4©6 SCIENCE PROGRESS. 



Fortunately, however, several important indirect methods 

 may be employed in dealing with this question. The mag- 

 nitude of the osmotic pressure may be connected with those 

 of other properties of solutions, properties which can be 

 readily measured for solutions in different solvents and 

 which may thus be employed to test the general validity of 

 the equation PV = '0819 T. Indeed, in the process of ac- 

 cumulating this indirect testimony the new theory of solu- 

 tions has rendered one of its most important services. For 

 here it has been emphasised that the properties involved 

 are correlated properties, and although this correlation is in 

 no way dependent upon any assumption as to the nature of 

 osmotic pressure or the state of dissolved substances, the 

 application of the conception of osmotic pressure must be 

 credited with giving for the first time due prominence to 

 this interdependence, the recognition of which goes far to 

 generalise the laws of dilute solutions. 



The property which in the first instance may be con- 

 veniently investigated, as giving indirect support to the 

 application of gaseous laws to dilute solutions, is vapour- 

 pressure. 



VAPOUR-PRESSURE. 



It can be readily shown theoretically that the partial 

 pressure of the vapour of a solvent over a solution must be 

 less than the vapour-pressure of the pure solvent at the 

 same temperature. Consequently, if the dissolved substance 

 be non-volatile, the vapour-pressure of the solution will be 

 less than that of the pure solvent. This conclusion is in 

 harmony with the well-known facts that at the same tempera- 

 ture the vapour-pressure of a solvent is lowered, and under 

 the same external pressure the boiling-point of a solvent is 

 raised, by the presence of a non-volatile dissolved substance. 



Experimental results. — The investigation of the vapour- 

 pressures of indifferent solutions has been carried out 

 almost entirely by F. M. Raoult, who in 1886 began to 

 communicate observations on the subject. His original 

 measurements were made by means of Dalton's statical 

 method, but eventually a dynamical or boiling-point 



