154 LIFE ON THE FARM. 



dormant in seeds and bulbs. Bacteria can exist in 

 this state for a long time, in many cases for years, 

 and then become active when the necessary mois- 

 ture is supplied. 



In the spore or resistant state, bacteria are float- 

 ing everywhere in large numbers in the dust of the 

 atmosphere. Their minute size makes this pos- 

 sible. The more dust, of course, the more germs. 

 The stirring up of dust on a street, in a house, or 

 in a stable sets afloat millions of bacteria, to settle 

 again after the air becomes quiet. In this manner 

 they are carried from place to place and from 

 object to object. They never rise and float away 

 from a moist or liquid surface. 



HOW BACTERIA MAY BE DESTROYED. 



Not only moisture, but a certain amount of heat 

 is necessary for the growth of bacteria. Too much 

 heat kills them. Boiling from a few minutes to 

 several hours destroys most kinds, but some are 

 able to resist continued boiling in water and are 

 killed only by a much greater heat. The best tem- 

 perature for rapid growth is a little less than mid- 

 way between freezing and boiling. Freezing stops 

 the growth but does not destroy the life of bacteria. 

 A substance in which they are growing may be 

 frozen for a long time, stopping their growth for 

 the period; but, on being thawed, their activity 

 begins anew. 



A great many chemical substances kill bacteria. 



