6 COLLOID CHEMISTRY AND THE 



The work, in particular, of J. Perrin and of The, Svedberg * 

 on colloids has demonstrated that in microscopically visible 

 systems the properties of fine dispersions can, in certain 

 directions, be brought into quantitative relation to those of 

 typical solutions. We must here consider in rather more detail 

 the work of Perrin, the upshot of which was that it is possible 

 directly to ascertain the gas constant, and, above all, Avoga- 

 dro's number for dispersions (as, for instance, of gamboge or 

 mastic) with astounding accuracy. 



The law of Boyle and Gay Lussac pv RT was expanded 

 by Avogadro into his incredibly fertile hypothesis that for the 

 same conditions of pressure and temperature the same volume 

 of any given gas contains the same number of molecules. This 

 development depended on the discovery that the weights of 

 equal volumes of two gases under the same temperature and 

 pressure conditions are proportional to the respective mole- 

 cular weights. Therefore the gram molecule (the molecular 

 weight in grams) of different gases contains the same number 

 of molecules. 



Van t'Hoff f has shown that the same law holds for dilute 

 solutions, provided the osmotic pressure of the solution against 

 the pure solvent is substituted for the gas pressure. The 

 constant R has the same value as for gases, and the solution of 

 gram molecules of different substances (provided no association 

 or polymerisation occurs) gives here once again the rule of 

 Avogadro that in each solution the same number of molecules 

 is present. This number is a well-known constant, and will 

 be indicated as Avogadro 's number N. 



The kinetic theory of gases, largely the work of Clausius and 

 Maxwell, has for its fundamental assumption the idea of move- 

 ment of the molecules. These movements increase with a 

 rise in temperature, and cease altogether at absolute zero. 

 Avogadro's hypothesis can be deduced from the gas laws by 



* " Die Existenz der Molekiile." Leipzig. 1912. 



f It is extraordinary that the fact that Graham used the analogy 

 between solutions and gases as the basis of his work has not been 

 emphasised. Of course, he only used the comparison for a special case, 

 and it was left to Van t'Hoff to develop it in a general form and apply it 

 quantitatively to dilute solutions. 



