A further consequence of this view is that no solid substance is entirely free from gas pressure or entirely in- soluble in a liquid, for it must be assumed that in a certain time some, even if few, molecules leave the solid and pass either into the gas-space or into the dissolving liquid. This conclusion, although impossible to prove experimentally in those cases where, by analytical methods, the presence of dissolved or gaseous substance cannot be recognised, is of extreme importance from a theoretical point of view.
Let us consider more closely a gas in contact with a liquid. A number of gas molecules pass into the liquid until the equilibrium between the gas and the saturated solution is reached. If now the number of gram-molecules in the gas-space be doubled, then in unit time twice as many mole- cules pass into the liquid as before, since the movements of the gas molecules are independent of each other. In order that equilibrium may exist, double as many gram-molecules must leave the solution in unit time as previously. This occurs when the concentration of the solution in gas mole- cules has been doubled. It is easy, therefore, to see that the concentration (partial pressure) of the gas must be proportional to the concentration of molecules dissolved ia the liquid (Henry's law). The general law of distribution can be derived in a similar manner.
Depression of Solubility. — Nemst's method (6) of determining the molecular weight by the depression of the solubility shows the analogy between a solution and a gas. Ether dissolves in water at 0° to such an extent that a solution is obtained which freezes at —3'85^ (about 2-normal). If to this solution a substance be added, like camphor, which is soluble in ether and practically insoluble in water, the vapour tension of the ether and its solubility in water will both be diminished, and both in the same proportion. When 1 gram-molecule of the substance to be investigated is dissolved in N gram-molecules of ether, the relative
lowering of the solubility of the ether is _-, and the freezing;
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