3+ LAWS OF AVOGADRO AND VANT HOFF. chap.
the palladium, so also in Pfeffer's experiment the pressure of the water is the same in A and B, The excess of pressure in J^ is in this case due to sugar, just as in the former case it is due to nitrogen. We generally conceive gas pressure as due to the impact of the molecules against the walls of the containing vessel ; in the same way the osmotic pressure of the sugar may be imagined to be due to impacts of the sugar molecules against the membrane. The impacts of the molecules of a substance exert the same action whether the substance be in the gaseous or dissolved (liquid) state.
It is, however, quite unnecessary to rely on the kinetic view. It is well known that a gas tends to expand so as to fill the volume placed at its disposal, and this tendency evidences itself in the pressure.
The dissolved sugar has a similar tendency to become evenly distributed over the solvent, water, and the measure of this is the osmotic pressure. This expansion tendency of gaseous and dissolved substances at the same temperature, and with the same number of molecules in unit volume, is the same for all substances; it increases directly with the absolute temperature and with the concentration.
From the preceding examples it will be seen what is meant by a semi-permeable membrane. It is a medium which is capable of taking up one component of a (gaseous or liquid) mixture and holding the other back. As a rule one of the components is water, the other a dissolved substance. The envelopes of protoplasm, copper ferro- cyanide, Prussian blue, etc., take up water, but not substances dissolved therein ; palladium dissolves hydrogen, but not nitrogen; caoutchouc dissolves carbon dioxide, but not (in appreciable amount) hydrogen.
The above definition of a semi- permeable wall con^esponds with two cases, the meaning of which we now come to. One case is the vacuum or a gas : the water may be taken out of a sugar solution in the form of vapour, but the sugar remains, being practically non- volatile. The other case is ice. If water
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