THE NEW THEORY OF SOLUTIONS. i8r 



the solvent, the dissolved substance is practically non- 

 volatile. Planck (1888) has theoretically investigated the 

 more Q-eneral case of solutions of volatile substances, and 

 Winkelmann (1890) has experimentally verified Planck's 

 conclusions. Nernst has more recently taken up this ques- 

 tion, and shown that as a consequence of the new theory it 

 is possible to obtain evidence regarding the molecular 

 weight of a volatile substance from the boiling-point of its 

 solutions. Because, if the molecular weight of the dissolved 

 substance be the same in the solution as in the state of 

 vapour, the partial pressure of its vapour should be propor- 

 tional to the strength of the solution, and the rise or fall 

 which it produces in the boiling-point of the solvent should 

 be proportional to the concentration ; whereas, if the mole- 

 cular weights differ in the two states, there should be no 

 proportionality. 



Observations on osmotic pressure may also be employed 

 to ascertain molecular weights, although the methods are in 

 no case so convenient as those just alluded to. The most 

 suitable of the indirect methods are based upon the use of 

 vegetable cells, or red blood corpuscles (see vol. i., pp. 19-2 2). 

 By the former, in 1888, De Vries concluded that of the 

 formulae which had been proposed for raffinose, the C ls -for- 

 mula was correct. This conclusion has since been confirmed 

 by purely chemical evidence. The plasmolytic methods are 

 of interest as they show how the microscope may be em- 

 ployed to determine molecular weights. They can only be 

 used, however, with solutions in which protoplasm can live. 



In 1889, Ladenburg announced that he was perfecting 

 a method of estimating molecular weights from direct 

 observations on osmotic pressure, but as yet no further 

 details have been published. 



The general kinetic theory of solutions. — Not only has 

 the new theory of solutions proved itself a powerful 

 stimulus to the development of such a practical subject as 

 the determination of molecular weights, but it has also 

 given rise to several important speculations regarding the 

 general kinetic theory of solutions. These have, in the 

 main, confirmed the hypothesis of the new theory. 



