DOUBLE SALTS 157 



chemical study of double salts deals with the generalisations 

 concerning their formulae. The record of achievement in this 

 province is but meagre. The double halides are the class of 

 double salts which have been most extensively used in this 

 connection, the reason being that the total number known is 

 very large, and that there is considerable diversity in the 

 combining ratios of the constituents, as many as two or 

 three different combinations between the same simple chlorides 

 being known. 1 Remsen, from a tabulation of a large number of 

 the double halides of the alkalies, has generalised to the effect 

 that the number of halogen atoms due to the alkali metal is 

 always less — or, at least, not greater — than that of the halogen 

 atoms in the salt of the polyvalent metal, e.g. CuCl 2 .2KCl, 

 FeCl 3 .3CsCl, etc. This generalisation, known as " Remsen's 

 rule," seems, however, to be far from absolutely valid. 2 



Greater interest attaches to the theoretical speculations of 

 Werner, 3 to the attempt made by him to find in the formulae of 

 double salts support for his view concerning the numerical values 

 of the valency manifestations of the elementary atoms. Double 

 salts have been amongst the chief of the representatives of so- 

 called molecular combinations, a class of substances about which 

 there has been a good deal of theoretical wrangling, and much 

 discussion barren of practical results. Molecular compounds 

 were, so to speak, merely invented, created in imagination in 

 response to a desire for simplification of valency conceptions ; 

 and though furnished from time to time with supposed charac- 

 teristics to serve as label, the attempt to differentiate molecular 

 from atomic combinations cannot be considered to have proved 

 successful. Werner is attempting to quite banish this con- 

 ception of molecular compounds, of combinations specifically 

 distinct from the atomic; and to do so he tries to remove by 

 some other means the difficulty for the solution of which they 

 had been created. Structural representation of the composition 

 of double salts, of salts of complex radicles, of crystalline 

 hydrates, etc., on the basis of atomistic combination, requires for 



1 Remsen, "Double Halogen Salts," Amer. Chem.J. u, 1889, pp. 291 et seq. 



2 Wells, "Generalisations on Double Halogen Salts," Amer. Chem. J. 26, 

 1901, p. 389. 



3 Werner and Miolati, "Beitrage zur Konstitution anorganischer Verbindungen," 

 Zs. physik. Chem. 12, 1893, pp. 35 et seq. ; Zs. anorg. Chem. 3, 1893, P- 2 &7 

 et seq. ; 9, 1895, p. 382 et seq. 



