especially as defined by Helmholtz. 27 



haloid compounds and their analogues are also capable of 

 forming still more complex molecular aggregates with the 

 halogens, &c. : they are therefore to be regarded as possessed 

 of residual affinity ; and that polyad elements, e. g. nitrogen, 

 phosphorus and sulphur, should still exhibit residual affinity 

 in such compounds is not surprising in the light of the 

 hypothesis advocated in this note ; but it is scarcely com- 

 patible with the assumption that the halogen in the ammo- 

 nium haloid compounds serves as the bond of union. On the 

 other hand, if it be assumed, as I think it should be, that the 

 formation of double metallic chlorides, &c, is the outcome of 

 the possession of residual affinity by the halogen, the com- 

 plete analogy which appears to exist between the ammonium 

 haloid compounds and those of the alkali metals would seem 

 logically to involve the inference that the halogen of the 

 ammonium compound does serve as the bond of union. I 

 see but one mode of escape from this conflict of evidence, 

 and that is to call in question the time-honoured assumption 

 that the radical ammonium is the true analogue of potassium 

 and sodium, which, be it remarked, is of necessity subject to 

 doubt if the hypothesis that the ammonium salts are molecular 

 compounds be entertained ; and evidence which supports the 

 conclusion that the per-haloid compound is formed by the 

 addition of the halogen to the nitrogen (phosphorus or 

 sulphur) is afforded by the observation that not only haloid 

 ammonium and sulphine compounds, but also the sulphates 

 combine with halogens (Dobbin and Masson, Chem. Soc. 

 Trans. 1885, p. 56 ; 1886, p. 846). 



It is now proved by abundant experimental evidence that, 

 whatever the order in which the radicals A, B, C, D, are 

 introduced in forming a tetralkylic ammonium compound 

 N(ABCD)X, one and the same end product always results. 

 This is commonly regarded as proof, not only that nitrogen 

 is pentad, but also that the five affinities of the nitrogen atom 

 are of equal value, and it would appear to favour the conclu- 

 sion that the ammonium salts are in truth " atomic " com- 

 pounds ; but I see no reason why isomeric change should not 

 occur at the moment of formation of a molecular compound : 

 why the integrant molecules, in fact, should not interchange 

 radicals. If the statement be confirmed that the compound 

 formed from dimethyl sulphide and ethyl iodide is different 

 from that obtained on combining methylethyl sulphide and 

 methyl iodide (Kriiger, Journ. pr. Chem. 1876, xiv. p. 193), it 

 will follow, not that sulphur is a tetrad, and that the four 

 affinities are of unequal value, but that there is little or no 

 tendency for isomeric change to occur in the formation of 



