MICROBES, MOULDS, AND YEAST. 20/ 



be seen, therefore, that about 82 per cent, of the 

 impurities are thus removed ; the resulting liquid has 

 no offensive odour, and is, in fact, so pure that fish can 

 be preserved in it. 



It is, however, those bacteria which cause infectious 

 diseases that are the most generally known and have 

 been most carefully studied. Many kinds of bacteria, 

 as has been said, are constant inhabitants of the body 

 of man and other animals. Some of them are quite 

 harmless ; a few of them, such as the Sarcina and some 

 of the many bacterial forms found upon the teeth, 

 may be of great importance, and even of real assistance. 

 Many others, however, are exceedingly harmful, and 

 are the cause of a great number of infectious or other 

 diseases, from toothache to typhoid. 



The human body contains a number of free wandering 

 cells, the Phagocytes. It is the function of these cells 

 to enclose or surround bacteria with their own bodies 

 and to digest them. If, however, the patient is unable 

 to produce a sufficient number of these defending pha- 

 gocytes to check the increase of the disease bacteria, 

 these latter multiply to such an extent, that the whole 

 system is thrown into disorder. In the body, the bac- 

 teria not only devour valuable substances, that is, sub- 

 stances the presence of which is necessary to health ; 

 but they also excrete into the body their own waste 

 matters; and these may be of a poisonous nature, 

 poisonous even to themselves. Inoculation, which is 

 now adopted as a preventive against many diseases, 

 depends upon the possibility of cultivating bacteria of 

 any kind in a pure condition, and under human control. 

 With proper precautions a colony of any sort (say 

 "Fowl-cholera") may be isolated, and grown, in a test- 

 tube on sterilised {i.e. bacterium free) gelatine, or on other 



