504 Dr. W. W. J. Nicol on the Vapour-pressures 



4. The most general case is a combination of the three 

 foregoing classes in the order given. At first the quantity of 

 salt diminishes rapidly with the concentration ; then more 

 slowly till it becomes practically a constant for a few degrees; 

 then it increases more and more rapidly up to the saturation- 

 point. K 2 C0 3 may be taken as an instance. From 100°- 

 101° C. 13 per cent, of salt are required ; from 101°-102° C. 

 9*5 per cent. For each degree between 113° and 118° C. 

 4*8 per cent, only are required ; and above this the amount 

 increases till from 130°-131° C. 5*4 per cent are required ; 

 and with other salts the increase is still more marked. K 9 C0 3 , 

 NaC 2 H 3 2 , K 2 C 4 H 4 6 , NH 4 C1, CaCl 2 , SrCl 2 , Ca(N0 3 ) 2 , 

 KC 2 H 3 2 ; that is, eight out of seventeen salts examined 

 belong to this class alone. 



It appears probable, as indicated above, that there is but a 

 single curve of the form indicated in the diagram below ; and 

 that it is complete in the case of some salts, while others cor- 

 respond to the first, second, or third parts of the curve. How 

 far this hypothesis is borne out by my experiments will be 

 shown later on. 



Percentage Salt , 



Apart from the general inaccuracy attending Legrand's 

 method of experiment, there is the serious objection to it that, 

 though the pressure was constant, the temperature was vari- 

 able. It would be difficult, if not impossible, to convert his 

 results into a form in which they would be available for the 

 purposes of the present paper. As it is, I believe they can 

 be used only qualitatively, and that even with hesitation. 



