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 lono- 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 17332 
per cent., while that at the end maintained at 80° C. was ld--0o per cent., 
instead of 143 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 tilling 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 1: ,H ; >o0 11 = 312). 
Taking the weight of a litre of hydrogen, at D C. and one atmosphere 
pressure, as -08956 grm., and the above concentration as -0581 grm. per litre. 
the gas pressure at 0° C, at the volume 100-6 c.c, is "619 atmosphere, and at 
the temperature ^ = 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. phys. et wit., Geneve, Ser. 3, tome ii. p. 48 ; Ann. de chim., Paris, Se>. 5, 
tome xxii. p. 293. 
2 Nernst's "Theoretical Chemistry,'' 1S95, Palmer's trans., p. 148. 
3 Loc. cit. 4 Pfeffer, loc. cit., p. 85. 
