CHEMISTRY is that branch of natural science which takes cognizance of the changes that bodies undergo when they are influenced by affinity (q v.). Changes that do not alter the nature and properties of substances—such as the falling of a body by gravity, or its expansion by heat—belongs to physics or natural philosophy. In chemical changes, again, the properties of the substances are permanently altered. Thus, when a piece of iron is left exposed to damp air, it is after a while converted into a reddish brittle sub stance (rust), owing to the union with it of the oxygen of the air. Chemistry, then, may be most simply defined as that branch of natural science which considers (1.) The combination of two or more substances to form a third body, with properties unlike either of its components; and (2.) The separation from a compound substance of the more simple bodies present in it: and considering that the steps of the combination and decomposition of substances can never be correctly understood without an intimate knowledge of the properties of substances, it follows that the science of C. must likewise take into notice the description of all the simplest as well as the most complex bodies.
When the science of C. is considered as a whole, including the properties of all the elements or substances, and the combinations and changes which they can under all cir cumstances undergo, it is distinguished by the title of pure, theoretical, or philosophical a Particular departments of C., where the science is confined to the examination of special objects, receive distinctive names; as physical C., or chemical physics, which considers phenomena bordering on natural philosophy and C.; mineralogical C., which takes cog nizance of the composition of minerals; physiological C., which includes the changes which food undergoes in its transit through the animal economy, and the transforma tions that take place in organic substances generally; medical a, which considers the composition and compounding of medicines; agricultural a, which relates to the com position of soils and manures, the ingredients in plants, and the best modes of supplying the food they require, etc. Inorganic 0. takes cognizance of dead matter, and the changes it undergoes, whilst organic C. considers the substances obtained from plants and animals.
C. ranks as one of the arts as well as one of the sciences, and the division of practi cal a comprehends the rules and processes which must be followed, and the mechani cal means which must be resorted to, for the successful prosecution of the art. Practi cal C. is subdivided into analytical C. (q.v.), which is occupied with the separation of simple substances from more complex—as chlorine (C1) and sodium (Na) from the chlo ride of sodium or common salt (NaCI)—and to the estimation of the quantities of the several ingredients; and synthetical a, which has for its object the union of simpler bodies to form more complex—as hydrogen (H) and oxygen (0) to form water (HO). The art of assaying (q.v.) is a department of analytical chemistry. Applied a includes the art of manufacturing the various substances employed in commerce and in domestic life, so far as chemical processes and application are required. It is subdivided into technical a, which relates to everything connected with the arts and manufactures; and pharmaceutical a, which takes cognizance of the substances used in medicine., History.—The Egyptians appear to have possessed the greatest amount of chemical knowledge of all the nations of antiquity. They preserved dead bodies from decay
(see Mummy), fixed colors in silk by means of mordants, prepared many medicines and pigments, as also soap, beer, vinegar, metals and metallic alloys, common salt, vitriol, soda, sat-ammoniac, glass, enamel, tiles, and painted earthenware. The Chinese were very early acquainted with the processes for dyeing, and the preparation of metallic alloys, the fabrication of niter, sulphur, gunpowder, borax, alum, porcelain, verdigris, paper, etc. From the Egyptians, the Greeks and Romans derived what chemical knowl edge they possessed; but they added little or nothing; and when the migration of the northern tribes, and overthrow of the Roman empire, took place, a stop was put for a time to the advancement of all science in Europe. The prosecution of chemical knowl edge was taken up by the Arabs before the 8th c., and was carried on by them and by their European scholars, the alchemists, with the results described under ALCHEMY. The first germs of a real science of C. seem to appear about the end of the 17th and begin ning of the 18th c., in the speculations of Becher (q.v.) and the phlogistic theory of Stahl (q.v.). After this, C. rapidly advanced. In 1718, Geoffrey brought out the first table of affinities; in 1732, Boerhaave published many original experiments on the chemical rela tions of heat and light; in 1724 Hales, and in 1756 Black, published researches on the._ air and atIriforin bodies, showing that the carbonic acid evolved during fermentation, respiration, and by the action of acids on chalk, was different from atmospheric air. In 1754-59, Margraff added to the then known earths—lime and silica—two others, alumnina and magnesia; he also extracted sugar from plants. In 1770, Priestley began to announce his discoveries of oxygen, ammoniacal, hydrochloric, and sulphurous acid gases, etc. In 1773-86, Scheele contributed chlorine, hydrofluoric, prussic, tartaric, and gallic acids; also barytes, phosphoric acid from bones, etc., and gave the first hints regarding a new doctrine of combustion. About the same time Bergman and Cavendish enlarged our knowledge of the gases. Lavoisier, between 1770 and 1794, reorganized much of the then known C., and founded a system of C. which still remains as the skeleton of the science. Berthollet, in 1787, contributed much to the doctrine of affinity, and made researches in chlorine, etc. Fourcroy and Vauquelin advanced organic C.; Klaproth gave many contributions to mineral C. Richter devoted himself to the doctrine of com bining proportion, which was afterwards perfected by Dalton, as noticed under atomic theory (q.v.). The discovery of galvanic electricity by Galvani, and its advancement by Volta, led sir Humpliry Davy, and others, to important researches in the metals and gases. Gay Lussac and Thenard advanced our knowledge regarding organic sub stances and the chemical relations of heat. Berzelius made laborious researches in mineral C., and gave an exactness to this department which is matter of astonishment to the chemists of the present day. He was also the author of the electrochemical theory, which has been almost perfected by the labors of Faraday, De la Rive, Bec querel, etc. Organic C. has latterly advanced most rapidly under the researches of Liebig, Wohler. Mitscherlich, Mulder, Laurent, and others. See ATOM, ATOMIC THEORY, ATOMIC 'VOLUME, ATOMIC WEIGHTS, AFFINITY.