~Definitions.~ When a substance disappears in a liquid in such a way as to thoroughly mix with it and to be lost to sight as an individual body, the resulting liquid is called a _solution_. The liquid in which the substance dissolves is called the _solvent_, while the dissolved substance is called the _solute_. ~Classes of solutions.~ Matter in any one of its physical states may dissolve in a liquid, so that we may have solutions of gases, of liquids, and of solids. Solutions of liquids in liquids are not often mentioned in the following pages, but the other two classes will become very familiar in the course of our study, and deserve special attention. SOLUTION OF GASES IN LIQUIDS [Illustration: Fig. 30] It has already been stated that oxygen, hydrogen, and nitrogen are slightly soluble in water. Accurate study has led to the conclusion that all gases are soluble to some extent not only in water but in many other liquids. The amount of a gas which will dissolve in a liquid depends upon a number of conditions, and these can best be understood by supposing a vessel B (Fig. 30), to be filled with the gas and inverted over the liquid. Under these circumstances the gas cannot escape or become mixed with another gas. ~Circumstances affecting the solubility of gases.~ A number of circumstances affect the solubility of a gas in a liquid. 1. _Nature of the gas._ Other conditions being equal, each gas has its own peculiar solubility, just as it has its own special taste or odor. The solubility of gases varies between wide limits, as will be seen from the following table, but as a rule a given volume of a liquid will not dissolve more than two or three times its own volume of a gas. _Solubility of Gases in Water_ 1 l. of water at 760 mm. pressure and at 0 deg. will dissolve: Ammonia 1148.00 l. Hydrochloric acid 503.00 Sulphur dioxide 79.79 Carbon dioxide 1.80 Oxygen 41.14 cc. Hydrogen 21.15 Nitrogen 20.03 In the case of very soluble gases, such as the first three in the table, it is probable that chemical combination between the liquid and the gas takes place. 2. _Nature of the liquid._ The character of the liquid has much influence upon the solubility of a gas. Water, alcohol, and ether have each its own peculiar solvent power. From the solubility of a gas in water, no prediction can be made as to its solubility in other liquids.