SOLUTIONS 23 



weight of any non-ionizable solute. Thus Bouchard lias been 

 able to determine by means of cryoscopy the mean molecular 

 weight of the substances eliminated by the urine. A weight x 

 of the substance is dissolved in a litre of water, and the lower- 

 in" of the freezing point is observed. The value thus found 

 divided by T85 gives us n, the number of gramme-molecules 

 per litre. The molecular weight M may be determined by 

 dividing the original weight x by /(. 



The study of osmotic pressure was begun by the Abbe 

 Nollet ; and one of his disciples, Parrot, at an early date thus 

 described its importance: "It is a force analogous in all 

 respects to the mechanical forces, a force able to set matter in 

 motion, or to act as a static force in producing pressure. It 

 is this force which causes the circulation of heterogeneous 

 matter in the liquids which serve as its vehicle. It is this 

 force which produces those actions which escape our notice by 

 their minuteness and bewilder us by their results. It is for 

 the infinitely small particles of matter what gravitation is for 

 heavy masses. It can displace matter in solution upwards 

 against gravity as easily as downwards or in a horizontal 

 direction." 



Thus the recognition of the fact that a substance in solution 

 is really a gas, has at a single stroke put us in possession of 

 the laws of osmotic pressure — laws slowly and laboriously 

 discovered by the long series of investigations on the pressure 

 of gases. 



Osmotic pressure plays a most important role in the arena 

 of life. It is found at work in all the phenomena of life. 

 When osmotic pressure fails, life itself ceases. 



