24 The Story of the Bacteria 



setting free moisture and gases, and working 

 the whole stuff over into the vegetable mould, 

 from which the farmer wins a living for us all. 

 Sometimes the farmer deliberately sets the 

 bacteria at work, through the silos in which his 

 crops are made more useful for the food of cat- 

 tle. He keeps them busy in his manure heaps, 

 which they "ripen" for the special needs of 

 the soil. He uses them in making vinegar. 



One of the most important of the chemical 

 elements in the growth of plants, as we shall 

 see more fully later, is nitrogen. In the water 

 and earth certain bacteria manipulate nitro- 

 gen in the most astonishing fashion, getting 

 it finally into proper shape for the uses of plant 

 life. 



It is bacteria which make milk turn sour. 

 Other forms give to butter and to cheese their 

 distinctive flavors. Some of them help the 

 farmer in the ways which we have spoken 

 of and in many others. But there are bacteria 

 which play him false. His fruit decays when 

 they get inside through bruises. They injure 

 his vegetables when they find a chance. His 

 meat and milk and eggs go bad, when the 



