STALL-MANURE AND STRAW. 



123 



emit no disagreeable odor during putrefaction ; this 

 kind of change is called, by way of distinction, 

 fermentation. Animal substances are richest in ni- 

 trogen, and amongst vegetable matters the seed; 

 hence the great difference in the odor, where pota- 

 toes, sawdust, sugar, etc., or flesh, cheese, pease, 

 etc., which have been sprinkled with water, are left 

 standing until they pass into fermentation or pu- 

 trefaction. 



Heat is generated by most chemical processes, 

 and in most considerable degree by those which re- 

 semble combustion. Digestion and respiration have 

 been shown to be such processes ; so also are putre- 

 faction and decay. For this reason, we perceive a 

 visible and spontaneous evolution of heat, wherever 

 considerable quantities of animal or vegetable mat- 

 ters putrefy, decay, or rot. Hence soil rich in humus 

 (for humus must be considered vegetable fibre un- 

 dergoing decay or slow combustion) will always pre- 

 serve a greater amount of warmth than soil which is 

 poor in this ingredient, and this the more, because on 

 account of its dark color it absorbs a larger propor- 

 tion of the rays of the sun than a soil of lighter color. 

 The heating of stall-manure is thus explained at 

 once ; it will be stronger in proportion to the larger 

 masses heaped on each other, and to the abundance 

 of azotized substances they contain, inasmuch as 

 these latter produce a brisker putrefaction ; in the 

 first case, however, the heat is better kept together, 

 and is constantly generated anew, because with in- 



