32 LAWS OF AVOGADRO AND VAN'T HOFF. chap.
whilst in A there is only hydrogen at \ atmo. pressure. The excess of pressure in B may be termed the osmotic pressure of nitrogen. In this case it is quite clear that the osmotic pressure of nitrogen in B (according to Dalton's law) is equal to the pressure which obtains if it alone occupied the volume B in the state of gas.
A similar experiment may be carried out at the ordinary temperature with carbon dioxide and hydrogen, if the pal- ladium be replaced by a caoutchouc membrane. Cai'bon dioxide is much more soluble — about 60 times — than hydrogen in caoutchouc, and consequently the carbon di- oxide passes from AUiB comparatively quickly, whilst the hydrogen almost all remains in B. Complete equilibrium is established in this case when the carbon dioxide and hydrogen have distributed themselves equally between A and B, But at the beginning a rapid rise Ls noticed at the manometer m2, which then sinks slowly after some time. With respect to
hydrogen and carbon dioxide, caoutchouc is, therefore, not a perfect semi-permeable mem- / V brane ; and a similar imper-
fection is to be foimd in all semi-permeable membranes.
The above experiment can also be carried out in the fol- Fio. 8. ' lowing way : The wide end of
a funnel, T (Fig. 8^), is covered with a sheet of rubber. The funnel is then filled with carbon dioxide, and the narrow end dipped into water or other liquid, F. The liquid rises against the external pres- sure, because the carbon dioxide diffuses more quickly out- wards through K than air diffuses inwards.
Osmotic Experiments with Liquids. — Dutrochet in 1826 carried out a similar experiment with a liquid. He closed a funnel with an animal membrane, H, and after filling the funnel with copper sulphate solution, dipped it into water. As water passes through the membrane more quickly than
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