322 



HEAT. 



by p. 320 and p. 311 



(2) 



dO 

 Por Ixr 



-7 + -zr 

 E? 



"Lp 



where L is the latent heat of the solvent, and 6 is the absolute tempera- 

 ture of the normal boiling-point. 



Lowering of the Melting-Point. Another effect of salt in solution 

 which has long been known is a lowering of the melting-point. When 

 the solution does begin to freeze, the solid which crystallises out is pure 

 solvent. At the new melting-point, when solution and solid solvent 



are in equilibrium with each 

 other, their vapour - pressures 

 must be equal. For if not, 

 T if, for example, the vapour of 

 S the solution were the greater, 

 a reversible one - temperature 

 cycle could be arranged giving 

 work. 



Let T, Fig. 184, represent 

 the triple point of the pure 

 solvent, TA the liquid solvent 

 vapour- pressure line, TB the 

 solid solvent vapour -pressure 

 line, SB the vapour-pressure 

 line of the solution, lower than 



TA by where P is the osmotic 

 P 



pressure of the solution ; o- the 



density of the vapour, and p that of the liquid, and let TB, SB meet in 

 B. B is the melting-point in the solution. Let it be dQ below T. 



We have AB = TS = 



P 



But AB is equal to the difference of pure solid and pure liquid solvent 

 vapour- pressures dO below the melting-point, whence, as on p. 311, 



Temperalure 



FIG. 184. 



where L is the latent heat from solid to liquid. Equating the two values 

 of AB 



P<r 



or 



dd = 



P 



P6 

 Lp 



(3) 



The quantity P, the osmotic pressure, or as we are now regarding it, the 



