Section III., 1904 [ 43 ] Tras. R. S. C. 



VII. — The Variation of the Valency of Elements with Temperature. 

 By E. H. Archibald and D. McIntosh. 



(Presented by Prof. B. J. Harrington, and read June 22, 1904). 



Of the many points in chemical theory ovei- which controversy lias 

 raged, none, perhaps, has been the centre of a greater struggle than 

 the question of changing valency — a question which has been definitely 

 settled only in the last few years. For a long time chemists ascribed 

 a definite valency to eac<h element, and where compounds were kli(^^\■n 

 which did not easily conform to this view, they were looked upon as 

 " unsaturated." Lately, with the discovery of many new compounds, 

 tJiis theory of the unsaturated element has been put farther and farther 

 in the background, so that now nearly all chemists are agreed that 

 valency is not a fixed property, but depends on certain conditions, of 

 which temperature is probably one of the most important. 



A great deal has been written concerning the variation of valeney 

 ^vith the atomic weight, as illustrated by the periodic arrangement of 

 the elements, according to Mendeléef, Meyer and others. For the 

 metallic elements this variation is in most cases regular, and the metals 

 of any one group show the same valency towards a groat many dill'ertnt 

 elements. In the case of the non-metallic elements, this regularity is 

 not so apparent. Sulphur, for instance, forms three compounds, HjS, 

 SO2 and SO3, in, which the sulphur functions as a dyad, a tetrad, and 

 a hexad, respectively. It is well to remember, as Ostwald points out, 

 that the compounds of these elements from their very nature do not 

 lend themselves to the same careful study as those of the metals. One 

 point seems strongly brought out from a study of the periodic table, 

 i.e., that the valency of an element is determined, to a very great ex- 

 tent, by the nature of the body with which it is united ; and so we have 

 comparatively few cases where two elements unite in various propor- 

 tions, and in these only rarely, as in the case of the chlorides of phos- 

 phorus, is the valency change referred to but one of the elements. 



The effect of heat on the pentachloride of phosphorus is well 

 known. It is broken up into phosphorus trichloride and chlorine; 

 that is, the phosphorus is changed from a pentad to a triad. Many 

 such cases might be cited in which heat has the effect of lowering the 

 valency. The decomposition of ammonium chloride, of ammonium 

 sulphide, of nitrogen peroxide, and the change, with tcniperaturo, in 

 the molecular weights of phosphorus, bromine and iodine, will occur 



