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The view that has met with almost general acceptance is that put forward by Frankland, according to which, whilst each element exhibits a maximum valency, it may manifest one or more subordinate valenCy. valencies,, the affinities in abeyance in cases in which only the lower valency is manifest mutually satisfying each other. By a logical extension of this view elements have been divided into those of odd and those of even valency, and it has been held that all elements belong either to one or other of these classes; it will be shown later, however, that this is not justified in practice. Kekule, whose services in this field rank with those of Frankland, maintained that valency could not vary. He considered that it was necessary always to make use of monad elements in determining valency, and moreover that the only compounds on which valid arguments could be based were those which could be volatilized without undergoing decomposition. In discussing this question Professor Japp goes so far as to say : “ Of all the doctrines which we owe to Kekule, that of fixed valency is probably the one that has met with least acceptance even among chemists of his own school. At the present day it is, so far as I am aware, without supporters.” But he adds, ayet Kekule held it to the last.” Such a fact cannot be overlooked. That he went too far in asserting that valency could not vary is probably true, but the essential feature in his objection —that in many cases valency was over-estimated by the Frankland school — cannot be so easily disposed of. Kekule, it is to be remembered, although not the originator of our system of formulae expressive of structure, may yet fairly be said to have done so much to promote and extend their use that he is to be regarded as practically the father of modern structural chemistry. He saw clearly that structure is the determining factor to be taken into account in all such discussions, and hence it was that he would only allow moraad elements to be used in determinations of valency. In objecting to the use of compounds such as ammonium chloride and phosphorus pentachloride as criteria of valency, Kekule took a further step, the importance of which still remains unappreciated, The conception of valency, introduced into chemistry by dividing compounds into two classes—atomic comby Frankland in 1852, may be said to be the dominant pounds and molecidar compounds. In the former the principle on which our entire system of rational or components are held together by atomic affinities or tine constitutional formulae is founded, and therefore the valencies; the latter are composed of atomic compounds subject is one of the utmost importance. It may be held together by “molecular affinities.” Views such defined as the doctrine of the functional activity of the as these have not proved acceptable, probably because elements, and includes the consideration not merely of the chemists have been unwilling to base any distinction extent to which, but also of the manner and mode in on stability, having in mind the great differences in which, an element or radicle enters into combination with stability met with among compounds, and also because others. The valency of an element is usually defined as they have been unwilling to believe in the existence merely its atom-fixing power relatively to that of of different orders of affinity. The discovery by Thorpe hydrogen—elements being classed as 'iin%-.) bt-y ter-, (juadn- in 1875 of a highly stable phosphorus pentafluoride is valent, or as monads, dyads, triads, tetrads, according as undoubtedly supposed to have placed Kekule’s objection their atoms unite with one, two, three, four atoms ^ of out of court. hydrogen or their equivalent. In this form the conception The first difficulty that arises in determining the valency is not only a narrow one, but a totally inadequate expres- of an element is in the selection of a measure. In the sion of the facts. Proof is not wanting that this is case of elements capable of forming volatile gradually being realized, and it may be hoped that ere hydrides an unexceptionable standard is at hand ^ ^fter- * long the complex character of the function will be more in hydrogen, but few come under this categoi}. mjnation. clearly recognized. The first step will then have been Hydrocarbon radicles such as methyl (CH3) taken towards a truer appreciation of the phenomena. are so entirely comparable with hydrogen that they foim An admirable although^ brief summary of the historical an equally efficient standard, and as many elements form development of the conception of valency is to be found methides which do not furnish volatile hydrides, some in the lecture delivered by Professor Japp in memory of assistance may be obtained by calling such radicles into Kekule {Trans. Chem. Soc. 1898, p. 97). requisition. But in cases to which these criteria are 1 A valuable summary of our knowledge of tbe metals, from the inapplicable the difficulty becomes very great; indeed, it points of view referred to in this paragraph, by Mr. F. H. Neville, is to is doubtful if a trustworthy standard can then be found. be found in the Report of the British Association for 1900. _ A table A difficulty of another order is met with in determining of “ intermetallic ” compounds is given in this Report which is of whether the atom-fixing power of an element is to be interest as bearing on intermetallic valency.

difficult to account for the extraordinary behaviour of iron in any other way.1 Numerous experiments have been made by Victor Meyer and others to determine, with the aid of his apparatus, the molecular weight of metallic chlorides at high tunPand' temperatures, such knowledge being of great molecular importance in discussing the valency of metallic compost- elements. The results as a whole favour the t,on ‘ view that in many cases the chlorides have the simplest possible composition at high temperatures, whilst they undoubtedly form more complex molecules at temperatures near to their boiling-point j stannous chloride, for example, is represented by the formula SnCl2 at high, but by Sn2Cl4 at low temperatures. The case of aluminium is of special interest. This element occurs below boron in the periodic system and therefore should behave as a triad, but, as shown by Deville many years ago, the vapour of aluminium chloride not far above its boiling-point would seem to consist of molecules of the formula A12C16 (compare Crafts, Trans. Chem. Soc. 1900, p. 993). Determinations made at much higher temperatures, however, give lower values, and the conclusion that aluminium is really a triad metal is apparently confirmed by the observations first made by Odling and Buckton in 1865, and supported by later experiments (cf. Quincke, Zeits. phys. Chem. 1889, p. 164), that aluminium methide, even at a temperature only 10 above its boiling-point, has a vapour density 20 per cent, lower than the value corresponding to the formula A10(CH3)6, and that as the temperature is raised the density approaches to that required by the formula A1(CH.5)3. But such inquiries need to be carried out in a much more systematic and exact manner than heretofore, especially now that a new field has been opened up by H. B. Baker’s recent important observations, which show that ammonium and mercurous chlorides may be volatilized unchanged if only the presence of moisture be most scrupulously avoided ; all previous observers had found that both undergo decomposition when volatilized {Trans. Chem. Soc. 1900, p. 646). Valency.