OSMOSIS. 
265 
In practice such membranes are formed in the interstices of an 
indifferent supporting structure, such as the pores of a porous battery 
pot (preferably previously soaked in gelatin), by placing one of the mem- 
branogens inside the pot, which is then Lowered into a solution of the 
other, so that the precipitate is formed within the structure of the 
earthenware where the two solutions come into contact. It is only by 
such an artifice that the membrane can be sufficiently supported to 
enable it to withstand the high pressure produced by the osmosis under 
the conditions. 1 
If, now, a battery pot with such a membrane in its pores lie filled 
with' a solution of sugar in water, hermetically sealed, and placed in 
a vessel of water, the water molecules will diffuse in either direction 
through the membrane, which is permeable to them ; the sugar molecules, 
on the other hand, cannot pass out, for to them the membrane is imper- 
meable. As a result of the presence of the sugar on the inner side of 
the membrane, in unit time, more water enters the pot than passes out, 
and the pressure rises until it is sufficient to bring about the condition 
of equality in the number of water molecules entering and leaving the pot. 
This pressure is called the osmotic jwessuir of the solution of sugar, 
under the conditions of concentration and temperature. That this 
pressure is comparable to that of a gas was first clearly pointed out 
by van 't Hoff. 2 
Thus the osmotic pressure of a dilute solution at constant tem- 
perature is proportional to its concentration | i.e. density of a gas in 
the law of Boyle). This is illustrated by the following table from 
Pfeffer : 3 — 
Cane Sugar Solutions at 13 c, 5 C. to 16 0- 1 C. 
Concentration of j Osmotic Pressure 
Solution. in Mm. of Hg. 
Osmotic Pressure 
Concentration. 
1 per cent. 
2 
2-74 
4 
6 
535 
1016 
1518 
2082 
3075 
535 
508 
554 
521 
513 
Again, at constant concentration of a dilute solution, the osmotic 
pressure is proportional to the absolute temperature (law of Charles). 
Thus, again, taking Pfeffer's data — 
1 per Cent. Cane Sugar Solution. 
Temperature. 
Observed Pressure. 
Calculated Pressure. 
(1) 
(2) 
32° 
14°-15 
36° 
15°-5 
Mm. Hg. 
544 
510 
567 
520-5 
Mm. Hg. 
512 
529 
1 For details of manufacture see Adie, Journ. C'hem. Soc, London, 1891, vol. lix. p. 344. 
-Arch, ncerl. d. sc. exactes, etc., 1885, Bd. xx. S. 239; Ztschr. f. physikal. C'hem. 
Leipzig, 1887, Bd. i. S. 479. 
s Loc. cit., p. 85. 
