Suppose a vessel of one litre capacity to be filled with oxygen gas at 0°, and under the atmospheric pressure of 76 centimeters of mercury. The oxygen will exert pressure on its walls equal to that of the atmosphere, for the vessel may be placed in communication with the atmosphere, in order to equalise pressure, before it is closed. Now let half a litre of hydrogen be introduced by means of a force-pump. As temperature and volume remain the same, the pressure will be increased to 76 cms. + 38 cms. Introduce another half-litre of hydrogen, and the initial pressure will be doubled ; it will now be 152 cms. Let another litre of hydrogen be introduced, and the initial pressure will be trebled. We might introduce a third gas, say nitrogen, into the vessel, and the pressure would be increased proportionately to the quantity introduced. Each constituent of the gaseous mixture, accordingly, exerts pressure on the walls of the containing vessel proportionally to its relative amount. For example, the pressure of the nitrogen of the oxygen and of the argon in air is proportional in each case to the amounts of these constituents, viz., oxygen, about 21 per cent. ; nitrogen, 78 per cent. ; and argon, 1 per cent. This statement is known as Dalton's law of partial pressures. If the pressure of the air is 76 cms., that of the nitrogen is x 76 ; of the oxygen, —~- x 76 ; and of the argon, —Ly x 76 cms. In the case of liquids, however, such a method fails. For while in some instances the volume of a solution is nearly equal to that of the solvent, plus that of the dissolved substance, in others the volume is less, and in a few instances greater.
A device has, however, been discovered, by which it is possible to measure the partial pressure of the dissolved substance ; and again an example will first be given from the behaviour of gases. The rare metal palladium is permeable at high temperatures by hydrogen, but not by other gases. Now, if a vessel made of palladium be filled with a gas that cannot escape through its walls—for example, with nitrogen at atmospheric pressure—equal to that of 76 cms. of mercury —and at a high temperature, say 300° C. ; and if it be then surrounded with hydrogen gas, also at atmospheric pressure, the pressure of the gases in the interior of the vessel will rise to two atmospheres, owing to the entry of hydrogen through the walls, which are permeable to that gas alone. The mercury in the gauge connected with the palladium vessel will rise, until it stands at a height of 76 cms., showing that the original atmospheric pressure has been doubled. As there is no opposition to the passage of the hydrogen inwards or outwards through the walls of the vessel, hydrogen will enter until the pressure of the hydrogen in the interior is equal to that on the exterior of the vessel. But the nitrogen cannot escape, hence it exerts its original pressure of 76 cms. of mercury.