CH.C12 CHCI,, and finally CC14; the fourth one being ihe familiar body chloroform. So also acetic acid, C.11.0., by substitution with chlorine, gives GH,C102, C1H.C40., and GHC130,; but here the process stops and can not be carried further. The one remaining atom of hydrogen in acetic acid is differently Combined from the others, and here we begin to see the sort of evidence by which differences of chemical structure may be determined. When we study great numbers of organic com pounds, especially with regard to their pos sibilities of chemical change, we find that cer tain combined atoms behave differently from other combined atoms of like kind. We also discover groups of atoms which can be shifted collectively from compound to compound; clusters which act almost like elements and are perfectly definite in their nature. The group NH, is so much like a metal in its compounds that it is given a distinct name, atninaabon, and is conventionally treated as if it were really a metallic body. CN, cyanogen, resembles chlorine in some of its relations, and such groups as CH,, methyl, and Calls, ethyl, are en countered at every turn. They do not exist separately, but only in combination, and are known as compound radicles. Every such radicle has valency, and this is essentially re sidual in its nature. Thus in methyl, three of the four units of affinity belonging to the carbon atom are satisfied by hydrogen, leaving one unit unemployed. Methyl, therefore, is a univalent radicle, and acts almost as if it were an independent element.
The chemical formula; which we have-so_far considered belong to the class known as em piricaLfg_rinulie; they give the number and kind of atoms in a molecule, but tell nothing as to their arrangement or mode of union. With the conception of valency and a knowledge of radi cles we are now prepared to construct rational or constitutional formula:, and from these it Is possible to infer what sort of changes a given molecule can undergo, and to understand where in isomeric bodies differ. For example, there are two bodies having the empirical formula GH.O• one, methyl ether, is a gas; the other, common alcohol,_ is liquid; the grEat differefice between them is evident at a glance. Their ra tional or structural formulae exhibit the cause of difference as follows: • Ethyl alcohol.
In the first, two methyl groups appear united by an atom of oxygen; in the other, the carbon atoms are directly combined, and the bivalent oxygen connects carbon and hydrogen. The lines which connect the symbols represent units of valency, and the groups CHI in one case and CM. in the other stand for well-known radi ries. For further illustration we may recur to the case of acetic acid, in which, as we have seen, three hydrogen atoms behave different), from the fourth. This condition is shown in the following structural expression: H