402 



On Electromotive Force and Osmotic Pressure. 



experiment, but are real fluctuations due to the combined 

 effect of the decreasing dissociation of the salt and the in- 

 creasing departure from Boyle's law. This complication can 



PV 



be partly eliminated by taking the quotient • the i being 



determined from the conductivity. Undoubtedly the values 

 of van't Hoff's factor i so arrived at are somewhat uncertain, 

 but they are probably near enough to the truth to serve the 

 immediate purpose of bringing out the character of the devia- 

 tions from the gaseous laws. Assuming the values of i to be 

 correct, the quotients PV/t should be constant so far as the 

 gaseous laws are applicable. The table shows that even for 

 a decinormal solution, with a pressure of 7 atmospheres the 

 deviation is marked. It is about as great as that tor sulphur 

 dioxide or ammonia, but in the opposite direction. 



Large deviation is, of course, to be expected on account of 

 the large size of the dissolved molecules, if for no other 

 reason. 



Zinc Sulphate. 



Gai.-equiv. /litre. 



Osmotic Pressure. 

 P (atmos). 



PV. 



PV 

 t 



o-oi 



0-396 



39-6 



24-12 



01 



3-34 



33-4 



239 



1 



240 



24-0 



19-6 



2 



477 



238 



20-3 



3 



88-5 



27-8 



24-5 



4 



137 



34-2 



31-0 



5 



196 



39 2 



36-4 



6 



263 



438 





It will be seen that there is a decided difference between 

 the behaviour of the chloride and the sulphate. In the latter 

 the values both of PV and TV/i show a well-marked 

 minimum. The sulphate therefore follows the same rule as 

 the ordinary gases, deviating first on the side of less elasticity 

 than a perfect gas : afterwards, however, as the specific volume 

 becomes very small, on the opposite side. 



(ix.) Summary. 



In the preceding pages an expression is obtained for the 

 E.M.F. of a concentration-cell, with or without migration, by 

 means of a thermodynamic cycle: the expression, which is 

 applicable to strong as well as weak solutions, is given first, 



