104: H. L. Wells — Color-Effect of Isomorphous Mixture. 



tive examination of the red product showed nothing unusual 

 in it except the presence of much lead. Then it was found 

 that it could be readily prepared by dissolving caesium, lead 

 and tellurium salts in hot aqua regia and cooling or evaporat- 

 ing to crystallization. It should be explained that the pres- 

 ence of the lead tetrachloride compound in the mother-liquor 

 from the fractional crystallization was due to the presence in 

 the hydrochloric acid solution of a considerable amount of 

 nitric acid which had been used in dissolving the crude tellu- 

 rium. The lead was an impurity in the latter. 



The separate salts Cs 2 PbCl 6 and Cs 2 TeCl 6 were prepared 

 repeatedly from aqua regia solution, but they gave invariably 

 pure yellow products. Under precisely the same conditions 

 when both tellurium and lead were present the products were 

 always red, and there was no very marked variation in this 

 red color when the proportions of lead and tellurium were 

 changed considerably. The red products formed octahedral 

 crystals like the yellow salts, and they were of similar size. 



It was suspected that the red substance might be a triple 

 salt of caesium, tellurium and lead, but analyses of several 

 crops showed that there was no constant relation between the 

 lead and tellurium, and that recrystallization from aqua regia 

 changed the composition of a product very much by increas- 

 ing the proportion of the lead compound. Therefore it must 

 be concluded that the products were isomorphous mixtures. 



The following analyses were made of separate crops of the 

 red mixture, where Y was obtained by a single recrvstalliza- 

 tion of IY : 



I II in IV v 



Cs 2 PbCl 6 41-2 55-3 47'7 57'0 88'4 



Cs 2 TeCl 6 57-6 44-3 51*3 43*5 11*4 



It is to be noticed that when the two yellow salts in separate, 

 very small crystals are mixed, either dry or under hydrochloric 

 acid, there is no development of any red color, so that it 

 appears that light in passing from one kind of crystal to the 

 other kind gives no unusual effect. Hence it is evident that 

 the effect under consideration is due to the crystallization of the 

 two things together. 



Sheffield Laboratory, New Haven, Conn., 

 December, 1911. 



