A direct result of the oxidizing action of bacteria is the production of heat. Under most conditions of bacterial growth this heat is not appreciable. It may become well marked. The “heating” of manure is one of the commonest 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 “spontaneous 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 disintegrating 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. 74). ABSORPTION OF FREE NITROGEN. [Illustration: FIG. 75.--Root tubercles on soy bean. × 3/7.] 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 that this property is due to bacteria. These plants, and several other kinds as well, have on their roots larger or smaller nodules (Fig. 75) spoken of as “root tubercles” which are at certain stages filled with bacteria. When conditions are favorable, these bacteria live in symbiotic relationship with the plant tissues, receiving carbonaceous and other food material from them and in return furnishing nitrogenous compounds to the plant. This nitrogenous material is built up from free nitrogen absorbed from the air by the bacteria. The utilization of this peculiar property through the proper cultivation of clover, alfalfa, soy beans and other legumes is one of the best ways of building up and maintaining soil fertility in so far as the nitrogen is concerned. The technical name of these bacteria is _Rhizobium leguminosarum_. [Illustration: FIG. 76.--Free-living nitrogen absorbing bacteria “Azotobacter.” Note their large size as compared with other bacteria shown in this book.]