FERMENTS AND FERMENTATION. 197 



vironment is more conspicuous than, and out of all proportion to 

 the change in the agent are in some cases known wo, fermen- 

 tations, and the agent effecting the change is described as a 

 ferment. Some ferments are organized or living, and some are 



B C G 



Fio. 105. Bacillus megaterium (X 600). Spore formation and germination. A, 

 a pair of rods forming spores, about 2 o'clock P.M. B, the same about an hour 

 later. C, one hour later still. The spores in C were mature by evening ; the one 

 apparently begun in the third upper cell of A and B disappeared ; the cells in 

 which did not contain spores were dead by 9 P.M. D, a five-celled rod with three 

 ripe spores, placed in a nutrient solution, after drying for several days, at 12.30, 

 P.M. E, the same specimen about 1.30 P.M. F, the same about 4 P.M. G, a pair of 

 ordinary rods in active vegetation and motion. (After De Bary.) 



unorganised or lifeless. Of the former the vinegar bacterium 

 and yeast are good examples. Of the latter the digestive fer- 

 ments, like pepsin, ptyalin, and trypsin, and certain vegetal 

 ferments, like diastase of malt are familiar instances. 



As a rule the bacteria seem to prefer neutral or slightly 

 alkaline nitrogenous foods. They therefore decompose more 

 readily meats, milk, and substances (such as beef-tea) made of 

 animal matters ; less readily acid fruits, timber, etc. If in the 

 course of their activity they decompose meats, or fish, eggs, etc., 

 with the production of evil-smelling gases or putrid odors, the 

 process is known as putrefaction. Rarely, bacteria invade the 

 animal (or plant) body and act upon the organic matters which 

 they find there. In such cases disease may result, and the 

 bacteria concerned are then known as disease germs. 



But while bacteria appear to prefer highly organized nitrog- 

 enous (proteid) food, they are by no means dependent upon it. 

 Experiments have shown that many . species can thrive upon 

 Pasteur's fluid, a liquid containing only ammonium tartrate and 

 certain purely inorganic substances ; and one bacterium, at least 

 (the "nitrous"), according to Winogradsky, can thrive upon 

 ammonium carbonate. If this proves to be true for other spe- 

 cies, it will show that bacteria can not only obtain their nitrogen 

 from the inorganic world, but their carbon also. Enough has 



