the Conductivity of Solutions of Double Salts. 515 



and have selected the double sulphate of potassium and 

 copper. 



As a full account of the experiments is being published 

 elsewhere *, we need not give details as to methods. The 

 following statement will be sufficient. The water employed 

 in preparing the solutions was carefully purified and tested by 

 conductivity measurements. The salts used were obtained as 

 chemically pure from reliable makers, were re-crystallized 

 several times, and tested. The double salt was prepared by 

 mixing equi-molecular solutions of the constituents and eva- 

 porating the mixture at a temperature below 70° C; and after 

 preparation it was analysed. The constitution of the solutions 

 was determined by the analysis of standard solutions, which 

 were subsequently diluted, the dilutions being checked by 

 occasional analytical tests. All solutions were prepared and 

 all observations made at 18° C, so that Kohlrausch's deter- 

 minations of the molecular conductivities of simple solutions 

 at infinite dilution might be used in the calculations, and his 

 observations at moderate concentrations used as checks. Obser- 

 vations of density were made both of the complex solutions 

 and of their mixture constituents, of sufficient accuracy to 

 show that no change of volume would occur on mixing the 

 latter such as would need to be taken account of in the 

 calculations. Kohlrausch's telephone method was used in 

 measuring the conductivity, his experience and that of other 

 observers with this method being fully utilized. The obser- 

 vations of conductivity might be in error by about 0*25 

 per cent. The ion ization-coefficients" were determined by the 

 method referred to above. 



The following are the observations made on simple solutions 

 for the purpose of securing the requisite data for the calcu- 

 lations. Dilutions are expressed in litres per gramme-equi- 

 valent. Conductivities are specific molecular conductivities 

 (i. e. per gramme-equivalent), and are expressed in terms of 

 10 -8 times the conductivity of mercury at 0° C. 



* Archibald, Trans. N. S. Inst. Sci. ix. p. 307 (1897-98). 



