especially as defined by Helmholtz. 25 



of the element in its gasefiable hydride*. In cases where 

 such hydrides are unknown, the determination of valency is 

 very difficult ; it can be but provisionally effected, and only 

 by most carefully weighing all the evidence relating to the 

 constitution of the compounds available for discussion. 



But if it be granted, for example, that nitrogen is a triad, 

 and that iodine is a monad, How are we to explain the fact that 

 the methyl compounds of these two elements unite to form 

 so well characterized a molecular compound as tetramethyl- 

 ammonium iodide ? How are such molecular compounds con- 

 stituted ? My own view has long been that the nitrogen and 

 iodine in such a case are both possessed of a certain amount 

 of residual affinity; and I would define a molecular compound 

 as one formed by the coalescence of two or more molecules, 

 unattended by redistribution of the constituent radicals, and 

 in which the integrant molecules are united by residual affi- 

 nities. In other words, the unit charge must be capable in 

 certain cases of directly promoting the association, not merely 

 of two, but of at least three, atoms. To put this hypothesis 

 into terms which cannot be misunderstood, let unit valency 

 or charge be represented by a unit line, and further be it 

 supposed that the charge penetrates the atom, then the atom 

 with its unit charge may be represented thus : — 



f /CN f 



i. e., the unit charge may be held to consist of three portions, 

 the buried portion a, and the free portion /+/'. The facts, 

 as they present themselves to me, also appear to necessitate 

 the assumption that, in the case of different elements, the 

 charge penetrates the atom ; and in the case of some polyad 

 atoms, different directions in the atom with varying degrees 

 of freedomf. The union of two atoms may then be pictured 

 as an overlapping of the unit lines. If the atoms are freely 

 penetrated by their charges, each atom may tend to move 

 out to the end of the line, leaving either no portion, or but a 

 very small portion, free ; a conception of this order would 



* If this be granted, it follows that the maximum number of cbarges 

 which an atom can carry is four ; in other words, that the possible 

 maximum valency is attained in tbe case of carbon. 



t This is practically but a modification of Helmholtz's statement that 

 "the phenomena are the same as if equivalents of positive and negative 

 electricity were attracted by different atoms, and perhaps also by the 

 different values of affinity belonging- to the same atom, with different 

 force." 



