592 VITAL ACTIONS OF THE LOWER FUNGI. 



Employment species of yeast. It is only within the last few years 

 yewrtin 1 * tn &t Hansen has heen able to obtain a completely pure 

 expTrhnents matei 'i a l> ^J using Koch's solid nutrient media, and we 



must await the results obtained by chemical analysis, in 



the experiments made with this yeast. 



Self ferment- Very special attention has been paid by many investigators 

 ation of yeast. to the so . ca n ed "self-fermentation" of yeast. This occurs 

 when large quantities of fresh, active yeast is left to itself in 

 the presence of a plentiful supply of water, insufficient 

 entrance of air, and a favourable temperature (25 to 30 C.). 

 Under these circumstances a large amount of carbonic acid 

 and alcohol are formed, the yeast becomes soft, and gives with 

 warm water an extract containing numerous substances, 

 which must be looked on as products of tissue change and of 

 decay. According to the investigations of Bechamp and 

 Schiitzenberger the water contains albuminoid substances, 

 gum, a ferment rotating to the left, a substance called zymase, 

 pseudo-leucin (with which a varying quantity of sulphur is 

 mixed), tyrosin, butalanin, carnin, xanthin, guanin, sarkin. 

 The substances last named are evidently derived from the 

 decomposition of albuminoid materials ; the production of 

 carbonic acid and alcohol, however, can only be explained 

 either by the presence of f ermentescible sugar in the yeast, or 

 by the ready conversion of some constituents of the yeast cell 

 into sugar ; in that case the substance from which the sugar 

 is derived will belong either to the group of carbo-hydrates, 

 such as cellulose or gum, or to the group of proteid substances. 

 According to Pasteur, material resembling sugar is always 

 present in yeast, this material being very difficult to extract 

 as such, but becoming converted into sugar by mineral acids, 

 for example ; and it is this, along with the cellulose of the 

 cell wall, which furnishes, according to Pasteur's view, the 

 fermentative products which arise in the self-fermentation. 

 Another important view was, however, advocated by Liebig ; 

 he found, in some experiments on the self-fermentation 

 of yeast, such large quantities of alcohol and carbonic acid 

 (8 to 13'5 per cent, of alcohol, of the total weight of the 

 dry yeast), that the whole amount of cellulose and other carbo- 

 hydrates contained in the yeast was not sufficient to furnish 

 this amount of fermentative products ; hence in order to ex- 

 plain the amount of these substances it was necessary to 

 assume the occurrence, to a large extent, of a decomposition 

 of albuminoid substances. Liebig laid the greatest stress on 

 this decomposition of albuminoid substances because he saw in 

 it the really essential and constant process which took place 

 in fermentation ; according to his view, the essence of the 



