266 



DIFFUSION, OSMOSIS, AND FILTRATION. 



The experiments of Soret, 1 again, show that in a solution, as in a gas, 

 the warmest part is the most dilute. Soret introduced a solution into 

 a long vertical tube and maintained a difference of temperature at the 

 two ends, the upper end being warmer than the lower. At the end 

 of several weeks the concentration of the solution at the warm end of 

 the tube was found to be lowered. Thus, with solution of copper 

 sulphate, the concentration at the end of the tube at 20 C. was 17'332 

 per cent., while that at the end maintained at 80 C. was 14'03 per cent., 

 instead of 14*3 per cent, as calculated by Charles' law. And, again, with 

 concentration of 29'867 per cent, at the 20 C. end, a concentration of 

 23'871 per cent, was found at the end warmed to 80 C. instead of 24'8 

 per cent, as calculated. 



Thus " the osmotic pressure of a dissolved substance is exactly the 

 same as the gas pressure, measured by the manometer, which one would 

 observe if he could remove the solvent, and leave the dissolved substance 

 as a gas filling the same volume." 2 The hypothesis of Avogadro then 

 is, according to van 't Hoff, not merely capable of extension by the law 

 of Henry to solutions of gases, but to solutions of matter which is not 

 gaseous under ordinary circumstances, and it may be stated that 

 equal volumes of gases or dilute solutions at the same gas or osmotic 

 pressure, and at the same temperature, contain equal numbers of 

 molecules. 



A marked concordance is seen in the table below, between the 

 observed osmotic pressures for sugar solution taken from Pfeffer 3 and 

 those calculated on the hypothesis of Avogadro and the law of Charles. 



One per cent, sugar solution contains 1 grm. of sugar in 100-6 c.c. of 

 solution. At the same temperature and pressure, -gf^ of a grm. of hydrogen 

 contains by hypothesis the same number of molecules (C 12 H 22 O n = 342). 



Taking the weight of a litre of hydrogen, at C. and one atmosphere 

 pressure, as '08956 grm., and the above concentration as '0581 grm. per litre, 

 the gas pressure at C., at the volume 100*6 c.c., is '649 atmosphere, and at 

 the temperature t= "649 (1 + -003670. 



The law of Dalton may also be applied, with certain restrictions, to 

 the osmotic pressure of solutions, the total pressure of a mixture of 

 substances being equal to the sum of the partial osmotic pressures of 

 the several components. 



1 Arch. d. sc. pliys. et nat., Geneve, S^r. 3, tome ii. p. 48 ; Ann. de chim., Paris, Se'r. 5, 

 tome xxii. p. 293. 



9 Nernst's "Theoretical Chemistry," 1895, Palmer's trans., p. 148. 



3 Loc. cit. 4 Pfeffer, loc. cit., p. 85. 



