He has also shown that the nitrophenols yield, in addition to the
colourless true nitrophenol ethers, an isomeric series of coloured
unstable quinonoid _aci_-ethers, which have practically the same
colour and yield the same absorption spectra as the coloured metallic
salts. He suggests that the term "quinone" theory be abandoned, and
replaced by the _Umlagerungs_ theory, since this term implies some
intermolecular rearrangement, and does not connote simply benzenoid
compounds as does "quinonoid." H. von Liebig (_Ann._, 1908, 360, p.
128), from a very complete discussion of triphenyl-methane
derivatives, concluded that the grouping
R R R
.. .. ..
A - A - A
was the only true organic chromophore, colour production, however,
requiring another condition, usually the closing of a ring.
The views as to the question of colour and constitution may be
summarized as follows:--(1) The quinone theory (Armstrong, Gomberg, R.
Meyer) regards all coloured substances as having a quinonoid structure.
(2) The chromophore-auxochrome theory (Kauffmann) regards colour as due
to the entry of an "auxochrome" into a "chromophoric" molecule. (3) If a
colourless compound gives a coloured one on solution or by
salt-formation, the production of colour may be explained as a
particular form of ionization (Baeyer), or by a molecular rearrangement
(Hantzsch). A dynamical theory due to E.C.C. Baly regards colour as due
to "isorropesis" or an oscillation between the residual affinities of
adjacent atoms composing the molecule.
_Fluorescence and Constitution._--The physical investigation of the
phenomenon named fluorescence--the property of transforming incident
light into light of different refrangibility--is treated in the article
FLUORESCENCE. Researches in synthetical organic chemistry have shown
that this property of fluorescence is common to an immense number of
substances, and theories have been proposed whose purpose is to connect
the property with constitution.
In 1897 Richard Meyer (_Zeit. physik. Chemie_, 24, p. 468) submitted
the view that fluorescence was due to the presence of certain
"fluorophore" groups; such groupings are the pyrone ring and its
congeners, the central rings in anthracene and acridine derivatives,
and the paradiazine ring in safranines. A novel theory, proposed by
J.T. Hewitt in 1900 (_Zeit. f. physik. Chemie_, 34, p. 1; _B.A.
Report_, 1903, p. 628, and later papers in the _Journ. Chem. Soc._),
regards the property as occasioned by internal vibrations within the
molecule conditioned by a symmetrical double tautomerism, light of one
wave-length being absorbed by one form, and emitted with a different
wave-length by the other. This oscillation may be represented in the
case of acridine and fluorescein as
CH CH CH
//\ //\ /\\ //\ /|\ /\\ //\ /\\ /\\
| | || | ==> | || | || | ==> | || | |
| | || | <== | || | || | <== | || | |,
\\/ \\/ \// \\/ \|/ \// \\/ \// \//
N N N
O O O O O
\\/ \ / \ / \OH HO/ \ / \ / \OH / \ / \ / \//
| | | | ==> | | | | ==> | | | |
| | | | <== | | | | <== | | | |
\ / \\/ \ / \ / \ / \ / \ / \// \ /
C C C
/ / \ /
C6H4.COOH C6H4 O C6H4.COOH
\ /
CO
This theory brings the property of fluorescence into relation
with that of colour; the forms which cause fluorescence being the
coloured modifications: ortho-quinonoid in the case of acridine,
para-quinonoid in the case of fluorescein. H. Kauffmann (_Ber._, 1900,
33, p. 1731; 1904, 35, p. 294; 1905, 38, p. 789; _Ann._, 1906, 344, p.
30) suggested that the property is due to the presence of at least two
groups. The first group, named the "luminophore," is such that when
excited by suitable aetherial vibrations emits radiant energy; the
other, named the "fluorogen," acts with the luminophore in some way or
other to cause the fluorescence. This theory explains the fluorescence
of anthranilic acid (o-aminobenzoic acid), by regarding the aniline
residue as the luminophore, and the carboxyl group as the fluorogen,
since, apparently, the introduction of the latter into the
non-fluorescent aniline molecule involves the production of a
fluorescent substance. Although the theories of Meyer and Hewitt do
not explain (in their present form) the behaviour of anthranilic acid,
yet Hewitt has shown that his theory goes far to explain the
fluorescence of substances in which a double symmetrical tautomerism
is possible. This tautomerism may be of a twofold nature:--(1) it may
involve the mere oscillation of linkages, as in acridine; or (2) it
may involve the oscillation of atoms, as in fluorescein. A theory of a
physical nature, based primarily upon Sir J.J. Thomson's theory of
corpuscles, has been proposed by J. de Kowalski (_Compt. rend._ 1907,
144, p. 266). We may notice that ethyl oxalosuccinonitrile is the
first case of a fluorescent aliphatic compound (see W. Wislicenus and
P. Berg, _Ber._, 1908, 41, p. 3757).
_Capillarity and Surface Tension._--Reference should be made to the
article CAPILLARY ACTION for the general discussion of this phenomenon
of liquids. It is there shown that the surface tension of a liquid may
be calculated from its rise in a capillary tube by the formula [gamma] =
1/2rhs, where [gamma] is the surface tension per square centimetre, r the
radius of the tube, h the height of the liquid column, and s the
difference between the densities of the liquid and its vapour. At the
critical point liquid and vapour become identical, and, consequently, as
was pointed out by Frankenheim in 1841, the surface tension is zero at
the critical temperature.
Relation to molecular weight.
Mendeleeff endeavoured to obtain a connexion between surface energy
and constitution; more successful were the investigations of Schiff,
who found that the "molecular surface tension," which he defined as
the surface tension divided by the molecular weight, is constant for
isomers, and that two atoms of hydrogen were equal to one of carbon,
three to one of oxygen, and seven to one of chlorine; but these ratios
were by no means constant, and afforded practically no criteria as to
the molecular weight of any substance.
In 1886 R. Eotvos (_Wied. Ann._ 27, p. 452), assuming that two liquids
may be compared when the ratios of the volumes of the liquids to the
volumes of the saturated vapours are the same, deduced that
[gamma]V^{2/3} (where [gamma] is the surface tension, and V the
molecular volume of the liquid) causes all liquids to have the same
temperature coefficients. This theorem was investigated by Sir W.
Ramsay and J. Shields (_Journ. Chem. Soc._ 63, p. 1089; 65, p. 167),
whose results have thrown considerable light on the subject of the
molecular complexity of liquids. Ramsay and Shields suggested that
there exists an equation for the surface energy of liquids, analogous
to the volume-energy equation of gases, PV = RT. The relation they
suspected to be of the form [gamma]S = KT, where K is a constant
analogous to R, and S the surface containing one gramme-molecule,
[gamma] and T being the surface tension and temperature respectively.
Obviously equimolecular surfaces are given by (Mv)^{2/3}, where M is
the molecular weight of the substance, for equimolecular volumes are
Mv, and corresponding surfaces the two-thirds power of this. Hence S
may be replaced by (Mv)^{2/3}. Ramsay and Shields found from
investigations of the temperature coefficient of the surface energy
that T in the equation [gamma](Mv)^{2/3} = KT must be counted
downwards from the critical temperature T less about 6 deg. Their
surface energy equation therefore assumes the form [gamma](Mv)^{2/3} =
K([tau] - 6 deg.). Now the value of K, [gamma] being measured in dynes
and M being the molecular weight of the substance as a gas, is in
general 2.121; this value is never exceeded, but in many cases it is
less. This diminution implies an association of molecules, the surface
containing fewer molecules than it is supposed to. Suppose the
coefficient of association be n, i.e. n is the mean number of
molecules which associate to form one molecule, then by the normal
equation we have [gamma](Mnv)^{2/3} = 2.121([tau] - 6 deg.); if the
calculated constant be K1, then we have also [gamma](Mv)^{2/3} =
K1([tau]-6 deg.). By division we obtain n^{2/3} = 2.121/K1, or n =
(2.121/K1)^{3/2} the coefficient of association being thus determined.
The apparatus devised by Ramsay and Shields consisted of a capillary
tube, on one end of which was blown a bulb provided with a minute
hole. Attached to the bulb was a glass rod and then a tube containing
iron wire. This tube was placed in an outer tube containing the liquid
to be experimented with; the liquid is raised to its boiling-point,
and then hermetically sealed. The whole is enclosed in a jacket
connected with a boiler containing a liquid, the vapour of which
serves to keep the inner tube at any desired temperature. The
capillary tube can be raised or lowered at will by running a magnet
outside the tube, and the heights of the columns are measured by a
cathetometer or micrometer microscope.
Normal values of K were given by nitrogen peroxide, N2O4, sulphur
chloride, S2Cl2, silicon tetrachloride, SiCl4, phosphorus chloride,
PCl3, phosphoryl chloride, POCl3, nickel carbonyl, Ni(CO)4, carbon
disulphide, benzene, pyridine, ether, methyl propyl ketone;
association characterized many hydroxylic compounds: for ethyl alcohol
the factor of association was 2.74-2-43, for n-propyl alcohol
2.86-2.72, acetic acid 3.62-2.77, acetone 1.26, water 3.81-2.32;
phenol, nitric acid, sulphuric acid, nitroethane, and propionitril,
also exhibit association.
_Crystalline Form and Composition._
The development of the theory of crystal structure, and the fundamental
principles on which is based the classification of crystal forms, are
treated in the article CRYSTALLOGRAPHY; in the same place will be found
an account of the doctrine of isomorphism, polymorphism and morphotropy.
Here we shall treat the latter subjects in more detail, viewed from the
standpoint of the chemist. Isomorphism may be defined as the existence
of two or more different substances in the same crystal form and
structure, polymorphism as the existence of the same substance in two or
more crystal modifications, and morphotropy (after P. von Groth) as the
change in crystal form due to alterations in the molecule of closely
(chemically) related substances. In order to permit a comparison of
crystal forms, from which we hope to gain an insight into the prevailing
molecular conditions, it is necessary that some unit of crystal
dimensions must be chosen. A crystal may be regarded as built up of
primitive parallelepipeda, the edges of which are in the ratio of the
crystallographic axes, and the angles the axial angles of the crystals.
To reduce these figures to a common standard, so that the volumes shall
contain equal numbers of molecules, the notion of molecular volumes is
introduced, the arbitrary values of the crystallographic axes (a, b, c)
being replaced by the topic parameters[18] ([chi],[psi],[omega]), which
are such that, combined with the axial angles, they enclose volumes
which contain equal numbers of molecules. The actual values of the topic
parameters can then readily be expressed in terms of the elements of the
crystals (the axial ratios and angles), the density, and the molecular
weight (see Groth, _Physikalische Krystallographie_, or _Chemical
Crystallography_).
_Polymorphism._--On the theory that crystal form and structure are the
result of the equilibrium between the atoms and molecules composing the
crystals, it is probable, _a priori_, that the same substance may possess
different equilibrium configurations of sufficient stability, under
favourable conditions, to form different crystal structures. Broadly
this phenomenon is termed polymorphism; however, it is necessary to
examine closely the diverse crystal modifications in order to determine
whether they are really of different symmetry, or whether twinning has
occasioned the apparent difference. In the article CRYSTALLOGRAPHY the
nature and behaviour of twinned crystals receives full treatment; here
it is sufficient to say that when the planes and axes of twinning are
planes and axes of symmetry, a twin would exhibit higher symmetry (but
remain in the same crystal system) than the primary crystal; and, also,
if a crystal approximates in its axial constants to a higher system,
mimetic twinning would increase the approximation, and the crystal would
be pseudo-symmetric.
In general, polysymmetric and polymorphous modifications suffer
transformation when submitted to variations in either temperature or
pressure, or both. The criterion whether a pseudo-symmetric form is a
true polymorph or not consists in the determination of the scalar
properties (e.g. density, specific heat, &c.) of the original and the
resulting modification, a change being in general recorded only when
polymorphism exists. Change of temperature usually suffices to determine
this, though in certain cases a variation in pressure is necessary; for
instance, sodium magnesium uranyl acetate, NaMg(UO2)3(C2H3O2)9.9H2O
shows no change in density unless the observations are conducted under a
considerable pressure. Although many pseudo-symmetric twins are
transformable into the simpler form, yet, in some cases, a true
polymorph results, the change being indicated, as before, by alterations
in scalar (as well as vector) properties.
For example, boracite forms pseudo-cubic crystals which become truly
cubic at 265 deg., with a distinct change in density; leucite behaves
similarly at about 560 deg.. Again, the pyroxenes, RSiO3 (R = Fe, Mg,
Mn, &c.), assume the forms (1) monoclinic, sometimes twinned so as to
become pseudo-rhombic; (2) rhombic, resulting from the pseudo-rhombic
structure of (1) becoming ultramicroscopic; and (3) triclinic,
distinctly different from (1) and (2); (1) and (2) are polysymmetric
modifications, while (3) and the pair (1) and (2) are polymorphs.
While polysymmetry is solely conditioned by the manner in which the
mimetic twin is built up from the single crystals, there being no change
in the scalar properties, and the vector properties being calculable
from the nature of the twinning, in the case of polymorphism entirely
different structures present themselves, both scalar and vector
properties being altered; and, in the present state of our knowledge, it
is impossible to foretell the characters of a polymorphous modification.
We may conclude that in polymorphs the substance occurs in different
phases (or molecular aggregations), and the equilibrium between these
phases follows definite laws, being dependent upon temperature and
pressure, and amenable to thermodynamic treatment (cf. CHEMICAL ACTION
and ENERGETICS). The transformation of polymorphs presents certain
analogies to the solidification of a liquid. Liquids may be cooled below
their freezing-point without solidification, the _metastable_ (after W.
Ostwald) form so obtained being immediately solidified on the
introduction of a particle of the solid modification; and supersaturated
solutions behave in a similar manner. At the same time there may be
conditions of temperature and pressure at which polymorphs may exist
side by side.
The above may be illustrated by considering the equilibrium between
rhombic and monoclinic sulphur. The former, which is deposited from
solutions, is transformed into monoclinic sulphur at about 96 deg.,
but with great care it is possible to overheat it and even to fuse it
(at 113.5 deg.) without effecting the transformation. Monoclinic
sulphur, obtained by crystallizing fused sulphur, melts at 119.5 deg.,
and admits of undercooling even to ordinary temperatures, but contact
with a fragment of the rhombic modification spontaneously brings about
the transformation. From Reicher's determinations, the exact
transition point is 95.6 deg.; it rises with increasing pressure about
0.05 deg. for one atmosphere; the density of the rhombic form is
greater than that of the monoclinic. The equilibria of these
modifications may be readily represented on a pressure-temperature
diagram. If OT, OP (fig. 6), be the axes of temperature and pressure,
and A corresponds to the transition point (95.6 deg.) of rhombic
sulphur, we may follow out the line AB which shows the elevation of
the transition point with increasing pressure. The overheating curve
of rhombic sulphur extends along the curve AC, where C is the
melting-point of monoclinic sulphur. The line BC, representing the
equilibrium between monoclinic and liquid sulphur, is
thermodynamically calculable; the point B is found to correspond to
131 deg. and 400 atmospheres. From B the curve of equilibrium (BD)
between rhombic and liquid sulphur proceeds; and from C (along CE) the
curve of equilibrium between liquid sulphur and sulphur vapour. Of
especial interest is the curve BD: along this line liquid and rhombic
sulphur are in equilibrium, which means that at above 131 deg. and 400
atmospheres the rhombic (and not the monoclinic) variety would
separate from liquid sulphur.
| P |D
| |
| B |
| / \
| / \ Liquid
| / \
| Rhombic / \
| / \ E
| / \ _ -
| / Monoclinic_ \ -
| / _ - C
| /_ -
| /A
| / Vapour
| /
|/
+----------------------------------
O T
FIG. 6.
Mercuric iodide also exhibits dimorphism. When precipitated from
solutions it forms red tetragonal crystals, which, on careful heating,
give a yellow rhombic form, also obtained by crystallization from the
fused substance, or by sublimation. The transition point is 126.3 deg.
(W. Schwarz, _Zeit. f. Kryst._ 25, p. 613), but both modifications may
exist in metastable forms at higher and lower temperatures
respectively; the rhombic form may be cooled down to ordinary
temperature without changing, the transformation, however, being
readily induced by a trace of the red modification, or by friction.
The density and specific heat of the tetragonal form are greater than
those of the yellow.
Hexachlorethane is trimorphous, forming rhombic, triclinic and cubic
crystals; the successive changes occur at about 44 deg. and 71 deg.,
and are attended by a decrease in density.
Tetramorphism is exhibited by ammonium nitrate. According to O.
Lehmann it melts at 168 deg. (or at a slightly lower temperature in
its water of crystallization) and on cooling forms optically isotropic
crystals; at 125.6 deg. the mass becomes doubly refracting, and from a
solution rhombohedral (optically uniaxial) crystals are deposited; by
further cooling acicular rhombic crystals are produced at 82.8 deg.,
and at 32.4 deg. other rhombic forms are obtained, identical with the
product obtained by crystallizing at ordinary temperatures. The
reverse series of transformations occurs when this final modification
is heated. M. Bellati and R. Romanese (_Zeit. f. Kryst._ 14, p. 78)
determined the densities and specific heats of these modifications.
The first and third transformations (reckoned in order with increasing
temperature of the transition point) are attended by an increase in
volume, the second with a contraction; the solubility follows the same
direction, increasing up to 82.8 deg., then diminishing up to 125.6
deg., and then increasing from this temperature upwards.
The physical conditions under which polymorphous modifications are
prepared control the form which the substance assumes. We have already
seen that temperature and pressure exercise considerable influence in
this direction. In the case of separation from solutions, either by
crystallization or by precipitation by double decomposition, the
temperature, the concentration of the solution, and the presence of
other ions may modify the form obtained. In the case of sodium
dihydrogen phosphate, NaH2PO4.H2O, a stable rhombic form is obtained
from warm solutions, while a different, unstable, rhombic form is
obtained from cold solutions. Calcium carbonate separates as hexagonal
calcite from cold solutions (below 30 deg.), and as rhombic aragonite
from solutions at higher temperatures; lead and strontium carbonates,
however, induce the separation of aragonite at lower temperatures. From
supersaturated solutions the form unstable at the temperature of the
experiment is, as a rule, separated, especially on the introduction of a
crystal of the unstable form; and, in some cases, similar inoculation of
the fused substance is attended by the same result. Different
modifications may separate and exist side by side at one and the same
time from a solution; e.g. telluric acid forms cubic and monoclinic
crystals from a hot nitric acid solution, and ammonium fluosilicate
gives cubic and hexagonal forms from aqueous solutions between 6 deg.
and 13 deg.
A comparison of the transformation of polymorphs leads to a twofold
classification: (1) polymorphs directly convertible in a reversible
manner--termed "enantiotropic" by O. Lehmann and (2) polymorphs in which
the transformation proceeds in one direction only--termed "monotropic."
In the first class are included sulphur and ammonium nitrate; monotropy
is exhibited by aragonite and calcite.
It is doubtful indeed whether any general conclusions can yet be drawn
as to the relations between crystal structure and scalar properties and
the relative stability of polymorphs. As a general rule the modification
stable at higher temperatures possesses a lower density; but this is by
no means always the case, since the converse is true for antimonious and
arsenious oxides, silver iodide and some other substances. Attempts to
connect a change of symmetry with stability show equally a lack of
generality. It is remarkable that a great many polymorphous substances
assume more symmetrical forms at higher temperatures, and a possible
explanation of the increase in density of such compounds as silver
iodide, &c., may be sought for in the theory that the formation of a
more symmetrical configuration would involve a drawing together of the
molecules, and consequently an increase in density. The insufficiency of
this argument, however, is shown by the data for arsenious and
antimonious oxides, and also for the polymorphs of calcium carbonate,
the more symmetrical polymorphs having a lower density.
_Morphotropy._--Many instances have been recorded where substitution has
effected a deformation in one particular direction, the crystals of
homologous compounds often exhibiting the same angles between faces
situated in certain zones. The observations of Slavik (_Zeit. f.
Kryst._, 1902, 36, p. 268) on ammonium and the quaternary ammonium
iodides, of J.A. Le Bel and A. Ries (_Zeit. f. Kryst._, 1902, 1904, et
seq.) on the substituted ammonium chlorplatinates, and of G. Mez (ibid.,
1901, 35, p. 242) on substituted ureas, illustrate this point.
Ammonium iodide assumes cubic forms with perfect cubic cleavage;
tetramethyl ammonium iodide is tetragonal with perfect cleavages
parallel to {100} and {001}--a difference due to the lengthening of
the a axes; tetraethyl ammonium iodide also assumes tetragonal forms,
but does not exhibit the cleavage of the tetramethyl compound; while
tetrapropyl ammonium iodide crystallizes in rhombic form. The
equivalent volumes and topic parameters are tabulated:
+---------+----------+------------+-----------+----------+
| | NH4I. | NMe4I. | NEt4I. | NPr4I. |
+---------+----------+------------+-----------+----------+
| V | 57.51 | 108.70 | 162.91 | 235.95 |
| [chi] | 3.860 | 5.319 | 6.648 | 6.093 |
| [psi] | 3.860 | 5.319 | 6.648 | 7.851 |
| [omega] | 3.860 | 3.842 | 3.686 | 4.933 |
+---------+----------+------------+-----------+----------+
From these figures it is obvious that the first three compounds form a
morphotropic series; the equivalent volumes exhibit a regular
progression; the values of [chi] and [psi], corresponding to the a
axes, are regularly increased, while the value of [omega],
corresponding to the c axis, remains practically unchanged. This
points to the conclusion that substitution has been effected in one of
the cube faces. We may therefore regard the nitrogen atoms as
occupying the centres of a cubic space lattice composed of iodine
atoms, between which the hydrogen atoms are distributed on the
tetrahedron face normals. Coplanar substitution in four hydrogen atoms
would involve the pushing apart of the iodine atoms in four horizontal
directions. The magnitude of this separation would obviously depend on
the magnitude of the substituent group, which may be so large (in this
case propyl is sufficient) as to cause unequal horizontal deformation
and at the same time a change in the vertical direction.
The measure of the loss of symmetry associated with the introduction of
alkyl groups depends upon the relative magnitudes of the substituent
group and the rest of the molecule; and the larger the molecule, the
less would be the morphotropic effect of any particular substituent. The
mere retention of the same crystal form by homologous substances is not
a sufficient reason for denying a morphotropic effect to the substituent
group; for, in the case of certain substances crystallizing in the cubic
system, although the crystal form remains unaltered, yet the structures
vary. When both the crystal form and structure are retained, the
substances are said to be isomorphous.
Other substituent groups exercise morphotropic effects similar to those
exhibited by the alkyl radicles; investigations have been made on
halogen-, hydroxy-, and nitro-derivatives of benzene and substituted
benzenes. To Jaeger is due the determination of the topic parameters of
certain haloid-derivatives, and, while showing that the morphotropic
effects closely resemble those occasioned by methyl, he established the
important fact that, in general, the crystal form depended upon the
orientation of the substituents in the benzene complex.
Benzoic acid is pseudo-tetragonal, the principal axis being remarkably
long; there is no cleavage at right angles to this axis. Direct
nitration gives (principally) m-nitrobenzoic acid, also
pseudo-tetragonal with a much shorter principal axis. From this two
chlornitrobenzoic acids [COOH.NO2.Cl = 1.3.6 and 1.3.4] may be
obtained. These are also pseudotetragonal; the (1.3.6) acid has nearly
the same values of [chi] and [psi] as benzoic acid, but [omega] is
increased; compared with m-nitrobenzoic acid, [chi] and [psi] have
been diminished, whereas [omega] is much increased; the (1.3.4) acid
is more closely related to m-nitrobenzoic acid, [chi] and [psi] being
increased, [omega] diminished. The results obtained for the (1.2) and
(1.4) chlorbenzoic acids also illustrate the dependence of crystal
form and structure on the orientation of the molecule.
The hydroxyl group also resembles the methyl group in its morphotropic
effects, producing, in many cases, no change in symmetry but a
dimensional increase in one direction. This holds for benzene and
phenol, and is supported by the observations of Gossner on [1.3.5]
trinitrobenzene and picric acid (1.3.5-trinitro, 2 oxybenzene); these
last two substances assume rhombic forms, and picric acid differs from
trinitrobenzene in having [omega] considerably greater, with [chi] and
[psi] slightly less. A similar change, in one direction only,
characterizes benzoic acid and salicylic acid.
The nitro group behaves very similarly to the hydroxyl group. The
effect of varying the position of the nitro group in the molecule is
well marked, and conclusions may be drawn as to the orientation of the
groups from a knowledge of the crystal form; a change in the symmetry
of the chemical molecule being often attended by a loss in the
symmetry of the crystal.
It may be generally concluded that the substitution of alkyl, nitro,
hydroxyl, and haloid groups for hydrogen in a molecule occasions a
deformation of crystal structure in one definite direction, hence
permitting inferences as to the configuration of the atoms composing the
crystal; while the nature and degree of the alteration depends (1) upon
the crystal structure of the unsubstituted compound; (2) on the nature
of the substituting radicle; (3) on the complexity of the substituted
molecule; and (4) on the orientation of the substitution derivative.
_Isomorphism._--It has been shown that certain elements and groups
exercise morphotropic effects when substituted in a compound; it may
happen that the effects due to two or more groups are nearly equivalent,
and consequently the resulting crystal forms are nearly identical. This
phenomenon was first noticed in 1822 by E. Mitscherlich, in the case of
the acid phosphate and acid arsenate of potassium, KH2P(As)O4, who
adopted the term isomorphism, and regarded phosphorus and arsenic as
isomorphously related elements. Other isomorphously related elements and
groups were soon perceived, and it has been shown that elements so
related are also related chemically.
Tutton's investigations of the morphotropic effects of the metals
potassium, rubidium and caesium, in combination with the acid radicals
of sulphuric and selenic acids, showed that the replacement of
potassium by rubidium, and this metal in turn by caesium, was
accompanied by progressive changes in both physical and
crystallographical properties, such that the rubidium salt was always
intermediate between the salts of potassium and caesium (see table;
the space unit is taken as a pseudo-hexagonal prism). This fact finds
a parallel in the atomic weights of these metals.
+---------+---------+---------+---------+---------+
| | V | [chi] | [psi] | [omega] |
+---------+---------+---------+---------+---------+
| K2SO4 | 69.42 | 4.464 | 4.491 | 4.997 |
| Rb2SO4 | 73.36 | 4.634 | 4.664 | 5.237 |
| Cs2SO4 | 83.64 | 4.846 | 4.885 | 5.519 |
+---------+---------+---------+---------+---------+
| K2SeO4 | 71.71 | 4.636 | 4.662 | 5.118 |
| Rb2SeO4 | 79.95 | 4.785 | 4.826 | 5.346 |
| Cs2SeO4 | 91.16 | 4.987 | 5.035 | 5.697 |
+---------+---------+---------+---------+---------+
By taking appropriate differences the following facts will be
observed: (1) the replacement of potassium by rubidium occasions an
increase in the equivalent volumes by about eight units, and of
rubidium by caesium by about eleven units; (2) replacement in the same
order is attended by a general increase in the three topic parameters,
a greater increase being met with in the replacement of rubidium by
caesium; (3) the parameters [chi] and [psi] are about equally
increased, while the increase in [omega] is always the greatest. Now
consider the effect of replacing sulphur by selenium. It will be seen
that (1) the increase in equivalent volume is about 6.6; (2) all the
topic parameters are increased; (3) the greatest increase is effected
in the parameters [chi] and [psi], which are equally lengthened.
These observations admit of ready explanation in the following
manner. The ordinary structural formula of potassium sulphate is
O
|
K--O--S--O--K.
|
O
If the crystal structure be regarded as composed of three
interpenetrating point systems, one consisting of sulphur atoms, the
second of four times as many oxygen atoms, and the third of twice as
many potassium atoms, the systems being so arranged that the sulphur
system is always centrally situated with respect to the other two, and
the potassium system so that it would affect the vertical axis, then
it is obvious that the replacement of potassium by an element of
greater atomic weight would specially increase the length of [omega]
(corresponding to the vertical axis), and cause a smaller increase in
the horizontal parameters [chi] and [psi]; moreover, the increments
would advance with the atomic weight of the replacing metal. If, on
the other hand, the sulphur system be replaced by a corresponding
selenium system, an element of higher atomic weight, it would be
expected that a slight increase would be observed in the vertical
parameter, and a greater increase recorded equally in the horizontal
parameters.
Muthmann (_Zeit. f. Kryst._, 1894), in his researches on the tetragonal
potassium and ammonium dihydrogen phosphates and arsenates, found that
the replacement of potassium by ammonium was attended by an increase
of about six units in the molecular volume, and of phosphorus by
arsenic by about 4.6 units. In the topic parameters the following
changes were recorded: replacement of potassium by ammonium was
attended by a considerable increase in [omega], [chi] and [psi] being
equally, but only slightly, increased; replacement of phosphorus by
arsenic was attended by a considerable increase, equally in [chi] and
[psi], while [omega] suffered a smaller, but not inconsiderable,
increase. It is thus seen that the ordinary plane representation of
the structure of compounds possesses a higher significance than could
have been suggested prior to crystallographical researches.
Identity, or approximate identity, of crystal form is not in itself
sufficient to establish true isomorphism. If a substance deposits itself
on the faces of a crystal of another substance of similar crystal form,
the substances are probably isomorphous. Such parallel overgrowths,
termed episomorphs, are very common among the potassium and sodium
felspars; and K. von Hauer has investigated a number of cases in which
salts exhibiting episomorphism have different colours, thereby clearly
demonstrating this property of isomorphism. For example, episomorphs of
white potash alum and violet chrome alum, of white magnesium sulphate
and green nickel sulphate, and of many other pairs of salts, have been
obtained. More useful is the property of isomorphous substances of
forming mixed crystals, which are strictly isomorphous with their
constituents, for all variations in composition. In such crystals each
component plays its own part in determining the physical properties; in
other words, any physical constant of a mixed crystal can be calculated
as additively composed of the constants of the two components.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
Fig. 7 represents the specific volumes of mixtures of ammonium and
potassium sulphates; the ordinates representing specific volumes, and
the abscissae the percentage composition of the mixture. Fig. 8 shows
the variation of refractive index of mixed crystals of potash alum and
thallium alum with variation in composition.
In these two instances the component crystals are miscible in all
proportions; but this is by no means always the case. It may happen
that the crystals do not form double salts, and are only miscible in
certain proportions. Two cases then arise: (1) the properties may be
expressed as linear functions of the composition, the terminal values
being identical with those obtained for the individual components, and
there being a break in the curve corresponding to the absence of mixed
crystals; or (2) similar to (1) except that different values must be
assigned to the terminal values in order to preserve collinearity.
Fig. 9 illustrates the first case: the ordinates represent specific
volumes, and the abscissae denote the composition of isomorphous
mixtures of ammonium and potassium dihydrogen phosphates, which
mutually take one another up to the extent of 20% to form homogeneous
crystals. The second case is illustrated in fig. 10. Magnesium
sulphate (orthorhombic) takes up ferrous sulphate (monoclinic) to the
extent of 19%, forming isomorphous orthorhombic crystals; ferrous
sulphate, on the other hand, takes up magnesium sulphate to the extent
of 54% to form monoclinic crystals. By plotting the specific volumes
of these mixed crystals as ordinates, it is found that they fall on
two lines, the upper corresponding to the orthorhombic crystals, the
lower to the monoclinic. From this we may conclude that these salts
are isodimorphous: the upper line represents isomorphous crystals of
stable orthorhombic magnesium sulphate and unstable orthorhombic
ferrous sulphate, the lower line isomorphous crystals of stable
monoclinic ferrous sulphate and unstable monoclinic magnesium
sulphate.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
An important distinction separates true mixed crystals and
crystallized double salts, for in the latter the properties are not
linear functions of the properties of the components; generally there
is a contraction in volume, while the refractive indices and other
physical properties do not, in general, obey the additive law.
Isomorphism is most clearly discerned between elements of analogous
chemical properties; and from the wide generality of such observations
attempts have been made to form a classification of elements based on
isomorphous replacements. The following table shows where isomorphism
may be generally expected. The elements are arranged in eleven series,
and the series are subdivided (as indicated by semicolons) into groups;
these groups exhibit partial isomorphism with the other groups of the
same series (see W. Nernst, _Theoretical Chemistry_).
Series 1. Cl, Br, I, F; Mn (in permanganates).
2. S, Se; Te (in tellurides); Cr, Mn, Te (in the acids
H2RO4); As, Sb (in the glances MR2).
3. As, Sb, Bi; Te (as an element); P, Vd (in salts); N,
P (in organic bases).
4. K, Na, Cs, Rb, Li; Tl, Ag.
5. Ca, Ba, Sr, Pb; Fe, Zn, Mn, Mg; Ni, Co, Cu; Ce, La,
Di, Er, Y, Ca; Cu, Hg, Pb; Cd, Be, In, Zn; Tl, Pb.
6. Al, Fe, Cr, Mn; Ce, U (in sesquioxides).
7. Cu, Ag (when monovalent); Au.
8. Pt, Ir, Pd, Rh, Ru, Os; Au, Fe, Ni; Sn, Te.
9. C, Si, Ti, Zr, Th, Sn; Fe, Ti.
10. Ta, Cb (Nb).
11. Mo, W, Cr.
For a detailed comparison of the isomorphous relations of the elements
the reader is referred to P. von Groth, _Chemical Crystallography_.
Reference may also be made to Ida Freund, _The Study of Chemical
Composition_; and to the _Annual Reports of the Chemical Society_ for
1908, p. 258.
BIBLIOGRAPHY.--_History_: F. Hoefer, _Histoire de la chimie_ (2nd ed.,
1866-1869); Hermann Kopp, _Geschichte der Chemie_ (1869),
_Entwickelung der Chemie in d. neueren Zeit_ (1871-1874); E. von
Meyer, _Geschichte der Chemie_ (3rd ed., 1905, Eng. trans.); A.
Ladenburg, _Entwickelungsgeschichte der Chemie_ (4th ed., 1907); A.
Stange, _Die Zeitalter der Chemie_ (1908). Reference may also be made
to M.M. Pattison Muir, _History of Chemical Theories and Laws_ (1907);
Ida Freund, _Study of Chemical Composition_ (1904); T.E. Thorpe,
_Essays in Historical Chemistry_ (2nd ed., 1902). See also the article
ALCHEMY.
_Principles and Physical._--W. Ostwald, _Principles of Inorganic
Chemistry_ (3rd Eng. ed., 1908), _Outlines of General Chemistry_,
_Lehrbuch der allgemeinen Chemie_; W. Nernst, _Theoretische Chemie_
(4th ed., 1907, Eng. trans.); J.H. van't Hoff, _Lectures on
Theoretical and Physical Chemistry_; J. Walker, _Introduction to
Physical Chemistry_ (4th ed., 1907); H.C. Jones, _Outlines of Physical
Chemistry_ (1903); D. Mendeleeff, _Principles of Chemistry_ (3rd ed.,
1905).
_Inorganic._--Roscoe and Schorlemmer, _Inorganic Chemistry_ (3rd ed.,
Non-metals, 1905; Metals, 1907); R. Abegg, _Handbuch der anorganischen
Chemie_; Gmelin-Kraut, _Handbuch der anorganischen Chemie_; O. Dammer,
_Handbuch der anorganischen Chemie_; H. Moissan, _Chimie minerale_.
_Organic._--F. Beilstein, _Handbuch der organischen Chemie_; M.M.
Richter, _Lexikon der Kohlenstoffverbindungen_ (these are primarily
works of reference); V. Meyer and P.H. Jacobson, _Lehrbuch der
organischen Chemie_; Richter-Anschutz, _Organische Chemie_ (11th ed.,
vol. i., 1909, Eng. trans.); G.K. Schmidt, _Kurzes Lehrbuch der
organischen Chemie_; A. Bernthsen, _Organische Chemie_ (Eng. trans.).
Practical methods are treated in Lassar-Cohn, _Arbeitsmethoden fur
organisch-chemische Laboratorien_ (4th ed., 1906-1907). Select
chapters are treated in A. Lachmann, _Spirit of Organic Chemistry_;
J.B. Cohen, _Organic Chemistry_ (1908); A.W. Stewart, _Recent Advances
in Organic Chemistry_ (1908); and in a series of pamphlets issued
since 1896 with the title _Sammlung chemischer und
chemisch-technischer Vortrage._
_Analytical._--For Blowpipe Analysis: C.F. Plattner, _Probirkunst mit
dem Lothrohr_. For General Analysis: C.R. Fresenius, _Qualitative and
Quantitative Analysis_, Eng. trans, by C.E. Groves (_Qualitative_,
1887) and A.I. Cohn (_Quantitative_, 1903); F.P. Treadwell, _Kurzes
Lehrbuch der analytischen Chemie_ (1905); F. Julian, _Textbook of
Quantitative Chemical Analysis_ (1904); A. Classen, _Ausgewahlte
Methoden der analytischen Chemie_ (1901-1903); W. Crookes, _Select
Methods in Chemical Analysis_ (1894). Volumetric Analysis: F. Sutton,
_Systematic Handbook of Volumetric Analysis_ (1904); F. Mohr,
_Lehrbuch der chemisch-analytischen Titrirmethode_ (1896). Organic
Analysis: Hans Meyer, _Analyse und Konstitutionsermittlung organischer
Verbindungen_ (1909); Wilhelm Vaubel, _Die physikalischen und
chemischen Methoden der quantitativen Bestimmung organischer
Verbindungen_. For the historical development of the proximate
analysis of organic compounds see M.E.H. Dennstedt, _Die Entwickelung
der organischen Elementaranalyse_ (1899).
_Encyclopaedias._--The early dictionaries of Muspratt and Watts are
out of date; there is a later edition of the latter by H.F. Morley and
M.M.P. Muir. A. Ladenburg, _Handworterbuch der Chemie_, A. Wurtz,
_Dictionnaire de chimie_, and F. Selmi, _Enciclopedia di chimica_, are
more valuable; the latter two are kept up to date by annual
supplements. (C. E.*)