360 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a similar complex ion is well known ; it has even been used by Cushman 

 under Sanger's direction as a means of separating cadmium from other 

 metals. In this case the complex ion was formed simply by adding an 

 excess of sodic chloride, which prevents cadmium from being precipitated 

 by hydrogen sulphide. Upon dilution the sulphide of cadmium hegins to 

 be precipitated, owing to the splitting apart of the ion in dilute solutions 

 according to the law of " mass " action. 



The same tendency has been used to explain the otherwise incom- 

 prehensible migration values of cadmium salts. Very recently Noyes has 

 shown that probably a similar ion, BaCl/',* exists in baric chloride so- 

 lutions ; and the migration values of concentrated calcic and magnesic 

 chloride solutions lead one to infer that in these cases yet a greater 

 concentration of CaC'l 4 " and MgCl/' may exist. 



It is interesting to note that the decomposition of the mercurous 

 halide is carried to a much greater extent under similar conditions in the 

 case of the bromide than in that of the chloride,f and yet further in 

 the case of the iodide. This may be due simply to the greater solubilities 

 of mercurous bromide and iodide, but besides this cause there may exist 

 a greater affinity of the molecule for the ion. The study of the migra- 

 tion values of cadmium salts seems to show that the iodide has a much 

 greater tendency to add to ionized iodine than the chloride has to add to 

 ionized chlorine ; and it is probable that the same relation exists in the 

 case of mercury. 



The facts recorded above show that an accurate quantitative analysis 

 of a mercurous salt by precipitation with a soluble chloride is not to be 

 expected, unless the chloride is added only in very slight excess, and 

 then the solubility of mercurous chloride itself must be considered. 

 When, however, a large excess of mercuric salt is present, as for example 

 in the recent work of Ogg,} it is obvious that the disturbing effect of the 

 side-reaction must be much hindered, according to the law of " mass " 

 action. 



It is possible that the medicinal action of calomel is due to the small 

 but definite concentration of mercuric complex salt produced by common 

 salt or hydrochloric acid in the alimentary canal. In any case, one is 

 disposed to recommend cautious medicinal use of other chlorides in con- 

 nection with calomel. 



Preliminary experiments with sulphates showed that with these salts 



* A. A. Noyes, J. Am. Chem. Soc, 23, 37-57 (1901). 



t Richards, loc. cit. 



t Ogg, Zeitschr. phys. Chem., 27, 291 (1898). 



