FERMENTS AND ARTIFICIAL FERMENTATIONS. 71 



Fig. 35. Tor-ula (Saccha- 

 romyces) cerevisia;, yeast 

 of beer (x 400 diam.). 



Chemists were at first unwilling to admit the 

 important part played by yeast in fermentations, and 

 in order to explain it, they assumed the existence 

 of a very obscure physico-chemical 

 phenomenon, to which the name 

 of catalysis, or action by presence, 

 was given. But in 1843 an illus- 

 trious French chemist, Dumas, 

 clearly explained the physiological 

 function of the living ferment, or 

 yeast. 



" Fermentations," he writes, "are always pheno- 

 mena of the same order as those which characterize 

 the regular accomplishment of the acts of animal life. 

 They take possession of complex, organic substances, 

 and unmake them suddenly or by degrees, restoring 

 them to the inorganic state. Several successive fer- 

 mentations are, indeed, often required to produce the 

 total effect. The ferment appears to be an organized 

 being ; . . . the part played by the ferment is played 

 by all animals, and by all but the green parts of 

 plants. All these beings and organs consume organic 

 substances, multiply and restore them to the simplest 

 forms of inorganic chemistry." 



Finally, Pasteur's memorable labours, which he 

 began to publish in 1857, confirmed the new theory 

 of fermentation, which no one now doubts. Pasteur 

 states that every fermentation has its specific ferment ; 

 in all fermentations in which the presence of an or- 



