80 Fermentation and Kindred Phenomena. 



Another very interesting point in the history of yeast 

 was, I think, also first brought to light by Pasteur, viz. : — 

 that the yeast cells when introduced into a liquid medium 

 containing oxygen absorb that element with great rapidity, 

 and develop a corresponding quantity of carbonic acid. 

 This is a veritable respiration, exactly resembling the respira- 

 tion of animals. Indeed, it has been proved that this respiratory 

 act of yeast is as energetic, and even more so, than the respira- 

 tion of fishes, which occurs in exactly the same manner, i.e., by 

 the absorption of dissolved oxygen from water. As fermentation 

 can take place in a proper medium without free oxygen, Pasteur 

 appears to have formed the theory that the fermenting character 

 of the yeast cell is due to the power it possesses of breathing at 

 the expense of the sugar, and that the latter's decomposition 

 into carbonic acid and spirit is the consequence of the act by 

 which the oxygen is removed from the sugar. In this case the 

 latter must suffer a far more complex change than is usually 

 supposed. 



From all these considerations we see that yeast is a very 

 simple form of plant life, the spores of which, owing to 

 their minute size and lightness, are widely distributed. We 

 also see that like other plants it requires a definite soil for its 

 growth and nourishment, and also that in growing it gives rise 

 to perfectly definite chemical products which are formed from 

 the nutritive material, viz., sugar. 



Does yeast stand by itself in these respects, or are there other 

 ferments similar to it in general functions ? 



To this question science has given a very decided answer in 

 the affirmative, and has shown beyond doubt that there are 

 almost countless ferments in air, dust, and water, which, while 

 resembling yeast in the nature of their functions, differ from it 

 in several essential particulars. And this leads me to the second 

 division of my subject, viz., the question of spontaneous genera- 

 tion. 



I may introduce this part of my lecture by some extracts 

 from an address by Professor Huxley given some years ago to 

 the British Association. 



" From the earliest times the doctrine prevailed that under 



