156 MICROBES, FERMENTS, AND MOULDS. 



CHAPTER Y. 



THE MICROBES OF HUMAN DISEASES. 



I. MICROBES OF AIR, EARTH, AND WATER. 



IT is generally admitted that the large majority of 

 epidemic and contagious diseases which affect men 

 and animals are caused by the introduction of certain 

 kinds of microbes into the organism. In reply to the 

 question how these microbes are introduced into the 

 body, and where they are before entering it, it is easy 

 to show that these microbes exist in immense numbers 

 they or their spores in the air we breathe, in the 

 water we drink, in the ground on which we tread, 

 and whence there rises, whenever it is dry, a fine dust 

 charged with all sorts of germs, which penetrate 

 together with the air into our mouths and lungs. 



For a long while we were almost completely 

 ignorant of the conditions of existence of these 

 microbes when they are in the soil or water. The 

 recent researches of Zopf, a German botanist, tend to 

 show that among the inferior algse termed Bacteria 



