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 as it was coming to be termed, by showing how vast numbers of organic compounds could be classified and grouped into homologous series. Starting, for example, with marsh gas, CH4, which is chemically known as methane, he showed how from this type methyl alcohol, CH4O, and formic acid, CH2O2, are formed. Ethane, C2H6, comes next in the series and ethyl alcohol and acetic acid follow just as methyl alcohol and formic acid follow from methane. The addition of CH2 to ethane gives propane; propyl alcohol and propionic acid following; another addition of CH2 results in butane with butyl alcohol and butyric acid; and the next type is pentane, with amyl alcohol and valeric acid in its train. Thus it was perceived that all the multitude of complex bodies included in the organic kingdom were compounded in an orderly system.

The English chemist Edward Frankland next put forward the doctrine of valency. According to this theory atoms possess one, two, three, four, or more links each, and require that number of other atoms of minimum combining capacity to "saturate" them in a molecule. Carbon, for example, is usually considered to be quadrivalent, and as shown in the instance of methane, requires four hydrogen atoms to saturate it. But how is it then that in the case of the next type, ethane, C2H6, the conditions are satisfied? The explanation is that the molecule is arranged in this manner:

H H   |  | H—C—C—H | |   H  H