THE NEW THEORY OF SOLUTIONS. 17 



and may, therefore, be neglected. Entrance of water 

 into the cell, is to be regarded as brought about by the 

 attraction which is exerted by the dissolved substance. 

 Indeed, it was the endeavour to obtain a measure of such 

 attractive forces and to investigate their bearing on pro- 

 blems in chemical mechanics which first caused van't Hoff 

 to direct his attention to Pfeffer's observations. 



If, as this reasoning indicates, osmotic pressure is of 

 kinetic origin, there ought to be a proportionality between 

 the number of impacts per unit time of the molecules of 

 the dissolved substance on the semi-permeable wall and 

 the number of molecules per unit volume of solution, that 

 is, the osmotic pressure ought to be proportional to the 

 concentration. The above hypothesis as to the nature of 

 osmotic pressure thus supports the validity of Boyle's 

 law as applied to dilute solutions. 



The identity of the osmotic pressure of a given volume 

 of sugar solution with the gaseous pressure of an equal and 

 equimolecular volume of gas, was regarded by van't Hoff 

 as but a particular instance of the applicability to dilute 

 solutions of an hypothesis similar to that of Avogadro. 

 The validity of this application he established thermo- 

 dynamically, taking the particular case of a soluble gas 

 obeying Henry's law, and as Pfeffer's. observations showed 

 that sugar solutions conformed to the hypothesis, the 

 latter was transferred to solutions in general. For these 

 reasons it is asserted that equal volumes of dilute solu- 

 tions at the same temperature and under the same osmotic 

 pressure contain the same number of molecules of dis- 

 solved substance. Moreover, this number of molecules 

 is the same as that in an equal volume of gas at the same 

 temperature, and under a pressure equal numerically to 

 the osmotic pressure of the solution. 



THE APPLICATION OF GASEOUS LAWS TO DILUTE 



SOLUTIONS. 



On the new theory of solutions, therefore, a dissolved 

 substance may be treated as if it were gaseous. Replac- 



