The Smallest of Living Things 241 



with a good dressing of lime. The lime neutralizes or 

 sweetens the acid much as soda sweetens sour milk in 

 the making of biscuits. The lime, besides encouraging 

 the action of bacteria, has several other important uses 

 in the soil that have already been considered in Chapter 

 Twelve. 



It is a good plan, every few years, to put in a crop of Legumes 

 leguminous plants (page 45) such as clover or beans to 

 rotate with the usual crops. The nitrogen-gathering 

 bacteria that live in the nodules on the roots of the 

 legumes will add to the store of plant-food material in 

 the soil. 



Milk, being a suitable food for many kinds of bacteria, Bacteria 

 seldom contains fewer of them than several hundred thou- ^^ 

 sand to a teaspoonful. These bacteria are usually per- 

 fectly harmless, most of them being of a variety that 

 merely causes milk to sour. The reason milk does not 

 sour when it is kept cold is that the bacteria that change 

 the milk sugar into acid are not active at a low tempera- 

 ture. It is often noticed, however, that if it is a little 

 too cool for milk to sour it develops a bad odor and 

 taste. This is due to the fact that the bacteria that 

 bring about decay can work at a lower temperature 

 than those can that cause the souring ; so milk should 

 never be kept long unless at a very low temperature. 



Most of the bacteria of which we have already spoken Bacteria 

 are harmless to us because they live only upon dead ^ are 

 animal and vegetable matter ; but there is a much smaller enemies 

 number that are capable of using living substance to 

 feed upon and so of causing disease in animals and in 

 plants. One kind when it develops in the lungs causes 



