THE NEW THEORY OF SOLUTIONS. 407 



method was also employed in estimating the vapour-pres- 

 sures. If, at any temperature, p be the vapour-pressure of 

 the solvent, and p' be the vapour-pressure of the solution of 

 a non-volatile dissolved substance, the ratio (p - p')jp, which 

 is now commonly termed the relative lozvering of the vapour- 

 pressure, was found to obey the following laws : — 



Influence of temperature. — Within the limits of the ex- 

 perimental errors Raoult's results were in agreement with 

 von Babds law. This law, deduced as early as 1847 from 

 observations on aqueous saline solutions, implies that the 

 relative lowering of the vapour-pressure is the same at all 

 temperatures. In testing this conclusion measurements were 

 made upon solutions in ether, benzene, alcohol, and acetic 

 acid, over temperature ranges varying between 20° and 40 . 



Influence of concentration. — In his first communication, 

 which dealt with solutions containing" from one to five gram- 

 molecules of substance dissolved in 5000 gr. of ether, 

 Raoult found that the relative lowering was almost propor- 

 tional to the weight of substance dissolved in a constant 

 weight of ether. Hence, expressing the concentration in 

 gram-molecular weights, if n gram-molecules of substance be 

 dissolved in N gram-molecules of solvent, we have that — 



(P-p')jp=Kn/N, 



where K is a constant. Wullners law (1856-60), derived 

 from measurements of the vapour-pressures of aqueous 

 solutions of simple and mixed salts, and which implies that 

 the relative lowering is proportional to the concentration, 

 embodied the same conclusion. 



Trustworthy experiments on more dilute solutions are 

 difficult to make on account of the large value of the ex- 

 perimental error, and in subsequent papers Raoult has, in 

 general, dealt with solutions which are more concentrated 

 than those usually employed in ascertaining the laws of 

 ideal solutions. From measurements on ethereal solutions 

 containing 17 to 70 gram-molecules of substance in 100 of 

 solution he was led to express the effect of concentration by — 



(p-p')/p=K'u/(N + n); 

 the relative lowering being proportional not to the number 



