The origin of these purely non-nitrogenous compounds was usually sought in the sugar of the liquid fermented, from which they were thought to be formed by the yeast itself or by the agency of bacteria [Emmerling, 1904, 1905; Pringsheim, 1905, 1907, 1908, 1909], whilst others traced their formation to the direct reduction of fatty acids. Felix Ehrlich has, however, conclusively shown in a series of masterly researches that the alcohols, and probably also the aldehydes, contained in fusel oil are in reality derived from the amino-acids which are formed by the hydrolysis of the proteins. The close relationship between the composition of leucine, (CH{3}){2}·CH·CH{2}·CH(NH{2})·COOH, and isoamyl alcohol, (CH{3}){2}·CH·CH{2}·CH{2}·OH, had previously led to the surmise that a genetic relation might exist between these substances, but the idea had not been experimentally confirmed. In 1903 Ehrlich discovered [1903; 1904, 1, 2; 1907, 2; 1908; Ehrlich and Wendel, 1908, 2] that proteins also yield on hydrolysis an isomeride of leucine known as isoleucine, which has the constitution CH{3}·CH(C{2}H{5})·CH(NH{2})·COOH, and therefore stands to /d/-amyl alcohol, CH{3}·CH(C{2}H{5})·CH{2}·OH, in precisely the same relation as leucine to isoamyl alcohol. This suggestive fact at once directed his attention to the problem of the origin of the amyl alcohols in alcoholic fermentation. Using a pure culture of yeast, and thus excluding the participation of bacteria in the change, he found that leucine readily yielded isoamyl alcohol, and isoleucine /d/-amyl alcohol when these amino-acids were added in the pure state [p087] to a solution of sugar and treated with a considerable proportion of yeast [1905; 1906, 2, 3; 1907, 1, 3]. The chemical reactions involved are simple ones and are represented by the following equations:-- (1) (CH{3}){2}·CH·CH{2}·CH(NH{2})·COOH + H{2}O Leucine = (CH{3}){2}CH·CH{2}CH{2}·OH + CO{2} + NH{3} Isoamyl alcohol (2) CH{3}·CH(C{2}H{5})·CH(NH{2})·COOH + H{2}O Isoleucine = CH{3}·CH(C{2}H{5})·CH{2}·OH + CO{2} + NH{3} /d/-Amyl alcohol