SUBSTITUTION is one of the three principal methods employed in examining the chemical composition of organic bodies, and in tracing their relation to other com pounds; the two other methods being those of oxidation and of reduction. Although the term is restricted to organic chemistry, the ordinary method of preparing insoluble inor ganic compounds by double decomposition is in reality a case of substitution of one base or one acid for another. If, for instance, solutions of nitrate of lime and sulpha* of soda are mixed together, the resulting compounds are sulphate of lime and nitrate of soda, in which the lime is substituted for the soda, and the soda for the lime. In some cases an element may be replaced (or, more correctly, displaced) by a compound group; thus, cyanogen, C,N, may take the place of oxygen. as, for example, in the reaction that ensues between hydrocyanic acid and red oxide of mercury, when cyanide of mercury and water are formed, as shown in the equation Hg0 = Hg (C2N) + HO. Similarly, the groups and may often be substituted for hydrogen. In various organic bodies, one or more atoms of hydrogen may be displaced by one or more atoms of chlorine, a fact. which was originally observed by Gay-Lussac , in noticing the action of chlorine on wax. The new product thus formed is almost always analogous in its nature to the compound from which it is produced; thus, accord ing as the substance acted on by the chlorine is an acid or a base, the resulting product is an acid or a base, and the number of atoms is always the same in the original substance and the product. The following examples will elucidate the above remarks: If acetic acid, C411404, be exposed to the action of chlorine, we obtain, according to the duration and modifications of the action, the two compounds, monochloracetic acid,C4H3C104, and trichloracetic acid, C4HC1304, in the former of which, one atom, and in the latter, three atoms of hydrogen, are displaced by a corresponding number of atoms of chlorine.
Hydrochloric ether, may be made to yield the following succession of com pounds, in which a gradually increasing amount of the hydrogen is displaced by chlo rine, until, in the final result, the hydrogen has altogether disappeared. The consecu tive compounds thus resulting from hydrochloric ether, C4H5C1, are (1) chlorinated ether, 04(FI4C1)C1; (2) dichlorinated ether, 04(H:C1,)01; (3) trichlorinated ether, Cl; (4) tetracliloiinated ether, 04(HC14)C1; and (5) sesquichloride of carbon, C4(C15)Cl. "The chlorine," says prof. Miller, "appears to have taken the place of hydrogen in the group without disturbing the relative position of the other elements which enter into its formation; just as a brick in an edifice may be conceived to admit of being removed, while its place- is supplied by a block of wood or of stone, without altering the form or symmetry of the building." Substitutions of bromine and iodine for hydrogen may be effected in the same way as has been shown to occur in the case of chlorine. The study of the artificial formation of organic bases has led to the discovery of many remarkable instances of substitution products. If, for example, bromide of ethyl. is heated in a sealed tube with a solution of ammonia in alcohol, hydrobromate 'of ethylia (or ethylamine) is formed, and on distilling this vapor with hydrate of potash, one of the products is a new base, ethylia, which may be regarded as ammonia, in which one atom of hydrogen has been displaced by one atom of ethyl, 04115. By a sim ilar proceeding, we may successively displace the second and the third atoms of the hydrogen in the ammonia; and we thus obtain two more complex bases, diethylia, C5H and tnethylia,