MICROBES. 187 



produced. It can easily be understood that the pres- 

 ence of such a large number of these beings must modify 

 in some manner the composition of the liquid in which 

 they exist, for their development can take place only by 

 the removal from the liquid of the elements required for 

 their formation. This is the secret of animal and vege- 

 table decomposition, of putrefactions, and of fermenta- 

 tions. 



In the acetic fermentation, which is the conversion 

 of wine, cider, or other alcoholic liquid into vinegar, a 

 microbe known as the mycoderma aceti does the work. 

 This is formed of straight microbes, only six one-hun- 

 dred-thousandths of an inch long, and one -third or one- 

 half as thick, a number being joined end to end, and it 

 lives by transforming the alcohol into acetic acid by the 

 aid of the oxygen of the air. In vinegar factories the 

 mycoderma, which forms a whitish membrane on the 

 surface of the wine being soured, is carefully collected 

 and transferred to fresh wine which it is desired to con- 

 vert into vinegar. It is called mother of vinegar. 



We have already said that many contagious diseases 

 are produced by the presence of certain microbes in the 

 blood or in other organs. It has been found that re- 

 peated culture causes the microbes to lose their viru- 

 lence, that is, if the microbe which it is wished to cul- 

 tivate be introduced into a suitable liquid, and the 

 product of this culture be collected and submitted to a 

 new culture, by repetition of the process an attenuated 

 microbe may be obtained which occasions only slight 

 troubles when introduced into the system. Much more, 

 its presence or its effects will prevent the development 

 of the virulent microbe. The attenuated microbe may 

 constitute a vaccine matter which will protect the sys-. 



