MEASUREMENT AND CALCULATION 43 



number of molecules which are dissociated. Thus, if one 

 out of every ten molecules were dissociated, a would equal ^. 

 Now, if each molecule forms k ions, and if i denote the 

 ratio of the actual osmotic pressure to that which would be 

 obtained in the same concentration of a non-electrolyte 

 solution, i may be found from the following: 



And if P denote the osmotic pressure developed in a 

 solution of a non-electrolyte, of the same concentration as 

 that whose pressure is to be found, P' being the required 

 pressure, then 



P' = Pi . 



The conductivities of a great many solutions are to be 

 obtained from published tables. 1 It is not necessary to give 

 the methods for determining these conductivities here. 

 They are thoroughly and completely discussed by Kohl- 

 rausch and Holborn. If the osmotic pressure is all that is 

 required, and data for the conductivity of the given solute 

 cannot be found in the published tables, then it is more expe- 

 dient to determine the pressure by means of one of the indirect 

 methods previously described than to determine the conduc- 

 tivity. If the proper concentration is not given in the 

 tables, the conductivity for it is found by interpolation 

 between the conductivities for the two concentrations nearest 

 to it. If the table is rather extensive for the solution in 

 question, so that conductivities for very low concentrations 

 are given, it is usually safe to take the highest conductivity 

 as X w . If the table is not so complete, a limit for the con- 

 ductivity has to be approximated from the trend of the given 

 data. In using the published tables, it is very important 

 that one bear in mind the difference between molecular and 



i W. C. D. WHETHAM, Solution and Electrolysis (Cambridge, 1895), pp. 218 ff.; 

 also F. KOHLBAUSCH UND L. HOLBORN, Das LeitvermQgen der Elekrolyte, Leipzig, 

 1898. 



