CHAPTER X. 



MICROBES, MOULDS, AND YEAST. 



The surface of the earth, and, especially, the upper 

 layers of the soil may be considered as the chief store- 

 house of bacteria or microbes. They seem always present 

 in air, but are least numerous after heavy rain. Even 

 at 10,000 feet altitude in Mexico, Dr. Weinzierl found 

 a ten minutes' exposure to the air produced colonies of 

 bacteria on his gelatine plates. They are found almost 

 everywhere, in water ; on the surface of plants ; on the 

 petals of flowers ; on coins and banknotes ; upon the 

 skin of animals ; even on the lips of human beings. 

 Some of them are comparatively harmless ; some are, 

 indeed, very useful ; but, on the other hand, a few are 

 capable of decimating a continent when given an oppor- 

 tunity to do so. Not only is this the case, but animals 

 carry about within their bodies germs of many different 

 kinds. Some of these seem absolutely essential to 

 animal life. Digestion, for instance, is probably carried 

 on by the help of Sarcina ventriculi; and it is said that 

 animals fed on sterilised food invariably die in a short 

 time. 



The soil bacteria are of very great importance in 

 agriculture. Some of these minute plants break up or 

 decompose the bodies of animals and of plants, and so 

 render the waste products available. Even those bacteria 

 most dangerous to man have their importance, because 



