3. _Influence of pressure._ It has been found that the weight of gas which dissolves in a given case is proportional to the pressure exerted upon the gas. If the pressure is doubled, the weight of gas going into solution is doubled; if the pressure is diminished to one half of its original value, half of the dissolved gas will escape. Under high pressure, large quantities of gas can be dissolved in a liquid, and when the pressure is removed the gas escapes, causing the liquid to foam or _effervesce_. 4. _Influence of temperature._ In general, the lower the temperature of the liquid, the larger the quantity of gas which it can dissolve. 1000 volumes of water at 0 deg. will dissolve 41.14 volumes of oxygen; at 50 deg., 18.37 volumes; at 100 deg. none at all. While most gases can be expelled from a liquid by boiling the solution, some cannot. For example, it is not possible to expel hydrochloric acid gas completely from its solution by boiling. SOLUTION OF SOLIDS IN LIQUIDS This is the most familiar class of solutions, since in the laboratory substances are much more frequently used in the form of solutions than in the solid state. ~Circumstances affecting the solubility of a solid.~ The solubility of a solid in a liquid depends upon several factors. 1. _Nature of the solid._ Other conditions being the same, solids vary greatly in their solubility in liquids. This is illustrated in the following table: _Table of Solubility of Solids at 18 deg._ 100 cc. of water will dissolve: Calcium chloride 71.0 g. Sodium chloride 35.9 Potassium nitrate 29.1 Copper sulphate 21.4 Calcium sulphate 0.207 No solids are absolutely insoluble, but the amount dissolved may be so small as to be of no significance for most purposes. Thus barium sulphate, one of the most insoluble of common substances, dissolves in water to the extent of 1 part in 400,000. 2. _Nature of the solvent._ Liquids vary much in their power to dissolve solids. Some are said to be good solvents, since they dissolve a great variety of substances and considerable quantities of them. Others have small solvent power, dissolving few substances, and those to a slight extent only. Broadly speaking, water is the most general solvent, and alcohol is perhaps second in solvent power. 3. _Temperature._ The weight of a solid which a given liquid can dissolve varies with the temperature. Usually it increases rapidly as the temperature rises, so that the boiling liquid dissolves several times the weight which the cold liquid will dissolve. In some instances, as in the case of common salt dissolved in water, the temperature has little influence upon the solubility, and a few solids are more soluble in cold water than in hot. The following examples will serve as illustrations: _Table of Solubility at 0 deg. and at 100 deg._ 100 cc. of water will dissolve: At 0 deg. At 100 deg.