ABSORPTION OF FREE NITROGEN 



103 



illustrations. The temperature in such cases may reach 

 70°. The heating of hay and other green materials is due 

 chiefly to bacterial action. This heating may lead to " spon- 

 taneous combustion." The high temperatures (60° to 70°) 

 favor the growth of thermophil bacteria, which cause a still 

 further rise. The heat dries out the material, portions of 

 which are in a state of very fine division, due to the dis- 

 integrating action of the organisms. The hot, dry, finely 

 divided material oxidizes so rapidly on contact with the air 

 that it ignites. 



A practical use of heat production by bacteria is in the 

 making of "hot-beds" for forcing vegetables (Fig. 68). 



Fig. 69. — Root tubercles on soy bean. 



Xf. 



ABSORPTION OF FREE NITROGEN. 



This is likewise one of the most important practical 

 activities of certain types of bacteria present in the soil. 

 The ability of plants of the legume family to enrich the 

 soil has been known and taken advantage of for centuries, 

 but it is only about thirty years since it was demonstrated 



