At the present day we mean by “organic compounds” simply the compounds of carbon. These are so numerous, and frequently so complex, that it is convenient to group them together and study them as a special section of the science. At the outset it was supposed that only very few elements entered into the composition of organic substances. This, indeed, was held to be a point of fundamental distinction between organic and inorganic compounds. Lavoisier was of opinion that all organic bodies were combinations of carbon, hydrogen, and oxygen. Berthollet first discovered the presence of nitrogen in a product of animal origin. Sulphur and phosphorus were detected later. There is apparently no _à priori_ reason why any element should not be associated with carbon, and enter into the composition of an organic compound. Lavoisier was one of the first to devise methods for ascertaining the composition of organic (carbon) compounds, and to indicate the general principles by which the proportion of the elements met with in these substances can be ascertained. So imperfectly, however, were these methods worked out that it was not established until the close of the first decade of the nineteenth century that organic compounds even obeyed the law of multiple proportions. Thanks to the efforts of Berzelius, Gay Lussac, and Thénard, and especially of Liebig, in 1830, methods of organic analysis were so far perfected that it became possible to ascertain the empirical composition of these compounds with certainty. This point reached, the development of this section of chemistry proceeded with unexampled rapidity. Not only was the composition of numbers of products, such as sugar, starch, the vegetable acids, certain alkaloids, etc., established, but altogether unlooked-for facts became manifest. One of the most surprising of these was that of _isomerism_. Up to the close of the first quarter of the nineteenth century it seemed self-evident that substances of the same percentage composition are necessarily identical. In 1823 Liebig showed that the silver cyanate of Wöhler had the same composition as silver fulminate. Faraday, in 1825, found a hydrocarbon in oil gas, which had the same composition as olefiant gas, but was otherwise different from it; and in 1828 Wöhler discovered that urea and ammonium cyanate—perfectly dissimilar substances—were identical in elementary composition. Lastly, Berzelius found this to be true of tartaric and racemic acids; and he thereupon proposed the term _isomerism_ to denote the general fact. He further pointed out that the phenomenon could only be explained by supposing that the relative positions of the atoms in isomeric compounds are different.