Base, in chemistry, a term used with several applications, varying according to the view taken of the constitution of compounds. As originally used in the exposition of the dualistic hypothesis, it signified the electro-positive oxide, sulphide, etc.; but in the new unitary hypothesis it must be applied to those electropositive elements or compound radicals which can be substituted for the hydrogen of acids. Alkalies and some other metallic oxides were formerly regarded as comprising all the strictly defined bases; but to these are now added a large class of organic substances existing in plants, which with acids form salts, and may be separated by the greater affinity of the acid for stronger bases. These vegetable bases or alkaloids consist of oxygen, hydrogen, and carbon, in combination with a certain proportion of nitrogen. The constant presence of this element has led to the supposition that the salifiable properties of these compounds may be attributed to it. The vegetable bases are usually in white crystals. The few animal bases or alkalies are volatile, liquid, and of oily consistency. The medicinal properties of plants reside in the bases extracted from them.

A crystal of aconitine contains the concentrated strength of numerous plants of the monkshood; and one of morphia combines that of a large quantity of opium, as one of quinine does of Peruvian bark. (See Alkali, Alkaloid, and Salt).