202 THE EARL OF BERKELEY, MR. E. G. J. HARTLEY AND DR. C. V. BURTON : 



corresponding difference in column (3). This ratio is practically the s of the term. 

 Further, it will be seen that the s for any given solution varies but slightly with the 

 pressure. The maximum difference is only 0'5 per cent. It was, therefore, considered 

 that for the purpose of evaluating the integral, sufficient accuracy would be attained 

 if we put 



fA+;) fA + ;i 



s dp = 5 J dp. = s (P-TT,,,}, 



ait <nr 



where the mean value of s between the limits is s, obtained in the manner just 

 indicated. 



These mean values are given in the following table* : 



In Prof PORTER'S ideal apparatus, it will be rememl)ered that there is supposed to 

 be a pressure p u on the pure solvent piston. In our actual experiments this p a 

 vanishes ; hence p = P, so that the term under discussion becomes sP. 



[A+w, 



(2) The term ?< dp. As the specific volume of water at C. does not differ 



appreciably from unity, even over a pressure range of one atmosphere, this term, 

 remembering that p = 0, reduces to TT M , which is a negligibly small quantity. 



(3) The term " v dp. There seem to be no data for the exact evaluation of this 



J ",m 



integral, but if we assume that both BOVLK'S law and the partial-pressure law apply 

 to the vapour pressure of water in air, we may put 



( 7 



" V dp = -2 log, -5? = -2 log, -, 



J it On D^f. f i 



where 



3 a o = vapour density of water vapour in air when the water is under the pressure A + TT M , 

 1 = observed loss of weight of the solution and water vessels, 

 Z, = observed loss of weight of the solution alone. 



The original equation thus reduces to 



" The solution of weight concentration 49 966 is taken because it is the solution for which we have the 

 lowering of vapour pressure. Sec p. 190, Table HA. 



