AVOGADRO'S LAW FOR SOLUTIONS 35 



since T is constant, we see that for equal values of V and v 

 we must have p _ ^ 



This is the result originally referred to, that any dissolved 

 body which obeys Henry's law, must have such properties 

 that for equal temperature and concentration, the gas 

 pressure and the osmotic pressure must be equal. 



But we know, by what precedes, that a vapour dis- 

 solving according to Henry's law must have the same 

 molecular character in the solution and the vapour, and 

 consequently we may draw all the conclusions as to 

 the osmotic pressure of dissolved bodies that have been 

 drawn as to gas or vapour pressure, i. e. we may apply 

 Avogadro's law to solutions, making use of the osmotic 

 pressure instead of the gas pressure. 



From this necessary equality of gas and osmotic pressure 

 for similarity of molecular constitution in solution, it 

 follows that the osmotic pressure obeys the gaseous laws, 

 i. e. the laws of Boyle and Gay-Lussac. A remark on the 

 nature of the osmotic pressure may be made here; if it 

 follows Gay-Lussac's law, and is so proportional to the 

 absolute temperature, it, like gas pressure, becomes zero 

 at the absolute zero of temperature, and consequently 

 vanishes when the molecular movements come to rest. 

 It is, therefore, natural to look for the cause of osmotic 

 pressure in kinetic grounds, and not in attractions. 



A second general remark may be made in connexion 

 with the gaseous laws as applied here. In this case too 

 they are to be regarded as limiting approximations, only 

 strictly true for infinite dilution. They refer, on account 

 of the kinetic nature of the osmotic pressure, to that part 

 of it which prevails on increase of dilution ; whilst, when 

 the concentration is greater, the attraction of the solvent 

 becomes of more consequence, and eventually must prevail, 

 since it increases in proportion to the square of the con- 

 centration, while the number of molecular collisions is only 

 proportional to the first power. 



C 2, 



