USE OF BACTERIA IN THE ARTS. 49 



difference between them, based upon their method 

 of multiplication, and therefore places them in 

 different classes (Fig. 2, page 19). In their gen- 

 eral power of producing chemical changes in their 

 food products, yeasts agree closely with bacteria, 

 though the kinds of chemical changes are differ- 

 ent. The whole of the great fermentative indus- 

 tries, in which are invested hundreds of millions 

 of dollars, is based upon chemical decompositions 

 produced by microscopic plants. In the great 

 part of commercial fermentations alcohol is the 

 product desired, and alcohol, though it is some- 

 times produced by bacteria, is in commercial 

 quantities produced only by yeasts. Hence it is 

 that, although the fermentations produced by 

 bacteria are more common in Nature than those 

 produced by yeasts and give rise to a much larger 

 number of decomposition products, still their com- 

 mercial aspect is decidedly less important than 

 that of yeasts. Nevertheless, bacteria are not 

 without their importance in the ordinary ferment- 

 ative processes. Although they are of no im- 

 portance as aids in the common fermentative 

 processes, they are not infrequently the cause of 

 much trouble. In the fermentation of malt to 

 produce beer, or grape juice to produce wine, it 

 is the desire of the brewer and vintner to have 

 this fermentation produced by pure yeasts, un- 

 mixed with bacteria. If the yeast is pure the 

 fermentation is uniform and successful. But the 

 brewer and vintner have long known that the 

 fermentation is frequently interfered with by ir- 

 regularities. The troubles which arise have long 

 been known, but the bacteriologist has finally 

 discovered their cause, and in general their rem- 

 edy. The cause of the chief troubles which arise 



