20 Proceedings. 



by which plant -life is mainly sustained, while others are inti- 

 mately concerned in the production and spread of infectious 

 diseases. 



I have in the first place to speak of Fermentation. This 

 term is primarily used to express that process by which 

 solutions of sugar, glucose, and maltine are converted into 

 alcohol and carbonic acid ; a change which is due to the 

 presence and growth of a minute plant, the Torula cerevisia or 

 Yeast Fungus. This plant consists of round or oval cells 

 filled with protoplasm, having some small granules dispersed 

 through it. These cells in growing produce an outgrowth or 

 bud, filled with the same contents, growing to the same size, 

 and budding in the same way as the mother- cell. These 

 daughter-cells remain for a time attached to the parent-cell, 

 and then separate from it. In this way the number of cells 

 is very rapidly increased, this increase gomg on till the sugar 

 is decomposed ; while the plants which cause the fermentation 

 become matted together, and, being buoyed up by bubbles of 

 carbonic acid gas, float on the surface as yeast. Besides 

 budding, yeast sometimes, but rarely, increases by the growth 

 of spores ; in this case the contents of the parent-cell become 

 re-arranged, so as to produce one, or oftener two, or four, 

 cells within the parent-cell. 



That fermentation depends on the growth of this fungus is 

 proved by its being present whenever that process is going 

 on — by our being able to set up that process at will by intro- 

 ducing a small quantity of the living plant into a proper 

 saccharine solution ; and again, if we divide such a solution 

 into two parts by a partition of filtering-paper, fermentation 

 will go on in that part of the solution to which yeast has been 

 added, and not in the other, although the liquid passes freely 

 through the paper from one portion to the other. 



I have now to proceed to the subject of Putrefaction. When 

 the dead flesh of an animal is exposed to the air, it grows 

 soft, moist, and emits an offensive smell. These are the 

 obvious signs of putrefaction. If we take the smallest possible 

 portion of the moisture from the surface of this putrefying 

 flesh and place it under a microscope of sufficient power, we 

 shall see in it thousands of particles, which reflect the light 



