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Originally appearing in Volume V06, Page 76 of the 1911 Encyclopedia Britannica.
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C5H4 Co o 'Cs Ha COONa (I) (a) Baeyer (Ber., 1905, 38, p. 569) and Silberrad (Journ. Chem. Soc., 1906, 89, p. 1787) have disputed the correctness of this explanation, and the latter has prepared melliteins and pyromelliteins, which are highly-coloured compounds produced from mellitic and pyromellitic acids, and which cannot be, formulated as quinones. Baeyer has suggested that the nine carbon atom system of xanthone may act as a chromophore. An alternative view, due to Green, is that the oxygen atom of the xanthone ring is tetravalent, a supposition which permits the formulation of these substances as ortho-quinonoids. The theories of colour have also been investigated by Hantzsch, who first considered the nitro-phenols. On the chromophoreauxochrome theory (the nitro group being the chromophore, and the hydroxyl the auxochrome) it is necessary In order to explain the high colour of the metallic salts and the colourless alkyl and aryl derivatives to assume that the auxochromic action of the hydroxyl group is only brought strongly into evidence by salt formation. Armstrong, on the other hand, assumed an intermolecular change, thus: /\OH =0 INO2 \/ =NO2Na. The proof of this was left for Hantzsch, who traced a connexion with the nitrolic acids of V. Meyer, which are formed when nitrous acid acts on primary aliphatic nitro compounds. Meyer formulated these compounds as nitroximes or nitro-isnitroso derivatives, viz. R•C(NO2)(NOH). Hantzsch explains the transformation of the colourless acid into red salts, which on standing yield more stable, colourless salts, by the following scheme: R.C\/O 0%N \ONa Colourless, stable. Coloured, labile. Colourless, stable. 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, 36o, p. 128), from a very complete discussion of triphenyl- methane derivatives, concluded that the grouping 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 b; R•C' NO2Na 72 salt-formation, the production of colour may be explained as a particular form of ionization (Baeyer), or by a molecular re-arrangement (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. I ; 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 eHHCOOH CG 4'ct BH•COOH 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, paraquinonoid in the case of fluorescein. H. Kauffmann (Ben., 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, Bee., 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 y = Irks, where 'y 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. Mendeleeff endeavoured to obtain a connexion between surface energy and constitution; more successful were the investigations Relation of Schiff, who found that the " molecular surface tension," to moleca- which he defined as the surface tension divided by the tar weight. 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 i886 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 yV1(where y is the surface tension, and V the molecular volume of the liquid) causes all liquids to have the same temperature [PHYSICAL 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 yS = KT, where K is a constant analogous to R, and S the surface containing one gramme-molecule, y and T being the surface tension and temperature respectively. Obviously equimolecular surfaces are given by (Mv)I, 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)3. Ramsay and Shields found from investigations of the temperature coefficient of the surface energy thatTin the equation y(Mv)3 =KT must be counted downwards from the critical temperature r less about 6°. Their surface energy equation therefore assumes the form y(Mv)l = K(r-6°). Now the value of K, y 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 y(Mnv)3=2.121(r—6°); if the calculated constant be K1, then we have also 7(Mv)3 = -6'). By division we obtain n3=2.121/Ki, or n= (2.121/K1)3, 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, N204, sulphur chloride, S2C12, silicon tetrachloride, SiC14, phosphorus chloride, PC13, phosphoryl chloride, POC13, 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 stand-point 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' (x, w), 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). 1 This was done simultaneously in 1894 by W. Muthmann and A. E. H. Tutton, the latter receiving the idea from F. Becke (see Journ. Chem. Soc., 1896, 69, p. 507; 1905, 87, p. 1183). CH O 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°) 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 mono-clinic and liquid sulphur, is thermodynamically calculable; the point B is found to correspond to 131 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 0 curve BD; along this line liquid and rhombic sulphur are in equilibrium, which means that at above 131 ° and 400 atmospheres the rhombic (and not the monoclinic) variety would separate from liquid sulphur. 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° (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° and 71°, and are attended by a decrease in density. Tetramorphism is exhibited by ammonium nitrate. According to O. Lehmann it melts at 168° (or at a slightly lower temperature in its water of crystallization) and on cooling forms optically isotropic crystals; at 125.6° 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°, and at 32.4° 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 in-creasing 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°, then diminishing up to 125.6°, 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 300), 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° and 13° Monoclinic sulphur, obtained by crystallizing fused sulphur, melts A comparison of the transformation of polymorphs leads to at 119.5°, and admits of undercooling even to ordinary temperatures, a twofold classification: (I) polymorphs directly convertible but contact with a fragment of the rhombic modification spontane- in a reversible manner—termed " enantiotropic " by O. Lehmann ously brings about the transformation. From Reicher's determina- tions, the exact transition point is 95.6°; it rises with increasing and (2) polymorphs in which the transformation proceeds in oressure about o.05° for one atmosphere; the density of the rhombic one direction only—termed " monotropic." In the first class 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(C2H302)o.9H20 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°, with a distinct change in density; leucite behaves similarly at about 56o°. Again, the pyroxenes, RSiO3 (R=Fe, Mg, Mn, &c.), assume the forms (I) monoclinic, sometimes twinned so as to become pseudo-rhombic; (2) rhombic, resulting from the pseudo-rhombic structure of (I) becoming ultramicroscopic; and (3) triclinic, distinctly different from (i) and (2); (I) and (2) are polysymmetric modifications, while (3) and the pair (I) 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 poly-morphs 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°, but with great care it is possible to overheat it and even to fuse it (at 113.5°) without effecting the transformation. Mono. clinic Rhombic Vapour 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 (Zell. 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 [loo} and tool}—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 x 3.86o 5.319 6.648 6.093 3.86o 5.319 6.648 7.851 w 3.86o 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 x and ', corresponding to the a axes, are regularly increased, while the value of w, 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 factthat, 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 remark-ably 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•C1=I.3.6 and I .3.41 may be obtained. These are also pseudotetragonal; the (1.3.6) acid has nearly the same values of x and V, as benzoic acid, but w is increased ; compared with m-nitrobenzoic acid, x and tG have been diminished, whereas w is much increased; the (I.3.4) acid is more closely related to m-nitrobenzoic acid, x and being increased, w diminished. The results obtained for the (1.2) and (I.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 (I.3.5-trinitro, 2 oxybenzene); these last two substances assume rhombic forms, and picric acid differs from trinitrobenzene in having w considerably greater, with x and 'G 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 (I) 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)04, 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 cr istallographical properties, such that the rubidium salt was always Inter-mediate 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 x >G w K 25004 64.9 4.6464 4.661 4'297 73.36 4 34 4 4 5 37 Cs2SO4 83.64 4.846 4.885 5.519 K2SeO4 71.71 4.636 4.662 5^I18 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: (I) the replacement of potassium by rubidium occasions all 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 x and 11. are about equally increased, while the increase in w is always the greatest. Now consider the effect of replacing sulphur by selenium. It will be seen that (I) 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 x and >d, which are equally lengthened. These observations admit of ready explanation in the following manner. The ordinary structural formula of potassium sulphate is 0 K-O-S-O-K. If the crystal structure be regarded as composed of 0 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 potassiun atoms, the systems being soarranged that the sulphur system is always centrally situated with respect to the other two, and the potassium systern 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 w (corresponding to the vertical axis), and cause a smaller increase in the horizontal parameters (x and u') ; 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 w, x and 1' being equally, but only slightly, increased ; replacement of phosphorus by arsenic was attended by a considerable increase, equally in x and tG, while w 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 demonstrat- ing this property of isomorphism. For example, episomorphs of white potash alum and violet chrome alum, of white mag- nesium 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 de- termining the physical pro- perties; in other words, any physical constant of a io zo 30 ao ao eo Zo eo so 100 mixed crystal can be cal- K2SOa=z00% K2SO4=o% (NHa)2SOa=o% (xH > so-zoa/ culated as additively corn- ,,, posed of the constants of Fig. 7 represents the specific volumes of mixtures of ammonium and potassium sulphates; the ordinates re-presenting specific volumes, and the abscissae the percentage composition of the mixture. Fig. 8 shows the K Alum=o% variation of refractive index Tl Alum-s00% of mixed crystals of potash FIG. 8. 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 (i) 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. io. Magnesium sulphate (orthorhombic) takes up ferroussulphate (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 isomor- phous crystals of stable monoclinic ferrous sulphate and unstable monoclinic magnesium sul hate. 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). As, Sb, Bi; Te (as an element) ; P, Vd (in salts) ; N, P (in organic bases). K, Na, Cs, Rb, Li; Tl, Ag. Ca, Ba, Sr, Pb; Fe, Zn, Mn, Mg; Ni, Co, Cu; Ce, La, Di, Er, Y, Ca; Cu, Hg, Pb; Cd, Be, In, Zn; TI, Pb. Al, Fe, Cr, Mn; Ce, U (in sesquioxides). Cu, Ag (when monovalent) ; Au. Pt, Ir, Pd, Rh. Ru, Os; Au, Fe, Ni; Sn, Te. C, Si, Ti, Zr, Th, Sn; Fe, Ti. Ta, Cb (Nb). 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. 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 (1 ith ed., s- se 03, 1%56 •408 K Alum=z00% Tl Alum= o% ao 00 KH2PO4zoo°, NH4H2PO,vz00% 3. 4.-5. 6. 7. S. 9• To. I I. vol. i., 1909, Eng. trans.) ; G. K. Schmidt, Kurzes Lehrbuch der organischen Chernie; 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 and chemisch-technischer Vortrdge. 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 and Konstitutionsermittlung organischer Verbindungen (1909) ; Wilhelm Vaubel, Die physikalischen and chemischen Methoden der quantitativen Bestimmung organischer Verbindungen. For the historical development of the proximate analysis of organic compounds sea 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, Encidopedia di chimica, are more valuable; the latter two are kept up to date by annual supplements. (C. E.*)
End of Article: C5H4

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