III.
��OSMOTIC PRESSURE.
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��called attention to the great similarity which exists between the gas pressure and the osmotic pressure of dissolved sub- stances. He expressed this by saying that the gas laws are also applicable to dilute solutions if the gas pressure be replaced by osmotic pressure. The law of Boyle, applicable to all gases at constant temperature, followed by Gay- Lussac's law for the single gases at all temperatures, then by Avogadro's law for all gases at all temperatures, and finally, by van't HofiTs generalisation for all finely dispersed material at every temperature, together form one of the most beautiful series of development in science.
Of all the laws of matter known to us, that of van't Hoflf is one of the most general.
Osmotic Pressure of Gases. — ^Experiments with semi- permeable membranes offer as a rule considerable difficulties, since the pressure equilibrium is only slowly established. The best results are obtained by working with gases. Bamsay (5) carried out the following experiment, first suggested by me : —
Two vessels, A and B (Fig. 7), each provided with a manometer, mi and m^, are separated by a palladium wall, P. -4 is filled with hydrogen and £ with nitrogen, both at atmospheric pressure and ordi- nary temperature; the mano- meters then indicate the same pressure in each vessel. The apparatus, but not the mano- meters, is then heated to 600". Hot palladium has the peculiar property of taking up hydrogen and allo^ang it, but not nitrogen, to pass through. Hydrogen, therefore, can pass from A to B imtil the hydrogen pressure is the same on both sides of the palladium wall — i atmo., if A and B are of equal volume. When the apparatus is now brought back to the ordinary temperature, there will be found in B nitrogen at 1 atmo. pressure and hydrogen at i atmo..
���Fio. 7.
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