The Nineteenth Century
In the nineteenth century, chemistry came of age as a science, both in terms of experimental procedures and scientific theory. On the practical side, scientists developed techniques to isolate and purify single compounds from natural extracts using such techniques as crystallization, solvent-solvent extraction and distillation. The active principle is the compound that is chiefly responsible for the biological effect of a plant or herbal extract. Other active principles isolated in the nineteenth century included nicotine, strychnine, caffeine, emetine, colchicine, codeine, atropine, and physostigmine. As active principles were isolated, chemists attempted to identify their structures. For the best part of the 19th century, medicines were derived from natural sources. As chemists carried out reactions on isolated active principles, a series of analogs were synthesized, some of which had useful pharmacological activity and which could also be used in medicine.