96 J. van't Hoff on the Function of Osmotic Pressure 



Solvent. 



Freezing- 

 point 



Heat of 

 fusion 



002 T 2 

 l ~ W ' 



Raoult's 

 molecular 





(T). 



(W). 





depression, 



Water . . . 



273 



79 



18-9 



18-5 



Acetic acid . 



273 + 16-7 



43-2*t 



38-8 



38-6 



Formic acid . 



273+ 8-5 



55-6*f 



28-4 



27-7 



Benzene . . 



273+ 4*9 



29-lf 



53 



50 



Nitrobenzene . 



273+ 5*3 



22-3f 



69-5 



70*7 



It may be added that, from the molecular depression of 

 ethylene bromide (117*9), the hitherto unknown heat of fusion 

 is found equal to 13 ; and Mr. Pettersson, at my request, 

 having experimentally determined this constant, obtained the 

 number 12*94. 



IX. Application of Avogadro's Law to Solutions. — The Law 

 of Guldberg and Waage. 



Having discussed the physical aspect of this problem in 

 order to furnish as many proofs as possible of the correctness 

 of its treatment, it now remains to apply it to chemical facts. 



The most evident application of AvogadiVs law in reference 

 to solutions is to the determination of the molecular weights 

 of dissolved substances. This has already been attempted ; 

 but here it is not the pressure which requires measurement, 

 as with gases, when the molecular weight is deducible from 

 volume, pressure, and temperature ; with solutions the osmotic 

 pressure must be measured, and as yet the practical method is 

 wanting. Yet this difficulty may be avoided by substituting 

 for a determination of pressure that of the related values ; as, 

 for example, the diminution of vapour-pressure, or the depres- 

 sion of the freezing-point. This proposal is Raoult's. He 

 divides the reduction of vapour-pressure of water containing 

 one per cent, of dissolved substance into 0*185 parts, or the 

 observed depression of freezing-point into 18*5 parts ; and 

 this method is comparable in respect of accuracy with deter- 

 minations of the density of gases, and is in itself a strong 

 presumption of the accuracy of Avogadro's law in its appli- 

 cation to gases. 



It is still more remarkable that the law of Guldberg and 

 Waage, so generally accepted for solutions, can be evolved 

 from the above laws regulating the behaviour of dilute solu- 

 tions. Again, a reversible cycle at constant temperature 

 must be imagined, which can be conceived for solutions 

 equally well with gases, by means of semipermeable 

 diaphragms. 



^ Berthelot, Essai de Mecanique Chimique. 



t Pettersson, Journ.f. prakt. Chem. [2] xxvi. p. 129. 



