SOURCES OF SOIL HEAT 147 



it is well known that sweet silage is produced by allowing 

 the mass of green matter to heat in the first place to a tempera- 

 ture at which the bacteria producing lactic acid are destroyed, 

 and then checking the action. Both in the silo and in the 

 hot-bed it is however quite easy to obtain temperatures 

 considerably exceeding those at which most organisms perish. 

 It is now known that a special class of bacteria exists in 

 the surface soil, and is probably widely distributed, which 

 is capable of living and performing energetic work, at 

 temperatures distinctly exceeding 70 C. (158 F. 1 ). These 

 bacteria doubtless take an active share in the chemical 

 changes producing heat both in the hot-bed and in the 

 silo. 



Dybowski (Annales agronomiques, 1887, 268) has made 

 careful experiments on the course of the development of heat 

 in hot-beds containing a 2 ft. layer of various materials. The 

 highest temperature, 75 C., was obtained from horse manure ; 

 the lowest, 37C., from a mass of mixed dead leaves. For 

 the maximum heat to be obtained the material must be in 

 a fresh state. 



Cases sometimes arise in which the actual ignition of 

 a haystack or manure heap occurs. The rise of tempera- 

 ture above 75 is clearly the result of chemical oxidation, 

 carried on without the aid of living organisms. We must 

 assume that fermentation has produced substances which at 

 the temperature of the rick are rapidly oxidized if air can 

 obtain access, and that the production of heat is thus 

 carried to the point of actual combustion. 



1 These high-temperature bacteria are readily separated from the other 

 organisms in soil by inoculating with soil tubes of broth maintained at 65 C. 

 Under these circumstances only the high-temperature organisms will develop. 



L 2 



