ON THE CHEMICAL NATURE OF ALLOYS. 



IN the preceding article I assumed for the sake of 

 simplicity that no chemical action takes place between 

 the metals of an alloy. The evidence on this point is very 

 imperfect, but in the majority of cases the balance of 

 probability is in favour of this view. Some metals, how- 

 ever, undeniably combine in definite proportions to form 

 chemical compounds. 



It has been already mentioned that the depression in 

 the freezing point of a solvent is proportional to the number 

 of molecular weights of foreign substance dissolved in 

 it. Moreover, the depression predicted by Van't Hoff's 

 law as due to one molecular weight, is in many cases pro- 

 duced by one atomic weight of a metal when dissolved in 

 other metals. Hence it seems probable that a metal is 

 separated into individual atoms during the process of 

 solution. It may be, however, that molecules are formed 

 containing an atom of the dissolved body associated with 

 one or more atoms of the solvent. Where the depression 

 in the freezing point is persistently one half the theoretical, 

 as in the case of aluminium dissolved in tin and of mercury 

 or bismuth in lead, it becomes, at least, probable that the 

 molecules in solution are AL_, Hg„ Bi 2 respectively. Tin 

 in lead appears to form molecules of the type Sn A . 



In these cases we do not know how much, if any, of 

 the solvent is associated with the molecules of the dissolved 

 metal. In fact the cryoscopic study of binary alloys throws 

 very little light on the question of chemical combination ; 

 but when we examine ternary alloys we get more evidence 

 on this point. 1 For, as a general rule, when two metals B 

 and C are simultaneously dissolved in a third, the total 

 depression in the freezing point is the sum of the effects 

 that B and C separately would produce. This fact indicates 

 that the atoms of B and C have not combined with each 



1 Heycock and Neville, Chem. Soc. Jour., 1891, p. 936, and 1S94, p. 

 65- 



