6 INTRODUCTION. I. 



the temperature down, while in air the conditions are just the 

 opposite. 



A still more extreme case is furnished by the many processes 

 of slow oxidation or combustion which occur so frequently in 

 nature. 



Under ordinary circumstances these processes generate heat 

 only at about the same rate as it is carried away by contact 

 with surrounding objects, consequently little or no elevation of 

 temperature occurs, but occasionally the rate of loss of heat 

 may be greatly diminished, when a decided and, in some cases, 

 a destructive rise of temperature ensues. Under particularly 

 favourable circumstances the loss of heat may be so small that 

 the actual ignition point (that is the temperature at which 

 rapid union with oxygen accompanied by flame occurs) is 

 reached and the mass takes fire. Such cases of " spontaneous 

 combustion" occur fairly frequently. Common causes are : 



1. Slow oxidation of drying oils, as in greasy waste in 



mills, &c. ; 



2. Fermentative changes produced by bacteria, e.g., 



in haystacks, in the manufacture of tobacco, &c. ; 



3. Slow oxidation of certain minerals, e.g., iron pyrites 



in coal ; 



and several others. Those of interest in agriculture will be 

 discussed later. 



Another instance of slow combustion producing only a very 

 slight elevation of temperature is afforded by the respiratory 

 processes of animals. Here the food acts as the combustible 

 and the process of union with oxygen takes place by means of 

 the blood, which absorbs the oxygen from the air in the lungs, 

 the chief product of combustion, carbon dioxide, being carried 

 by the blood to the lungs and there returned to the atmosphere. 

 In this case, as in all others, the amount of heat produced is 

 doubtless exactly proportional to the amount of the food 

 materials oxidised, though its measurement is complicated by 

 many other processes involving heat changes going on in the 

 body. 



Nitrogen is present in large proportion in the air, where it 

 exists in the free etate. In combination nitrogen occurs but 



