Affinity, in chemistry, that force by means of which two or more substances unite to form a compound in which the properties of each substance are lost; as, oxygen and hydrogen unite to form water, and hydrogen and chlorine to form hydrochloric acid. We do not know the nature of this force, but it is present to a greater or less extent in all substances. In some elements, such as oxygen and chlorine, it is strong, and these unite to form a large number of compounds; in others, like nitrogen and argon, it is very weak, and these have but few compounds. Elements unite only in definite proportions, as atom for atom in the case of hydrogen and chlorine, or two atoms of one to one of the other, as in case of hydrogen and oxygen in forming water. Some elements unite in proportion of three atoms of one to two of another, and so on. The proportions are always the same for the same elements, but they may vary by multiples (See ATOMIC THEORY). The action resulting from chemical affinity usually produces more or less heat. Heat, also, may destroy this force and separate the compound into its elements, as, when steam is passed through a red-hot tube it is separated into oxygen and hydrogen.