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,
