Proof of Van't Hoff's Constant. 119 



of experiment. These and many other points are receiving 

 consideration, and will be discussed in a subsequent com- 

 munication. We must, in conclusion, express our great 

 obligations to Mr. D. K. Morris, as well as to Mr. C. 

 Jakeman, for the valuable assistance they have rendered 

 to us in the laborious work of taking and reducing the 

 observations above recorded, as well as in the construction of 

 some of the apparatus used in this research. 



VI. Experimental Proof of Van't Hoff's Constant, of 

 Arrhenius's Generalization, and of Ostwald's Law of 

 Dilution in very Dilute Solutions. By Dr. Meyer 



WlLDEEMANN *. " 



I. Proof of Van't Hoff's Constant. 



IT is well known that it was Van't Hoff who first drew 

 attention to the fact that the equations representing 

 the generalizations arrived at by Boyle, Gay-Lussac, and 

 Avogadro in the case of gases are equally applicable to dis- 

 solved substances,, if the osmotic pressure of the molecules 

 of the dissolved substance be substituted for the pressure 

 of the gas. 



Van't Hoff deduced these laws for solution from thermo- 

 dynamical considerations — a method which gives them in- 

 creased validity, — and illustrated them from the osmotic 

 experiments of Pfeffer and de Vries. 



At the same time Van't Hoff was able to establish a thermo- 

 dynamics! relation between the osmotic pressure of a dissolved 

 substance and the molecular lowering of vapour-pressure ; 

 molecular lowering of freezing-point of the solution thereby 

 furnishing a rational basis for the empirical generalizations 

 of Raoult and of Babo and Wullner, who had previously 

 investigated the same point. 



In Van't Hoff's thermodynamical argument the solutions 

 are assumed to be very dilute, and hence their experimental 

 verification is specially important for the case of very dilute 

 solutions. 



The determination of the molecular depression of the 

 freezing-point is the safest and most convenient method of 

 testing the validity of these generalizations, and this has been 

 done for moderately dilute solutions by Van't Hoff himself 



* Communicated by the Author : read before the British Association 

 on the 13th of August, 1894. 



