ON VITAL PRODUCTION 79 



them, so that frozen meat will putrefy if it thaws. This is the 

 rationale of all processes of preservation of meat and other food 

 substances: heating destroys the "germs"; salting and drying 

 and freezing arrest their activities ; and rigorous exclusion of air, 

 dust, and other media which contain the organisms may delay 

 or prevent the putrefaction. Filtration of water through porous 

 earthenware keeps back the germs, and exposure to strong sun- 

 light, or, better still, to the rays from an electric mercury vapour 

 lamp, is said to destroy them. 



The Mode o! Action of Miero-Organisms. — In fermentation and 

 putrefaction much the same chemical processes occur as when 

 food substances are digested in the alimentary canal of an animal. 

 In fact, the micro-organisms form enzymes similar in effect to 

 the enzymes that are found in the stomach and intestine. The 

 enzymes split up proteids into amino-acids and ferment carbo- 

 hydrates, but the processes of disintegration of proteids and 

 carbohydrates go much further in bacterial action than in the 

 digestive operations. These processes are very complicated, and 

 are far from being fully understood. We call the breaking-down 

 of fats and carbohydrates by bacteria, yeasts, and moulds 

 fermentation, and that of proteids putrefaction. Many species 

 of bacteria, etc., are concerned in each process, and the rapidity 

 with which the latter occurs depends on the temperature and 

 other conditions. 



The final results are quite clear and well known. In all cases, 

 and if there is time enough, fats, carbohydrates, and cellulose, 

 such as the vegetable substances in grains, grass, leaves, fruits, 

 woody fibre, etc., are decomposed, with the result that their 

 chemical substance transforms into carbonic acid and water. 

 Putrefaction of proteid substance is accompanied or succeeded 

 by what is called nitrification — that is, other organisms, called 

 " nitrifying bacteria," also play their part. In the end the 

 proteid putrefies to form ammonia compounds, and then the 

 nitrifying bacteria oxidise the ammonia, converting this into 

 nitrous acid and the nitrous into nitric acid. The lime, soda, and 

 potash in the soil or in streams, rivers, etc., combine with the 

 nitric acid to form nitrate, and this is the way in which Chili 

 saltpetre and other natural stores of nitrate have been formed. 



Thus the action of micro-organisms on all dead organic matter 

 is to convert the latter into carbonic acid, water, and nitrate. 

 This is, in general, the fate of the excretions of animals and of all 



