Gerhardt's major contributions to chemistry were his concept of homology, his reform of equivalents, his redefinition of acids based upon their displaceable hydrogen, and his new type theory, in which all organic compounds were considered as substitutionary derivatives of either water, ammonia, hydrogen, or hydrogen chloride. This last had a powerful unifying effect on the development of chemical theory. Gerhardt formulated four inorganic types: water H2O, ammonia NH3, hydrochloric acid HCl, and hydrogen H2, from which all organic compounds could be derived. The sulfides, tellurides, oxides, acids, bases, salts, alcohols, ethers, etc., belong to the water type; chlorides, bromides, iodides, fluorides and cyanides to the hydrochloric acid type; nitrides, phosphides, arsenides, etc., to the ammonia type; metallic hydrides and metals to the hydrogen type. More complicated compounds are formed by substitution of radicals for hydrogen in the types. One of the great attributes of Gerhardt's theory was that unknown compounds could be predicted in large numbers by this scheme of classification. Within the space of twenty years organic compounds had been removed from what Wöhler had described as a primeval forest and had been transplanted in decorous straight lines.
DSB; Partington IV, p456; Norman Library of Science, 897.
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