1 84 SCIENCE PROGRESS. 



to the conclusion that if iodine were gasified it would obey 

 Henry's law, its solubility in either solvent being proportional 

 to the gaseous pressure. Now van't Hoff has shown that 

 for a substance obeying Henry's law the osmotic pressure 

 is equal to the corresponding gaseous pressure, hence it 

 follows from the constancy of the partition-coefficient that 

 if one gram of iodine be dissolved in the same volume of 

 water or carbon bisulphide, the osmotic pressure will be 

 the same in each case (Nernst, 1891). 



Correlation of the different modes of proving the validity 

 of the gas-equation. — We have now dealt in detail with five 

 different modes of determining molecular weights and of 

 proving that the gas-equation applies to dilute solutions. 

 Why each of these methods leads to the same result 

 depends upon the fact already stated, that the properties 

 to which they refer are correlated properties. A simple 

 mode of showino- the reason for the coincidence in the 

 conclusions to which they lead, and of indicating the 

 nature of their correlation, has been given by Nernst, who 

 has called attention to the fact that they are all concerned 

 with the separation of solvent and dissolved substance. In 

 the case of osmotic pressure this separation is effected by 

 directly forcing the solvent through a semi-permeable 

 membrane, in the case of the freezing-point the solvent is 

 made to crystallise out, in the case of vapour-pressure and 

 boiling-point it is made to volatilise, and finally in the 

 solubility-method, a solvent A is made to separate from a 

 liquid B by dissolving in B a substance insoluble in A. 

 Each method furnishes a means of ascertaining the work 

 required to bring about this separation, and hence each is 

 but a different mode of attacking the same problem. 



In the above methods the solvent is made to separate 



from the solution. It is of course possible to devise 



parallel methods in which the dissolved substance is made to 



separate — by crystallisation, evaporation, etc. — and already 



van't Hoff, Nernst, Deventer and Stadt, and others, have 



laid the foundation of the development of this side of the 



question. 



J. W. Rodger. 



{To be continued.) 



