280 HUMAN PHYSIOLOGY CHAP. 



idly that a single one may produce over sixteen million 

 in twenty-four hours, under favorable conditions. Like 

 fine dust or tiny spores they are carried about every- 

 where by the wind and water. Some have a wonderful 

 power to resist heat, cold, and moisture, and can live 

 under influences which destroy all other forms of life. 



Some kinds of bacteria are the causes of diseases 

 among the lower animals and in man, while the major- 

 ity are among man's most useful agencies in many 

 ways. We shall see how they are useful to man and 

 what relation they have to diseases. 



191. Bacteria as our Friends. Some of these little 

 organisms are very useful to us. All decay or decom- 

 position of vegetable and animal matter is due to the 

 chemical action of bacteria. In this way all dead or- 

 ganic matter is broken up into simpler substances that 

 can be used by the growing plants. They serve as 

 scavengers to clean up all dead organic wastes and at 

 the same time send into the atmosphere a large amount 

 of carbon dioxid which growing plants can use for 

 food. The amount of carbon dioxid exhaled by man 

 and the lower animals is not sufficient to supply the 

 demands of the vegetable kingdom. The plants retain 

 only the carbon and exhale the oxygen which we in 

 turn breathe. The bacteria produce important commer- 

 cial products, as many organic acids, and most of the 

 saltpeter. The bacteria in the soils of field and garden 

 are at work constantly changing and decomposing cer- 

 tain materials and preparing the food supply for the 

 growing crops. The leguminous plants, as clovers, 



