afforded a further indication of the
existence of real small differences of angle between the members of
truly isomorphous series. These two minerals form exceptionally
beautiful crystals belonging to the trigonal system, the hexagonal prism
being always a prominent form, terminated by the primary and other
rhombohedra, scalenohedra and various pyramidal forms, many of the
crystals being exceedingly rich in faces. When the crystals are freshly
obtained from the dark recesses of the silver mine they are very
lustrous and transparent, but they are gradually affected by light, like
many silver compounds, and require to be stored in the dark in order to
preserve their transparency. A magnificent crystal of proustite from
Chili is one of the finest objects in the British Museum at South
Kensington, but is rarely seen on account of the necessity for
preservation from light. Pyrargyrite is generally dark grey in
appearance, and affords a reddish-purple “streak” (colour of the powder
on scratching or pulverising). Proustite, however, possesses a beautiful
scarlet-vermilion colour, and affords a very bright red streak.
Now these two beautiful minerals are obviously analogous compounds of
the same metal, silver, with the sulpho-acid of two elements, arsenic
and antimony, belonging strictly to the same family group, the
nitrogen-phosphorus group, of the periodic classification of the
elements according to Mendeleéff. Consequently, they should be perfectly
isomorphous. Miers has shown in a most complete manner that they are so,
that they occur in very perfect crystals of similar habit belonging to
the same class of the trigonal system, the ditrigonal polar class, both
minerals being hemimorphic, that is, showing different forms at the two
terminations, in accordance with the symmetry of the polar class of the
trigonal system. But the angles of the two substances were not found to
be identical, although constant for each compound within one minute of
arc, there being slight but very real differences, which are very well
typified by the principal angle in each case, that of the primary
rhombohedron. In the case of proustite it is 72° 12′, while the
rhombohedron angle of pyrargyrite is 71° 22′.
This interesting and beautiful investigation of Miers thus gave us an
inkling of the truth, that small angular differences do exist between
the members of isomorphous compounds. It paved the way for, and indeed
partly suggested, the author’s systematic investigation of the
sulphates, selenates, and double salts of the alkali series of metals, a
brief account of the main results of which will be given in Chapter X.