SOLID SOLUTIONS 



others form with them homogeneous or isomorphous 

 mixtures. Von Hauer and Lehmann observed in this respect 

 that the tendency of hydrated salts to effloresce, which 

 indicates a somewhat high vapour pressure, is reduced by 

 isomorphous mixture ; this was observed for lead sulphite 

 in mixture with the corresponding calcium and strontium 

 salts, iron alum with aluminium alum, copper formate with 

 the formates of barium and strontium. The essential point 

 is that each component shows a greater tendency to effloresce 

 than the compound. 



This reduction in saturation pressure corresponds to 

 the observation already mentioned (p. 61), that when the 

 < Unsolved substance separates out along with the solvent 

 as an isomorphous mixture., 

 the depression is abnormally 

 small. Thus e. g. in Fig. 1 1 

 let AB and BC be the vapour 

 pressures of the solid and 

 liquid solvent respectively, so 

 that B, their point of inter- 

 section, represents the melting 

 point T! . If now the solution, 

 by taking up any non- volatile 

 body, comes to have the smaller 

 vapour pressure B 2 C 2 , the de- Fig. n. 



pression produced is T 2 T T ; 



if, however, the solid solvent also takes up some of the 

 dissolved body, and so possesses the smaller vapour pressure 

 A, BO, the depression is also less, viz. T 3 T 1? always on the 

 assumption that the freezing point is the temperature at 

 which solid and liquid have the same vapour pressure, 

 which is certainly the case when the dissolved body is 

 non- volatile. 



It may be remarked that just on this account the treat- 

 ment of isomorphous mixtures is less simple. The close 

 relation of the two bodies, as benzene and thiophene *, 



1 Van Bjlert, Zcitschr.f. Phys. Chem. 8. 343. 



To T. T, 



