Henceforward discoveries and theories based on them, or propounded to explain them, so crowd the field that even in bulky volumes the story is only told in outline. But several of the famous theories or laws or expositions, on which modern chemistry relies, have been so fertile in consequences that they must be very briefly mentioned.
Substitution.
Before 1840 the famous French chemist J. B. A. Dumas developed the theory of substitution, or “metalepsy,” showing that the hydrogen atoms in organic substances can be removed one by one from their molecules, other atoms being substituted for them. A simple illustration of this process is manifest in the action of potassium on water, though this is not an example of organic substitution. The water, H2O takes up one atom of potassium, K, in place of one of its hydrogen atoms, becoming caustic potash, KOH. It is further possible by an indirect method to replace the remaining hydrogen atom by another of potassium, yielding potassium oxide, K2O. Changes of organic bodies are always proceeding on these lines, and Frankland said the recognition of the process had contributed more to the progress of the science than any other generalisation.
Homologues.
About 1850 C. F. Gerhardt, one of Liebig’s pupils who settled in France (and died in 1856 at the age of 40), gave the next great impetus to the development of organic chemistry, or the chemistry of carbon compounds, 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.
Valency.
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:
each carbon atom having three hydrogen atoms attached to it, the fourth bond uniting it with the other carbon atom. This and other difficulties led to the theory of