CHEMISTRY, historical section.) Secondly, thetic methods may, and very often do, serve to reveal the complex chemical constitution of com pounds of carbon. It is explained under CARBON COMPOUNDS (q.v.) that one of the basal princi ples employed in determining the constitution of compounds is the assumption. itself well founded on facts, that when a given compound undergoes a moderate chemical change, the greater part of each of its molecules remains unaffected: an atom, or a group of atoms, may be replaced by another, but the linking of the remaining atoms is not changed. Thus, by the action of chlorine on acetic acid, the latter may be gradually trans formed into mono-chloracetie acid, di-chloracetic acid, and tri-chloracetic acid, one, two, or three atoms of hydrogen in the molecule of acetic acid being replaced by one, two. or three atoms of chlorine; but the remaining atoms (carbon. hy drogen. and oxygen) are linked in the chlor acetic acids in the same manner as in acetic acid itself. On this principle, the constitution of a compound becomes known if it is produced by synthesis from compounds of known constitution. In this connection it may be well to observe that the definition of the term synthesis must not be taken to mean the production of compounds from their elements directly. Thus, when it is said that ordinary alcohol can be produced syn thetically, it does not mean that alcohol can be made by the direct union of carbon, hydrogen. and oxygen. The synthesis of alcohol consists of the following steps: carbon and hydrogen are caused to unite into acetylene (q.v.) : acetylene is caused to unite with hydrogen into ethylene; ethylene is caused to unite with sulphuric acid to form ethyl-sulphuric acid; and the latter by reaction with water yields alcohol. The two compounds employed, viz, water and sulphuric acid, can also be made by starting with their elements; and so it may be said that ordinary alcohol can he made with nothing to start with but elementary substances, and this is what is meant by 'the synthesis of alcohol from its elements.' Finally,
many synthetic processes have acquired great im portance industrially. In IStIS Graebe and Lie bermann built up by synthetical processes the substance called alizarin (q.v.), which occurs naturally in madder root, and which gives the latter its chief value as a dye-stuff. This arti ficial alizarin is now made on an enormous scale and has almost entirely displaced madder in dyeing and calico-printing. In ISSO Adolf Baeyer accomplished the synthesis of indigo. In 1857 Ladenburg succeeded in the synthesis of coniine, C.,H„N, the poisonous alkaloid found in hemlock. Vanillin, the aromatic flavoring prin ciple of the vanilla bean. has been made arti ficially by several chemists. Antipyrin. a sub stanee much used in medicine. and saccharin. a substance several hundred times sweeter than sugar, are entirely the products of synthesis.
Vet while most of the known carbon compounds can he produced synthetically, some of the most important organic substances. including the pro teins and many carbohydrates, still remain un synthesized, and hence but imperfectly under stool, and must still be obtained ready-made from nature. The synthetic methods employed in the preparation of a large number of carbon compound-5 are mentioned under the special names of those compounds. For further informa tion. consult: Berthelot, La synthi'se chinnque (Paris. 1876; Ger. trans., Leipzig, 1877) ; Louise, Syntlu•se d'hydrocarburcs, d'acetoncs, d'acides, d'aleools. d'ethcrs et do quinones dens In serve aromatique ( Paris, 1885) ; LeIlmann, I'rincipicn der organischen Synthese (Berlin, 1557) ; and especially the excellent work of Elbs, Die syn thrtisrlien Darstellangsmethoden der Kohlenstoff rerbindungen (Leipzig, I839-9I). See CHEM ISTRY ( historical section) and CARBON COM POUNDS.