THE NEW THEORY OE SOLUTIONS. 179 



consequently the osmotic pressure of a solution is a matter 

 of extreme slowness. 



Molecula r weights and the properties of solutions. — M uch 

 of the testimony which substantiates the applicability of 

 the gas-equation has been accumulated in the endeavour 

 to utilise the properties of dilute solutions as methods of 

 estimating the molecular weights of soluble substances which 

 cannot be volatilised. It has again to be noted here that 

 these methods are purely empirical. Direct and indirect 

 experiments show that, for substances of known molecular 

 weight, osmotic pressure is equal to the corresponding 

 gaseous pressure. If this rule be extended, and we assert 

 that the osmotic pressure of a soluble non-volatile substance 

 may still be regarded as the pressure which it would exert 

 if it could be vaporised, even in such cases we may 

 estimate vapour density and hence molecular weight. But 

 apart from any correlation of the other properties of solu- 

 tions with osmotic pressure, it would be rational to employ 

 observations on solutions to fix doubtful or unknown 

 molecular weights. Oft-repeated experiment has proved 

 that in the case of substances of known molecular weight, 

 solutions of the same molecular concentration have the 

 same vapour-pressure, boiling-point, and freezing-point ; it is 

 therefore only necessary to extend these results to solutions 

 in general, in order to obtain the molecular weight of any 

 non-volatile substance which is soluble in a suitable 

 solvent. Although the use of these methods may thus be 

 justified apart from theoretical considerations, it was not 

 until van't HofT's theory had been published that their 

 value became widely recognised. Raoult's first series of 

 observations on the freezing-points of indifferent solutions 

 appeared in 1882. In 1884 he generalised his results and 

 laid especial stress on the ease with which the observations 

 lent themselves to the determination of molecular weights. 

 The first attempt to turn the method to practical account 

 was not made, however, till 1886, when Paterno and 

 Nasini estimated the molecular weights of paraldehyde and 

 dicyanamide from the cyroscopic behaviour of their solu- 

 tions. In 1888 Victor Meyer drew attention to the great 



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