CHEMISTRY is that branch of science which deals with the composition of mat ter and the changes which it undergoes through the action of various agencies. In considering this definition, however, it is necessary to distinguish between physical changes and chemical changes. Thus, when water is frozen, it becomes a solid, and the change from the liquid state is a physical one. No alteration has been brought about in the essential nature of the water, as can be shown by melting the ice, when it again be comes water. When, however, a piece of paper is burned, it is changed into ash and a gaseous mixture, and the change which it undergoes is perma nent, its essential nature being altered. In this case, the change is a chemical one, although, like all chemical changes, it is accompanied by physical changes. It is not always easy to distinguish be tween physics and chemistry, as the two sciences come very near to one another, but it can be said that chemistry is con cerned with those phenomena which in volve a change in the molecular struc ture of a substance. The science is divided into two great branches—Inor ganic and Organic. The former includes the study of all substances of a mixed character. The latter formerly con cerned itself with substances of animal or vegetable origin, but this classifica tion is no longer recognized, and it may now be said to cover the chemistry of the compounds of carbons, excluding such obviously inorganic substances as mineral carbonates and some similar compounds.
Particular departments of Chemistry, where the science is confined to the exam ination of special objects, receive dis tinctive names, as Physical Chemistry, or Chemical Physics, which considers phe• nomena bordering on Physics and Chem istry; Mineralogical Chemistry, which takes cognizance of the composition of minerals; Physiological Chemistry, which includes the changes which food under goes in its transit through the animal economy, and the transformations that take place in substances of organized be ings generally; Agricultural Chemistry, which relates to the composition of soils and manures, the ingredients in plants, and the best modes of supplying the food that they require, etc. Two classes of chemical work may be accepted as typical of the science. One is analysis, signify. ing unbinding; and the other is synthesis, or putting together. By the first process the chemist ascertains the composition of a substance; by the second process he forms a substance by bringing together, and combining the constituents. Analysis has been applied to almost all substances that exist on the earth, as well as to meteorites, and it has been found that they are all composed of about 80 con stituents which arc called elements. But of these only 12 enter largely into the composition of the earth.
History.—The Egyptians, of all na tions of antiquity, appear to have had the greatest amount of chemical knowl edge. They skillfully preserved dead bodies from decay, 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, sal ammoniac, glass, enamel, tiles, and painted earthen ware. The Chinese were very early ac
quainted with the processes for dyeing and the preparation of metallic alloys, the fabrication of niter, sulphur, gun powder, borax, alum, porcelain, verdigris, paper, etc. From the Egyptians the Greeks and Romans derived what chem ical knowledge they possessed, but they added little or nothing; and at the migra tion of the northern tribes, and the over throw of the Roman Empire, a stop was put for a time to the advancement of all science in Europe. The prosecution of chemical knowledge was taken up by the Arabs before the 8th century, and was carried on by them and by their European scholars, calling themselves alchemists. The first germs of the real science of chemistry appear about the end of the 17th and beginning of the 18th century, in the speculations of Becher and Stahl. After this chemistry rapidly advanced. In 1718 Geoffrey brought out the first table of affinities; in 1732 Boerhaave published many orig inal experiments on the chemical rela tions of heat and light; in 1724 Hales, and in 1756 Black, published researches on the air and aeriform bodies, showing that the carbonic acid evolved during fer mentation, respiration, and by the ac tion of acids on chalk was different from atmospheric air. In 1754-1759 Margraff added to the then known earths—lime and silica—two others, alumina and mag nesia; he also extracted sugar from plants. In 1770 Priestley began to an nounce his discoveries of oxygen, am moniacal, hydrochloric and sulphurous acid gases, etc. In 1772 Rutherford dis covered nitrogen, and in 1773-1786 Scheele contributed chlorine, hydrofluoric, prussic, tartaric, and gallic acids; also baryta, 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 chemistry, and founded a system which still remains the frame-work of the science. Berthol let, 1787, contributed much to the doc trine of affinity, and made researches in chlorine, etc. Fourcroy and Vauquelin advanced Organic Chemistry. Klaproth gave many contributions to Mineral Chemistry. Richter devoted himself to the doctrine of combining proportion in the molecule afterward perfected by Dalton. The discovery of galvanic electricity by Galvani, and its advance ment by Volta, led Sir Humphrey Davy and others to important researches in the metals and gases. Gay-Lussac and Thenard advanced the knowledge of or ganic substances and the chemical rela tions of heat. Berzelius made laborious researches in mineral chemistry, and gave an exactness to this department which is an astonishment to the chem ists of the present day. He was also the author of the electro-chemical theory, which was almost perfected by the labors of Faraday, De la Rive, Becquerel, etc. Organic chemistry advanced most rap idly under the researches of Liebig, Wohler, Mitscherlich, Mulder, Laurent, and, in more recent years, Bunsen, Men deleef, Ostwald, Van't Hoff, and many others too numerous to mention.