386 LECTURE XXIV. 



the case of Yeast, which decomposes sugar into alcohol and carbonic acid, glycerine, 

 and succinic acid. Naegeli found ^ that i gram of bottom-Yeast (dry weight) in 

 a 10 per cent, solution of cane-sugar containing tartrate of ammonia, as food 

 material, and through which air was continuously passed, fermented about 70 gr. 

 of sugar in twenty-four hours at 30° C, the weight of the Yeast itself increasing to 

 more than 2-5 gr. during the twenty-four hours. On the average, therefore, 1-7 gr. 

 of Yeast were effective during twenty-four hours, and decomposed forty times that 

 weight of cane-sugar. For further criticism of this process, into which however we 

 cannot here enter in detail, the additional remark may be made that the 

 volume of a cell of Beer-yeast amounts to something like 10,0^,000 *^^^ of ^ cubic 

 millimetre, which corresponds to a weight of about , 0,000,000 ^^^ of a milligram. 

 Naegeli calculates further that in the fermentation of i kilogr. of cane-sugar (or, 

 after its inversion, of 1-0526 kilogr. of grape-sugar), whereby 0-51 kilogr. of alcohol 

 are produced, 146-6 thermal units of heat appear. The temperature of a fer- 

 menting sugar solution may, if no loss of heat occurs, be raised more than i4^C. 



The greater part of the sugar is broken up in fermentation into alcohol and 

 carbon dioxide. Pasteur showed, however, that about 5 per cent, of the sugar 

 breaks up into succinic acid, glycerine, and carbon dioxide. Although, according to 

 Naegeli, the ferment-action of Yeast is more energetic on the access of oxygen, it 

 nevertheless takes place when oxygen is completely excluded from the fermenting 

 liquid. In the arts this is in fact the common experience : the vats containing the 

 wine-must are provided with special valves at the bung-hole, which allow of the exit 

 of the enormous quantities of carbon dioxide, but of no entrance of oxygen ; and in 

 experiments on a small scale it is also easy to convince ourselves that fermentation 

 proceeds energetically in a sugar solution utterly deprived of oxygen. 



Of the species oi Mucor, the mycelium oi M. racemosus especially breaks up in 

 a saccarine liquid into spheroidal cells, which multiply by budding like Yeast, and 

 which, though to a much smaller extent, effect alcoholic fermentation. If a few 

 spores of Mucor are placed on a perfectly sound apple from a small part of which 

 the skin has been removed, the mycelium of this Fungus grows through the whole of 

 the parenchyma of the apple in a few days ; the latter is thereby converted into 

 a soft and subsequently deliquescent mass, the odour and taste of which demonstrate 

 the formation of alcohol and ethereal oils. 



A peculiar form of fermentation is produced by the putrefactive Fungi (Schizomy- 

 cetes — Bacteria). These are much smaller than the cells of Yeast, and are spheroidal 

 or rod-shaped, and remarkable chiefly in that they also decompose saccharine solu- 

 tions, in the presence of proteid matters, in a manner which differs according to the 

 species of Fungus. They convert grape-sugar into lactic acid, and the lactic acid into 

 butyric acid, with the formation of carbon dioxide and hydrogen ; and if some grape- 

 sugar still remains over in this fermentation, it is converted into mannite. These 

 decompositions also may run their course without direct co-operation of the oxygen 

 of the atmosphere. In the formation of acetic acid from dilute alcohol, by means of 



' Naegeli's comprehensive treatise (' Theorie der Gährimg^) appeared in Abhandl. der kgl. 

 bayr. Akad. d. Wiss., B. XII. Abth. 2 (München, 1879), ^^id is indispensable to any one interested 

 in Fermentation. 



