1. Every liquid has its own _boiling_ point which under the same conditions of _pressure_ is always the same. 2. The temperature of the boiling liquid remains at the boiling point until all the liquid is changed into vapor. 3. The boiling point rises with increased pressure and falls if the pressure is diminished. 4. A boiling liquid and the vapor formed from it have the same temperature. On cooling, a vapor will liquefy at the boiling point. [Illustration: FIG. 155.--Distilling apparatus.] 5. The solution of solid substances in a liquid raises its boiling point, additional energy being needed to overcome the adhesion involved in the solution. The boiling point is also affected by the character of the vessel containing the liquid. In glass the boiling point is 101°. [Illustration: FIG. 156.--A vacuum pan.] =187. Distillation of Water.=--Usually when solids are dissolved in liquids the vapor coming from the liquid contains none of the dissolved solid. Thus by evaporating salt sea water, and collecting and condensing the vapor, pure water is obtained. _Distillation_ is the process of boiling a liquid and condensing the vapor formed back again into a liquid. (See Fig. 155.) The liquid to be distilled is placed in vessel _F_ and boiled. The vapor is conducted into the tube _J_ which is surrounded by a larger tube containing cold water. The vapor is condensed on the cold walls of the tube. The resulting liquid is collected in the vessel _R_. Distillation is employed for two purposes: (a) To remove impurities from a liquid (water is purified in this way). (b) Mixtures of different liquids having different boiling points may be separated by distillation. The one having the lower boiling point will be vaporized first. Thus a mixture of alcohol and water, on distillation yields a distillate having a much larger percentage of alcohol than at first. Repeating this process which is called _fractional distillation_ yields alcohol of increasing strength of purity. Distilled liquor such as alcohol, brandy, and whisky are made by distilling fermented liquor, alcohol being made from fermented grains. Gasoline and kerosene are distilled from crude petroleum. Sometimes as in the production of sugar or evaporated milk the object is to remove the water by evaporation in order to obtain the solid material. Since the two substances named are injured by heating, the syrup, or milk is evaporated under reduced pressure in a _vacuum pan_, that is in a boiler from which air and vapor are removed by an air pump. (See Fig. 156.)