CHAPTER VIII CARBON AND THE HYDROCARBONS

in general, C n H 2 n +2 , and these contain the greatest possible amount of hydrogen. Unsaturated hydrocarbons, containing less hydrogen, are evidently only formed when the increase of the new molecule derived from methane proceeds from one of the other forms of substitution. When the methyl substitution alone takes place in methane, CH 4 , it is evident that the saturated hydrocarbon formed is C 2 H 6 or (CH 3 )(CH 3 ). This is called ethane . By means of the methylene substitution alone, ethylene , C 2 H 4 , or (CH 2 )(CH 2 ) may be directly obtained from CH 4 , and by the acetylene substitution C 2 H 2 or (CH)(CH), or acetylene , both the latter being unsaturated hydrocarbons. Thus we have all the possible hydrocarbons with two atoms of carbon in the molecule, C 2 H 6 , ethane, C 2 H 4 , ethylene, and C 2 H 2 , acetylene. But in them, according to the law of substitution, the same forms of substitution may be repeated—that is, the methyl, methylene, acetylene, and even carbon substitutions (because C 2 H 6 will still contain hydrogen when C replaces H 4 ) and therefore further substitutions will serve as a source for the production of a fresh series of saturated and unsaturated hydrocarbons, containing more and more carbon in the molecule and, in the case of the acetylene substitution and carbon substitution, containing less and less hydrogen. Thus by means of the law of substitution we can foresee not only the limit C n H 2 n +2 , but an unlimited number of unsaturated hydrocarbons, C n H 2 n , C n H 2 n -2 ... C n H 2( n-m ) , where m varies from 0 to n -1, and where n increases indefinitely. From these facts not only does the existence of a multitude of polymeric hydrocarbons, differing in molecular weight, become intelligible, but it is also seen that there is a possibility of cases of isomerism with the same molecular weight. This polymerism so common to hydrocarbon compounds is already apparent in the first unsaturated series C n H 2 n , because all the terms of this series C 2 H 4 , C 3 H 6 , C 4 H 8 ... C 30 H 60 ... have one and the same composition CH 2 , but different molecular weights, as has been already explained in Chapter VII. The differences in the vapour density, boiling points, and melting points, of the quantities entering into reactions, and the methods of preparation also so clearly tally with the conception of