CHEMISTRY
746
atom suffice, triphenylmethane, CHPh3, being colourless, although abnormally refractive. It is also noteworthy that benzil, Ph.CO.CO.Ph, if coloured at all, is at most a very pale yellow substance; if not a consequence of its symmetrical structure, this may be due to the fact that the compound is one in which there is but little residual affinity. The colour of substances such as azobenzene, Ph.N : JSf.Ph, may be ascribed to the co-operation of the two phenyl systems with the diazo N : N system. Quinone and its homologues, and indeed the simple quinones generally, are at most either yellow, or orange, or orange-red in colour; as a rule para-quinones intensifies- are yellow, and the unsymmetrical orthocoiour
quinones orange to red. The introduction of oxygen (as OH) or halogens usually has the effect of deepening the colour—a result, it may be supposed, of the introduction of a further slight co-operative effect. Colours other than yellows and reds—blue shades of red, greens and blues—are met with only in mixed auinonoids, i. e., compounds in which the two C : II groups are dissimilar, one at least being a more or less complex group. The colours of the rosaniline group, for example, are all derivatives of the complex R" : C6H4 : CPh2, R" being usually either NH or NR', and either NH2 or a substituted NH2 group being present in each of the Ph groups. The colour is much intensified by the association of several quinonoid or pseudo-quinonoid systems. Thus the simple diazo - dyes, formed from the diazo - benzenes and either phenol or aniline, or their homologues, are all derivatives of azobenzene, C6H5.N :N.C6H5, and resemble this compound in being yellows. The derivatives of azonaphthalene, C10H7.N :KC10H7, are much deeper in colour, many pronounced reds being met with amongst them j the intensification of the colour is doubtless a consequence of the greater influence exerted by the naphthyl group. By diazotizing the amido-derivative of a diazo-dye, and then coupling the diazo - compound with amines or phenols, disazo-compounds are produced in which the azobenzene or azonaphthalene complex is contained twice, fetill more complex substances are easily prepared by repeating such a series of operations. Many of these polyazo-dye stuffs absorb light so completely that they are almost black. The hydroxy-derivatives of anthraquinone afford another striking example of the influence exercised by accumulated effects. The lower derivatives are orange-red or red, the higher greens and blues ; it is impossible to account for the difference on the assumption that it is due to the mere accumulation of hydroxyl groups in the one molecule, but it is easily understood if it be granted that anthraquinone itself is not the basis of the colour to which it gives rise, but an isodynamic compound. Thus the formation of alizarin may be assumed to involve the isodynamic change OH O O OH ///H
O Dihydroxyanthraquinone.
/// o Alizarin.
The corresponding heteronucleal tetrhydroxy anthraquinone may be supposed to undergo change, thus :— OH O ///oh HO
/// O OH OH O In the one compound a quinonoid system is developed on only one, but in the other on both sides of the complex.
If the argument here adduced be accepted as valid, there is no difficulty in understanding why a limited meaning must be attached to the conception of atomic constants. It would seem that, except perhaps in the paraffin series, we are called upon to study the behaviour of more or less elastic systems—not of atoms which can freely exert a constant individual influence. In such systems the affinities are more or less deflected and controlled and subject to change according to the influence which is brought to bear upon them, and therefore they exercise a varying effect. Still more important, probably, is the variation due to the degree of influence which the systems exercise by acting in co-operation. A similar doctrine may be found to apply to metallic salts and to inorganic metallic compounds generally. That a profound difference exists between metals and non-metals there can be no doubt, and that the latter tend to form complexes comparable in sa/ts> <£C. some cases with the ethenoid and centric hydrocarbon systems is also in the highest degree probable. But the investigation of metallic salts is complicated and rendered much more difficult than that of carbon compounds by the tendency they display to form complex molecules, and also by the readiness with which they enter into combination with water. The conclusion that coloured carbon compounds are quinonoids, using this term in a broad sense, if extended to elements, would involve the conclusion that the structure of these is in many cases comparable with that of quinonoids.1 The increase in the intensity of the colour as atomic weight increases in the case of the halogens; the intense blue colour of ozone in comparison with that of oxygen • and especially the intense colour and extraordinarily complicated absorption spectrum of nitrogen dioxide, are of interest from this point of view. In all these cases intensity of colour is associated with a high degree of chemical activity, and the conclusion is irresistible that the existence of residual affinity determines the appearance of colour. In any case it should be worth while to take into account the apparent simplicity of the mechanism by which colour is produced in carbon compounds, and to consider whether it may not be sufficient even to account for the complicated absorption spectra which characterize many compounds : it may well be that these have their origin in the superposition of effects produced at a very limited number of absorbing centres, and that colour is the outcome of intramolecular interferences. In this article the attempt has been made to pass briefly under notice what appear to the writer to be the more important problems in chemistry which have been _ the subject of discussion during the period 1875- c/„s/on. 1900, and besides indicating the directions in which progress has been made, to give some idea of the problems which press for solution. The end the chemist has in view is to gain clear conceptions of the nature of the primary materials, i.e., the elements, with which he has to deal, and an understanding of the operations into which they enter. He is called upon, however, to study their behaviour in crowds and companies in circumstances of great difficulty, in order that he may form an opinion of their behaviour as individuals and depict their charactere. That so much success has already attended his labours is proof that the methods in use are essentially sound, but the opportunities awaiting future inquirers are infinite. (h. e. a.) 1 It is a question of interest whether the blue colour of oxygen he not due to the presence of complexes (02)x. Abney’s observations show that solutions of colourless salts in water are less absorptive m the infra-red region than in water per se; this may be regarded as proof that the colour of water is at least in part conditioned by tbe presence of molecular complexes.
