BACTERIA 



263 



they enter our houses to walk about on our food, you 

 can see why we should do all we can to get rid of 

 them. The fly is now com- 

 monly called the " typhoid 



fly." 



If we have pure milk and 

 water, plenty of fresh air 

 and exercise, and keep our- 

 selves and our houses clean, 

 we need not worry about 

 bacteria. 



Useful bacteria. There 

 are very many useful bac- 

 teria. We could not get 

 along without the bacteria 

 of decay. We need the aid 

 of bacteria in making cheese 

 (page 390), vinegar, hay, 

 silage, and many other 

 things. Each year more 

 and more time and money 

 are spent in studying the 

 useful bacteria, and learning 

 how to make the most of 

 their assistance. 



Soil bacteria. Among the 

 most useful bacteria are 

 those of the soil. Some of 



FIG. 155. Bottle containing 

 sterilized gelatin over which a 

 fly was allowed to walk. Each 

 footprint of the fly is marked 

 by a white growth of bacteria. 

 Some bubbles of gas are seen, 

 which show the presence of 

 the colon bacillus. 



