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CHEMISTRY acid two molecules of hydrogen fluoride are associated with the molecule of silicon tetrafluoride through the agency of fluorine atoms; and it is worth while noticing that the stability of silicon tetrafluoride in presence of water is increased in a remarkable manner through its association with hydrogen fluoride. In view of these considerations, it is impossible to accept phosphorus pentafluoride as proof that phosphorus is a pentad. The still more remarkable compound sulphur hexafluoride, a gas which is not acted upon even by potash, recently discovered by Moissan (C. R. 1900, 130, p. 865), in like manner cannot be regarded as proof that sulphur is a hexad. It may almost be said that its extraordinary inertness is only compatible with the view that the sulphur and fluorine atoms form a closed chain or ring. Sulphur is commonly supposed to be capable of acting not only as a dyad, as in sulphuretted hydrogen, but also as a tetrad on the evidence afforded by the sulValeacy of compounds, and as a hexad in consequence of the existence of the oxide S03. The sulphonium compounds are the sulphur analogues of the quaternary ammonium compounds, and according as the latter are regarded as atomic or molecular compounds, so must the former be equally so regarded. Most important observations bearing on this problem have been recently made by Pope and Peachey (Trans. Chem. Soc. 1900, p. 1072), who have shown that it is possible to obtain the sulphonium compound which is formed on combining methylethyl sulphide with bromacetic acid in isomeric optically-active forms—a discovery which appears to prove that sulphur can function as a tetrad much as carbon does. Inasmuch as we are at present unable to determine the structure of sulphur trioxide, its existence cannot be accepted as proof that sulphur functions as a hexad. As bearing on this subject, the following striking passage may be quoted from Mendeleeff’s Faraday lecture (p. 653):— The periodic law has demonstrated that the maximum extent to which different non-metals enter into combination with oxygen is determined by the extent to which they combine with hydrogen, and that the sum of the number of equivalents of both must be equal to 8. Thus chlorine which combines with 1 atom, or 1 equivalent of hydrogen, cannot fix more than 7 equivalents of oxygen, giving C1207 ; while sulphur, which fixes 2 equivalents of hydrogen, cannot combine with more than 6 equivalents, or 3 atoms of oxygen. It thus becomes evident that we cannot recognize as a fundamental property of the elements the atomic valencies deduced from their hydrides ; and that we must modify, to a certain extent, the theory of atomicity if we desire to raise it to the dignity of a general principle capable of affording an insight into the constitution of all compound molecules. In other words, it is only to carbon, which is quadrivalent with regard both to oxygen and hydrogen, that we can apply the theory of constant valency and of bonds, by means of which so many still endeavour to explain the structure of compound molecules. Mendeleeff in these remarks altogether leaves out of account the possibility that the oxygen atoms in the oxides are not always separately attached, and apparently he does so advisedly; yet the properties of ruthenium and osmium tetroxide and of the perchlorates, for example, almost necessitate such an admission. As ‘an empirical generalization, the limit he fixes to oxidation is of remarkable interest and importance, and it is noteworthy, bearing in mind the resemblance fluorine bears to oxygen, that the sum of the equivalents in the highest fluoride is the same as in the highest oxide of sulphur. May it be that the same “ law ” applies to the oxides and fluorides of nonmetallic elements ? It remains to point out that an argument of great weight in favour of the view that iodine functions as a triad is supplied by Victor Meyer’s discovery in 1889 of the iodonium compounds, which apparently bear the same relation to iodine that the sulphonium compounds bear to sulphur,

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and the ammonium compounds to nitrogen. Diphenyl. J . . fC6H5 . lodomum hydroxide, Ij C«H5. is readily soluble in water, forming a strongly alkaline solution. On adding potassium iodide to a solution of the base, the corresponding iodide is thrown down as a yellow precipitate closely resembling lead iodide. When heated, this iodide is very readily converted into iodobenzene, heat being developed. It is very difficult, indeed impossible, to apply the molecular compound hypothesis to such substances. From the above sketch, in which only a few of the main points are touched on, it will be obvious that the problems to be solved are many and difficult. At present we have little but shadowy impressions to guide us; certain dogmas are taught in the schools, and these influence our judgment, if indeed we can be said to exercise any judgment in the matter. It is time the facts were more carefully correlated, and that systematic inquiries were instituted, as the determination of the valency of elements may be regarded as in a measure a determination of their structure. Fortunately a new beginning has been made in this direction by the recent extension of stereo-chemical conceptions to elements other than carbon; indeed, the facts brought to light are already sufficient to carry the problem a stage farther. As sulphur is found to yield optically-active sulphonium derivatives, and therefore to resemble carbon, it must be supposed that this element can act as a tetrad ; and on similar grounds the nitrogen in the ammonium compounds must be allowed to rank as a pentad. But seeing that the formation both of ammonium and of sulphonium compounds takes place only within very narrow limits, and that, as previously pointed out, nitrogen is basic only under special conditions, it is clear that in the case of the four affinities of the sulphur and of the five of the nitrogen atom, two of the former and three of the latter differ greatly from the remainder, and it would seem both in magnitude and character. A similar argument applies to other elements the valency of which apparently varies. If such a conclusion be warranted, the problem after all remains very much what it was when the question was raised by Kekule. In recent years the attempt has been made—largely on the authority of Helmholtz (Faraday lecture, Trans. Chem. Soc. 1881, p. 277)—-to give a more precise form to the conception of valency, by regarding it from an neImholtz electrical standpoint—by assuming that definite ° va ency' indivisible charges of electricity, positive or negative, are associated with the atoms of matter, the number of charges corresponding with the number of units of affinity manifested by the atom, i.e., its valency. These charges are supposed to condition the formation of compounds, the atoms clinging to the charges, and the charges of opposite sign clinging to each other. This hypothesis is the basis of the ionic dissociation hypothesis, to be discussed later, which has become so popular in recent years. But the new view is but a paraphrase of the old view. If the nitrogen atom, for example, carry five unit charges, it is difficult to understand why two of them should so often remain in abeyance; on the other hand, if only three charges are associated with the atom, it becomes necessary to admit that the unit charge may be divided—a supposition which is contrary to the hypothesis. The hypothesis does not take into account the fact that the fundamental molecules, even of so-called atomic compounds—water molecules, for example,—rarely, if ever, behave as saturated, but more or less readily and firmly unite with other molecules to form molecular aggregates. Helmholtz, seeing the difficulty, said that he did not suppose that the existence of other molecular forces, working directly from atom to atom,