Water may be purified by means of distillation. This consists in boiling the water and condensing the vapor by passing it through a tube which is kept cool by surrounding it with cold water. By means of distillation most substances in solution in water can be eliminated. Substances, however, which evaporate like water, will, of course, pass off with the water vapor. Aboard ship salt water is distilled and thus made fit for drinking. In chemical laboratories ordinary water is distilled in order to purify it for chemical work. USES OF WATER IN CHEMISTRY [Sidenote: Action of water in physiological chemistry] Water is termed by the chemist a stable compound. This means that it is difficult to get it to act chemically. Being thus inactive chemically, we find that water does not combine with most substances. There are exceptions to this, however, especially in physiological chemistry, an instance being that starch combines with water when it is changed to sugar in the process of digestion. [Sidenote: Water as a solvent] Water is the universal solvent. A greater number of substances dissolve in it than in any other liquid. Chemical operations are frequently carried on in solution, that is to say, the substances which are to act chemically upon each other are first dissolved in water. The object of this is to get the substances into as close contact as possible. If we rub two solids together, the particles remain slightly separated, no matter how finely the mixture may be powdered. If, however, the substances are dissolved and the solutions poured together, the particles of the liquid move so freely among each other that they come in direct contact, thus aiding chemical action. In some cases substances which do not act on each other at all when brought together in dry condition, act readily when brought together in solution. There is a limit to the amount of any substance which can be held in solution at a given temperature. [Sidenote: Chemical meaning of solution] The question will probably arise in the mind of the student as to whether a substance dissolved in water has chemically united with the water, or is merely mixed. Solution is in reality a process about half way between mixing dry substances and forming chemical combinations. The chemist considers that the water does not form a compound with the substance dissolved, when he can, by evaporating the water, get the substance back into its original form. IMPORTANCE OF SOLUTION TO THE FOOD SCIENTIST [Sidenote: Relation of solution to assimilation]