H. L. Wells — Color-Effect of Isomorphous Mixture. 103 



Art. XIII. — On a Color- Effect of Isomorphous Mixture ; by 

 Horace L. Wells. 



It appears to be a general rale that crystals composed of iso- 

 morphous substances, in case either or each of these is colored, 

 assume a color intermediate between the colors of the com- 

 ponents, and so far as the writer is aware, no unexpected 

 colors have been observed in such mixtures. For instance, it 

 was found in this laboratory several years ago* that the yellow 

 salt Cs 2 PbCl 6 and the deep blue salt Cs 2 SbCl 6 gave green mix- 

 tures, as would be expected. 



I have recently observed a curious color-effect when the 

 Cs 2 PbCl 6 ,f which was mentioned above, crystallizes with 

 Cs 2 TeCl 6 4 Both of these salts form bright yellow crys- 

 talline precipitates. The lead compound has the color of sul- 

 phur, while the tellurium compound has a slightly brighter 

 tint, but their colors are so nearly alike that they can hardly be 

 distinguished without direct comparison. Both of the salts 

 crystallize in isometric octahedra, like K 2 PtCl 6 and many other 

 compounds of the same type. 



These two yellow salts of lead and tellurium are sparingly 

 soluble in hydrochloric acid, and, therefore, are easily prepared. 

 When the conditions are such that both are deposited at the 

 same time from a solution, the product always has a bright 

 orange-red color, which is the peculiarity of the isomorphous 

 mixture. 



This bright red mixture was first observed in connection 

 with the fractional crystallization of about 600 g. of the salt 

 Cs 2 TeCl 6 . The object of this operation was to find if in this 

 way any separation of tellurium into different elements could 

 be effected, and it is sufficient to say that the results of a very 

 extensive fractionation were entirely negative in regard to any 

 such separation. 



The salt used for this systematic crystallization was prepared 

 from crude tellurium, as it was considered best to purify care- 

 fully the tellurium of the end-products, rather than the whole 

 of it. When the fractionation had been carried on to a con- 

 siderable extent, it was observed that the products at the solu- 

 ble end showed a bright red color, and since this substance was 

 not recognized as any known caesium double chloride, it 

 attracted attention as a possible indication of the presence of 

 the much-sought impurity in tellurium. However, a qualita- 



* Wells and Metzger, Amer. Chem. Jour., xxvi, 268, 1901. 

 f H. L. Wells, this Journal (3), xlvi, 180, 1893. 

 JH. L. Wheeler, ibid., xlv, 267, 1893. 



