the Conductivity of Solutions of Double Salts. 519 



ences in their case were due to the errors in the values of the 

 a's and /Xoo's, we might expect to find similar differences, 

 though possibly not so large, in at least one of the latter 

 series of mixtures. It seems probable, therefore, that the 

 differences observed both in the double salt solutions and the 

 equivalent mixtures, are not due to defects in the data for the 

 calculations. 



The case therefore stands thus : — 



(1) In all cases investigated so far the conductivity of 

 solutions containing two electrolytes with a common ion is 

 calculable by means of the dissociation theory up to a con- 

 centration of about 1 gramme-equivalent per litre. 



(2) The conductivity of solutions of the double sulphate of 

 potassium and copper is similarly calculable only up to a 

 concentration of about 0*1, and has at concentration 1 a con- 

 siderably smaller value than that calculated on the assumption 

 that no double molecules are present. 



^3) The solutions of double salt have at concentration 

 unitv an appreciably smaller conductivity than the equivalent 

 mixtures. 



(4) The conductivities of mixtures of equi-molecular simple 

 solutions of zinc and copper sulphates and of potassium and 

 sodium sulphates are calculable within the limits of obser- 

 vational error up to a concentration unity; and therefore the 

 non-calculability in the case of the double salt solutions is 

 probably not due to defective data. 



(5) The differences between the observed and calculated 

 values of the conductivity of the double salt solutions and of 

 the equivalent mixtures, and between the observed values in 

 these two sets of solutions, are such as would be accounted for 

 by the presence of double molecules in both, and their presence 

 in slightly greater number in the double salt solutions than in 

 the equivalent mixtures. 



A complete study of the existence of complex molecules in 

 solutions requires the application of other methods as well as 

 the conductivity method. What we wish to draw attention 

 to in the present paper is the increased utility of the con- 

 ductivity method due to the possibility of calculating the 

 conductivity of mixtures of solutions, by the aid of the disso- 

 ciation theory, in cases in which complex molecules are not 

 formed. 



