THERMODYNAMICS OF CHANGE OF STATE, ETC. 323 



hydrostatic pressure .which must be put on the solution to restore its 

 vapour-pressure to equality with that of the solvent, can be found from 

 either of these effects, lowering of melting-point or raising of boiling- 

 point, or going back to equation (1) it may be found from the difference 

 of vapour-pressures. 



Though much work had been done by earlier observers * the effect of 

 solution on vapour-pressure, and on melting-points was first put into a 

 satisfactory form by Raoult, who worked not only with salts in water, 

 but with solutes in other solvents. Herein lay the success he attained in 

 formulating laws. For aqueous solutions are generally electrolytic and for 

 these the osmotic pressure is irregular, probably for a reason which we 

 shall give below. But with other solvents it is easy to make great 

 numbers of non-conducting solutions, and these give more regular values 

 leading to definite laws. Raoult showed f that 



1. or the proportionate lowering of the vapour-pressure, is 



a) 



independent of the temperature for a particular solution, ether being the 

 solvent. 



2. is proportional to the amount of salt or solute dissolved. 



T grammes dissolved in 100 of solvent 



molecular weight of solute 



= "gramme molecules " of salt per 100 grammes of solvent 

 then in the case of ether as solvent 



0) (I)' 



-T- n is constant, 



0) 



or the lowering per gramme molecule is the same for different substances. 

 Later he showed J that if n is the number of molecules of solute in N of 

 the solvent, then for very dilute solutions 



a) to n i 



= very nearly. 



0) 



Still later he found that 



would hold for much stronger solutions if c were 0*9, and that as the 

 dilution increased c tended towards 1. 



For electrolytic solutions, c may be very much more than 1, rising 

 towards 2 for such salts as NaCl or KC1 in water, and towards 3 for such 

 salts as Ca C1 2 or Ba C1 2 , in water. 



. The law = implies that the lowering of the vapour-pressure 



* See Ostwald's Solution, chap, vii., for a history of the subject. 



t C.R., ciii. p. 1125, 1886. 



J C.R., civ. p. 976, 1887. 



Zeit.fiir Phys. Chem., ii. p. 353, 1888 



