DOUBLE SALTS 157 
chemical study of double salts deals with the generalisations 
concerning their formule. 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.' 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,.2KCl, 
FeCl;.3CsCl, etc. This generalisation, known as ‘“ Remsen’s 
rule,” seems, however, to be far from absolutely valid.’ 
Greater interest attaches to the theoretical speculations of 
Werner,’ to the attempt made by him to find in the formule 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. 11, 1889, pp. 291 e¢ seg. 
2 Wells, ‘‘Generalisations on Double Halogen Salts,” Amer. Chem. /. 26, 
1901, p. 389. 
3 Werner and Miolati, “Beitrage zur Konstitution anorganischer Verbindungen,” 
Zs. physik. Chem. 12, 1893, pp. 35 ef seg.; Zs. anorg. Chem. 3, 1893, p. 267 
et seg.; 9, 1895, p. 382 ef seg. 
