1889.] on Yeast. 575 



but there is probably a difference in their molecular grouping which 

 influences their respective actions towards polarised light. If 

 ordinary yeast — a member of the family of sprouting fungi — be 

 submerged, under proper conditions, in a mixture of equal quantities 

 of these two glucoses, it will be found that it is capable of dis- 

 tinguishing the one from the other, and of exercising a selection with 

 respect to assimilation ; for it will entirely decompose the dextrose 

 before attacking the l^evulose. Indeed, if the fermentation be stopped 

 at the proper jDoint, it is a very good way of obtaining Isevulose. 

 This is by no means the only case in point, as will be seen by reference 

 to the following table : — 



Selective Action of some Sprouting Fungi for Carbohydrate Food. 



Saccharomyces cerevisiee .. Can invert cane sugar, and subsequently select 



dextrose from Isevulose. Can ferment maltose. 

 Cannot decompose malto-dextrin. 

 „ pastorianus ..\ Can invert sugar, ferment maltose, and decom- 



„ ellipsoideus .. / pose malto-dextrin. 



„ exiguus.. .. Cannot ferment maltose. Can ferment glucose. 



„ ajDiculatus .. Cannot ferment maltose. Cannot invert cane 



sugar. Can ferment dextrin and Isevulose. 



Torula Can ferment maltose and glucose. Cannot invert 



cane sugar. 



Monilia Candida Cannot invert cane sugar, but can ferment it 



without previous conversion into glucose. 



In order that yeast, or any fungus capable of exciting alcoholic 

 fermentation, may flourish and reproduce, it is necessary that it shall 

 have an available supply of oxygen. This has to discharge a duty 

 which Pasteur deems analogous to respiration. If it is deprived of 

 oxygen, and is not introduced into a fermentable medium, the yeast 

 will not reproduce, but in a short time will become weak and shrivelled ; 

 but if, before it is too late, a supply of oxygen be introduced, the 

 exhausted organism will revive and again be capable of manifesting 

 all its vitality. 



If the alcoholic ferment be deprived of a sufficiency of oxygen, and 

 in such circumstances be kept immersed in a medium containing suit- 

 able carbohydrate in solution, it will decompose the latter, in order, 

 among other things, to obtain the necessary oxygen. This decom- 

 position will result in the production of alcohol and carbonic acid gas 

 in strictly definite proportions. 



2C,H,,06 = 4aH,0 + 400^. 



(Glucose) (Alcohol) (Carbonic 



anhydride) 



On looking at this equation, one fails to see the available oxygen, 

 although it may be that the decomposition itself suffices for the 

 requirements of what has been termed " the respiration " of the 

 yeast. But what is more likely is, that it is furnished by other 

 decompositions by which the reaction is always attended. Thus 

 Pasteur has proved that in addition to alcohol and carbonic anhydride, 



