1889.] on Yeast 573 



engagement of carbonic acid gas. It is throngh its agency that the 

 production of a light and vesicular bread is ensured. The yeast 

 being brought into intimate contact with carbohydrates suitable for 

 decomposition, effects the generation of carbonic acid gas, which, as it 

 is disengaged, makes an effort to force its way through the dough. 

 This is resisted by the yery plastic gluten of the flonr. The result 

 is to effect what is termed the raising of the '•' sponge." 



It will be unnecessary to remind yon that this decomposition of 

 carbohydrates into alcohol and carbonic acid gas by means of yeast 

 constitutes one phase of the phenomenon of alcoholic fermentation. It 

 is somewhat difficult to ascertain when it was first conjectured that 

 this was due to the growth of yeast ; but it is clear that Caignard de 

 la Tour had formed a strong opinion on the subject more than sixty 

 years ago. for lie wrote, " if yeast could thus ferment sugar, it must 

 surely be by some influence due to its yegetation and its life." Indeed, 

 there are many passages in bis works, as well as in those of that great 

 observer, Schwann, which indicate that about that period a very firm 

 grip had been obtained of the theory of fermentation as now accepted ; 

 but it had to be relinquished, as it clashed with the more speculative 

 views of Liebig and his supporters. So widespread was the influence 

 of this great school, that operations and experimental observations 

 contradictory of its doctrines were so neglected as to be forgotten, or 

 were combated by so strong a combination of intellect as to be prac- 

 tically untenable. 



It was reserved for Pasteur to rediscover the growth of yeast 

 previonsly noted by Caignard de la Tour and Schwann ; to add to 

 their observations by an abundance of fresh investigations : and to 

 formulate a theory of alcoholic fermentation, which, has at least the 

 merit, in contradistinction to others, of being a logical interpretation 

 of accurate experiment ; one, moreover, which, in my himible judg- 

 ment, is quite capable of ultimate reconciliation witb some at least of 

 the more important views of the German chemist. 



Pasteur's experiments placed it beyond doubt tbat tbe decom- 

 position of carbohydrates by means of yeast is neither more nor less 

 than one of tlie manifestations of vitality by yeast, a ftmgoid organ- 

 ism capable of growth and reproduction. Since his results have been 

 published it has been almost proved that this decomposition takes 

 place witbin the organism, and in any case it is proved that Eilcohol 

 and carbonic acid are to be regarded as products excreted dnring 

 phases of its life-history. This being the case, it manifestly becomes 

 necessary to study the biology of the organism, in order to acquire 

 further information respecting the nature of the chemical reactions 

 involved in the decomposition. 



But before doing this, it is advisable, at any rate for the purposes 

 of this discourse, to restrict the signification of the t^rm ferment : 

 otherwise confusion must inevitably result. Even to-day we are 

 accustomed to speak of two classes of ferments — organised and soluble. 



The alcoholic ferment, yeast, for example, is a cryptogam of definite 



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