52 SECTIONAL ADDRESSES. 



An important systematic advance was made by C. W. Scheele 

 (1742-1786), whose contributions to organic chemistry are almost as 

 important as his discovery of oxygen. Scheele was a pharmacist, and 

 most of the early chemists were trained as such, or as physicians, from 

 the iatro-chemical period onwards. This old connection between 

 chemistry and medicine was, however, hardly a biological one. Joseph 

 Black's work on fixed air and the mild alkalies indeed originated in 

 medicine, from his M.D. dissertation, ' De humore acido a cibis orto et 

 magnesia alba,' but the subsequent developments of Black's work were 

 not biological in character. Again, although Berzelius was trained as a 

 physician, his work had little connection with biology. The use of 

 vegetable drugs, however, led pharmacists to examine the constituents 

 of plants, and thus the foundations of descriptive biochemistry were laid. 

 Scheele investigated a number of organic acids in the wet way. He 

 obtained tartaric acid in 1769, and later benzoic acid by boiling gum 

 benzoin with lime. He first prepared lactic acid (1780) from sour milk, 

 and mucic acid by oxidation of milk sugar. When soon afterwards 

 mucic acid was also obtained from gum tragacanth, it became evident 

 that one and the same substance may be derived from both animal and 

 vegetable sources. Oxalic acid was obtained from the acid potassium 

 salt in Oxalis acetosella, and shown to be identical with an oxidation 

 product of cane sugar. Scheele also obtained citric, malic, and even 

 gallic acid by crystallisation from solution. Of more general biological 

 interest is his discovery of uric acid, of glycerol and of hydrocyanic acid ; 

 the last (acidum berolinense) by heating potassium ferrocj^anide with 

 dilute sulphuric acid. Scheele's discovery that esterification is greatly 

 furthered by the presence of mineral acids later became important in 

 connection with catalysis, but theoretical speculations were foreign to 

 his nature, and he was not greatly concerned with the essential character 

 of acids. Such questions appealed more strongly to Lavoisier, who 

 improved the nomenclature of organic acids and also investigated alcoholic 

 fermentation, a biochemical process which early engaged the attention 

 of chemists. 



Of course both Scheele and Lavoisier benefited biology more by dis- 

 covering and investigating oxygen than by their contributions to organic 

 chemistry. These latter contributions already illustrate two trends in 

 organic chemical research. There were those who, like Lavoisier, were 

 attracted by theoretical questions. Such were Gay-Lussac, Bunsen, and 

 Frankland, who investigated radicles ; Dumas, Gerhardt, and Laurent, 

 who evolved the theory of substitution and of types ; Wurtz, Hofmann, 

 and Williamson, the forerunners of Kekule in establishing the theory of 

 structure, which soon became the common ground of all organic chemists. 

 A knowledge of structure gave a great impetus to organic synthesis, 

 not only in the laboratory, for theoretical purposes, but also in the factory 

 for practical uses ; the manufacture of dyes, of synthetic drugs, of 

 explosives became an important industry. The number of known carbon 

 compounds grew at an enormous rate. In 1883 some 20,000 were regis- 

 tered in Eichter's Lexicon, in 1899 74,000, in 1910 144,000, and the number 

 now is probably not much short of a quarter of a million. More than 

 half of these are derived from coal tar, and only a small proportion occur 



