146 RELATIONS OF SOIL TO HEAT 



If in place of a log of wood we deal with some vegetable 

 matter which more easily undergoes chemical change, the 

 rise of temperature during fermentation and decay may 

 become very perceptible ; we have an excellent example 

 of this in the case of the heating of a damp hayrick or of 

 a silo. For the evolution of heat it is by no means essential 

 that oxidation to water and carbonic acid should take place. 

 An extreme oxidation, having these final products, will indeed 

 produce the whole of the heat which the fuel substance is 

 capable of yielding ; but heat is also produced by fermenta- 

 tive changes in which oxidation plays an insignificant part. 

 In the alcoholic fermentation of sugar, by far the greatest 

 part of the sugar is simply split up into two bodies, alcohol 

 and carbonic acid, no oxidation of the sugar occurring, and 

 the action taking place in the absence of air, yet the pro- 

 duction of heat is well marked throughout the whole opera- 

 tion. The evolution of heat in the stack, the silo, and the 

 hot-hed, is generally the result of fermentive changes. Only 

 a portion of the stable manure or hay is capable of rapid 

 fermentation ; the active production of heat thus soon ceases, 

 and the subsequent changes become very slow, if a secondary 

 action does not occur leading to ignition. 



Nearly the whole of the processes bringing about the 

 natural destruction of organic matter, within the soil or 

 out of it, and whether by means of fermentation or oxidation, 

 are the work of living organisms animals, fungi, yeasts, 

 bacteria; that this is the case is proved by these actions 

 ceasing when the organisms present have been destroyed. 

 As these agents can only work within certain limits of 

 temperature, a rise of temperature beyond a certain point 

 destroys the agent, and brings the work to an end. Thus 



