408 SCIENCE PROGRESS. 



of molecules of substance in a constant number of molecules 

 of solvent (as Wiillner's law requires), but in a constant num- 

 ber of molecules of solution. The value of K' was found to 

 vary with the nature of the solution ; in the case of six solu- 

 tions in ether, benzene, and alcohol, the extreme values were 



73 an d '97- 



For more dilute solutions containing from 3 to 1 5 gram- 

 molecules per 100 of solution K' in all cases approximated 

 to the value 1 , so that here the general law appeared to be — 



(p -p')lp = »/(N + 11). 

 For the most dilute of the above solutions and for still 

 more dilute solutions n/(N + n) is practically the same as 

 n/N. Raoult's observations may therefore be taken to 

 indicate that for the most dilute solutions (p — p')jp = n/N, 

 or that Wiillner's law is approximately obeyed. 



Measurements of the boiling-points of solutions (vide 

 infra), where smaller concentrations may be used, lead to a 

 similar conclusion, which, on employing the ordinary method 

 of denoting the concentration, may be expressed by stating 

 that \ig grams of substance be dissolved in 100 gr. of solvent 

 then (p - p')/pg is approximately constant ; in general, as g 

 increases the value of this quantity slowly diminishes, 

 although in some cases its value slowly increases. 



Influence of the chemical nature of the dissolved substance. 

 — The most interesting result arrived at by Raoult was that 

 dilute solutions in the same solvent, having the same molec- 

 ular concentration, have the same vapour-pressure. His 

 numbers showed that if M' be the molecular weight of the 

 dissolved substance, M' (p — p') Ipg = C was a constant. 

 This quantity he termed the molecular lowering of the 

 vapour-pressure ; it is evidently the lowering which would 

 be produced by the dissolution of a gram-molecular weight 

 of substance in 100 gr. of solvent if Wiillner's law were valid 

 for such a strong solution. That Wullner's observations on 

 aqueous saline solutions gave some indication that the mo- 

 lecular depression was approximately constant had previously 

 been pointed out by Ostwald in 1884. The following table 

 contains the data given by Raoult to demonstrate the con- 

 stancy of the molecular lowering in the case of ethereal 



