SOLUTIONS 181 



The apparatus used in such work is described in all the best 

 text-books, where also an account will be found of the method 

 based on the observation of the boiling-points of solutions. The 

 latter method is a little more difficult to carry out, and is not 

 so commonly resorted to as the freezing process. Obviously 

 the boiling-point of a liquid is directly related to its vapour 

 pressure, for a liquid boils when the pressure of the vapour pro- 

 duced just exceeds the pressure of the atmosphere on the surface. 

 Another method is occasionally used which consists in observing 

 the change in the vapour pressure of the solvent at a fixed 

 temperature when a known quantity of a substance is dissolved 

 in it. And by another formula based on Kaoult's work the 

 molecular weight of the dissolved substance can be determined. 

 The rules relating to the vapour pressures of dilute solutions are 

 similar to those relating to freezing-points. 



An important point in regard to these practical experimental 

 methods is the relation in which they all stand to osmotic 

 pressure, for molecular proportions of different substances when 

 dissolved in the same quantity of the same solvent exert equal 

 osmotic pressures, and raise the boiling-point or lower the 

 freezing-point and the vapour pressure to the same extent. 

 These effects are dependent on the number of particles present 

 without regard to their composition. 



Many other problems connected with solutions might be 

 discussed if space permitted. The application of the process of 

 dialysis or selective diffusion of dissolved substances discovered 

 by Graham more than half a century ago has been utilised in the 

 extraction of sugar from the beet. At the present time there is 

 much discussion about the cultivation of the sugar beet in 

 England. In France, Belgium, and Germany the crop and the 

 production of sugar have long been matters of great national 

 importance, while England, relying largely on Colonial sugar, 

 has been obliged also to purchase large quantities of beet sugar 

 from other countries. 



Beet root contains 14 to 18 per cent of cane sugar together 

 with a variety of other substances such as vegetable acids, 

 asparagine and albuminous matter. In order to separate the 

 sugar as much as possible from these substances, which if 

 expressed along with it would interfere with its crystallisation 

 when the juice is evaporated, the beet roots after being washed 

 are cut into very thin slices, which are laid in warm water. 



