Page:Elementary Text-book of Physics (Anthony, 1897).djvu/232

218 was announced by Dalton that the quantity of vapor which saturates a given space, and consequently the maximum pressure of that vapor, is the same whether the space be empty or contain a gas. Regnault has shown that, for water, ether, and some other substances, the maximum pressure of their vapors is shghtly less when air is present.

204. The Vapor Pressure of Solutions.—The pressure of the saturated vapor formed from an indifferent solution, or one which is not an electrolyte, is always less than the vapor pressure of the pure solvent. Raoult discovered that the diminution of vapor pressure is proportional to the concentration, provided the solutions are very dilute and that, for any one solvent, the diminution of vapor pressure is the same, whatever be the dissolved substance, provided the solutions are equimolecular, .that is, contain the same number of molecules in equal volumes of the solutions. It may be shown on theoretical grounds that the diminution of vapor pressure depends upon the density of the vapor and the osmotic pressure and density of the solution.

Solutions which are not electrolytes, or which are not indifferent, exhibit a diminution of vapor pressure proportional to the concentration, but the amount of change is greater than in indifferent solutions. This difference is explained by assuming a partial or complete dissociation of the molecules of the dissolved substances into their constituent ions (§285).

205. Ebullition.—As the temperature of a liquid rises, the pressure which its vapor may exert increases, until a point is reached where the vapor is capable of forming, in the mass of the liquid, bubbles which can withstand the superincumbent pressure of the liquid and the atmosphere above it. These bubbles of vapor, escaping from the liquid, give rise to the phenomenon called ebullition, or boiling. Boiling may, therefore, be defined as the agitation of a liquid by its own vapor.

Generally speaking, for a given liquid, ebullition always occurs at the same temperature for the same pressure; and, when once commenced, the temperature of the liquid no longer rises, no