This change can be effected in the laboratory only by heating with hydriodic acid. Biologically it has been observed [E. and H. Salkowski, 1879] when aspartic acid is submitted to the action of putrefactive bacteria, and almost quantitatively when /Bacillus coli communis/ is cultivated in a mixture of aspartic acid and glucose [Harden, 1901]. In this case a well-defined source of hydrogen exists in the glucose, which when acted on by this bacillus yields a large volume of gaseous hydrogen, which is not evolved in the presence of aspartic acid. Some such source is also available in the case of yeast, although it cannot be chemically defined, for this organism is known to produce many reducing actions, which are usually ascribed to the presence of reducing ferments or reductases in the cell. A similar action would convert glutamic acid, COOH·CH{2}·CH{2}·CH(NH){2}·COOH, into glutaric acid, COOH·CH{2}·CH{2}·CH{2}·COOH, which also is found among the products of fermentation, whilst the monamino-acids would pass into the simple fatty acids. On submitting these ideas to the test of experiment, however, Erhlich found that the addition of aspartic acid did not in any way increase the yield of succinic acid, and that of all the amino-acids which were tried only glutamic acid, COOH·CH{2}·CH{2}·CH(NH{2})·COOH, produced a definite increase in the amount of this substance. Further experiments showed that glutamic acid was actually the source of the succinic acid, the relations being quite similar to those which exist for the production of fusel oil. Succinic acid is formed whenever sugar is fermented by yeast, even in the absence of added nitrogenous matter, and amounts to 0·2 to 0·6 per cent. of the weight of the sugar decomposed, its origin in this case being the glutamic acid formed by the autolysis of the yeast protein. When some other source of nitrogen is present, such as asparagine or an ammonium salt, the amount falls to 0·05 to 0·1. If glutamic acid be added it rises to about 1 to 1·5 per cent. but falls again to about 0·05 to 0·1 when other sources of nitrogen, such as asparagine or ammonium salts, are simultaneously available, either in the presence or [p091] absence of added glutamic acid. As in the case of fusel oil, the production does not occur in the absence of sugar, and is not effected by yeast-juice or zymin. The chemical reaction involved in the production of succinic acid differs to some extent from that by which fusel oil is formed, inasmuch as an oxidation is involved:-- COOH·CH{2}·CH·CH(NH{2})·COOH + 2O = COOH·CH{2}·CH{2}·COOH + NH{3} + CO{2}. From analogy with the production of amyl alcohol from leucine, glutamic acid would be expected to yield γ-hydroxybutyric acid:-- COOH·CH{2}·CH{2}·CH(NH{2})·COOH + H{2}O = NH{3} + CO{2} + COOH·CH{2}·CH{2}·CH{2}·OH.