Chemical Types, a term used to designate the characteristics of chemical substances which are supposed to have an analogous molecular architecture, or are built up of elements which, although unlike, bear a certain relation to each other, by reason of which the materials of one part of the chemical fabric may be replaced by others without altering the general structure. Thus, hydrochloric acid, HC1, may be taken as a type of the chlorides in general, which may be regarded as derived from it by substitution; as for example, chloride of potassium, KC1, when the constituents are both monatomic elements; BaCl2, in which barium is diatomic, and demands two atoms of chlorine; and SbCl3, in which antimony (stibium) is triatomic, and requires three atoms of chlorine. The history, of the development of the theory of types may be briefly stated as follows: GayLussac observed that wax bleached by chlorine gave up oxygen and absorbed an equal volume of chlorine. Dumas observed the same action with regard to oil of turpentine, and from other observations he was led to the conclusion that a body containing hydrogen, subjected to the action of chlorine, bromine, iodine, or oxygen, takes up an atom of such element for every atom of hydrogen removed.

In 1839 he arrived at a "theory of types," which may be enunciated as follows: 1. The elements of a compound may, in numerous cases, be replaced in equivalent proportions by other elements, and by compound bodies which play the part of elements. 2. When this substitution takes place in equal numbers of equivalents, the body in which the substitution occurs retains its chemical type, and the elements which have entered into it play therein the same part as the element which has been abstracted. The chemical type included bodies containing the same number of atoms of their elements, and resembling each other in their principal chemical properties, such as chloroform, bromoform, and iodoform. But when the number of elements varied while the number of atoms of the substance remained the same, they were regarded as belonging to the same molecular type, as marsh gas, CH4, formic acid, CH2O2, chloroform, CHCI3, and chloride of carbon, CC14. Berzelius opposed this theory, holding that acetic acid and trichloroacetic acid, which Dumas regarded as belonging to the same type, did not so belong, but that acetic acid is an oxide of a radical, while trichloroacetic acid consists of oxalic acid copulated with chloride of carbon; and so of other substitution compounds.

But to maintain this position it was necessary to invent many radicals for compounds which were evidently analogous; and therefore when Melsens showed that acetic can be produced from trichloroacetic acid by the action of sodium amalgam, the idea that these two bodies were not built upon the same type could no longer be maintained. Gerhardt, by the introduction of his " theory of residues," reconciled the radical and substitution or type theories by supposing that a radical can be substituted for an element in a compound without altering the type; not, however, by direct substitution, but by the formation of a body of elements from each of the bodies brought together, by which residues are formed that subsequently unite. The discovery of the alcoholic ammonia bases by Wurtz and Hofmann in 1850, by which it was shown that the hydrogen in ammonia, NH3, may be replaced by equivalents of the radicals ethyle and methyle, favored the union of the two theories. This conclusion was still further strengthened in the same year by Williamson's explanation of the general relations of the ethers and alcohols by referring them to the type of water, H20. Four principal types are now recognized, which are illustrated by the union of one, two, three, and four molecules of a menatomic element respectively with another monad, dyad, triad, or tetrad, as HH, OH2, NH8, CBU; and nearly all organic compounds may be regarded as formed by a combination of two or more of these types.

The same compound may often be referred to different types. Thus, trichloride of phosphorus, PC13, may be formed from three molecules of hydrochloric acid, 3HC1, by the substitution of one molecule of the triad phosphorus for three hydrogen monads; or from one molecule of ammonia, NH3, by the equivalent substitution of phosphorus for hvdrogen.