I. INTEODUCTIOX 5 



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 com- 

 bustion " occur fairly frequently. Common causes are 



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



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

 in the manufacture of tobacco ; 



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 digested food acts as the combustible and the pro- 

 cess of union with oxygen takes place in the tissues 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 thence returned to the atmosphere. In this case, as in 

 all others, the amount of heat produced is doubtless exactly propor- 

 tional to the amount of the food materials oxidised, though its measure- 

 ment 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 state. In combination, nitrogen occurs but rarely, save in 

 substances which owe their origin to animal or vegetable life. Indeed, 

 of purely mineral substances containing it we know very few, if any. 

 The deposits of nitrates, e.g., of sodium and potassium nitrates, which 

 are found in certain hot climates and which are largely used as sources 

 of combined nitrogen, have almost certainly been formed by the same 

 agencies which produce nitrates in all fertile soils bacteria and prob- 

 ably from the same sources organic nitrogenous bodies. Unlike 

 other elements, nitrogen appears to occur only on the outermost parts 

 of our globe, i.e., either in the atmosphere or, if underground, within 

 a very short distance of the surface of the earth, the only noteworthy 

 exception to this being the occurrence of coal and carboniferous shale, 

 which usually contain about 1 per cent of combined nitrogen. It must 

 be remembered, however, that these deposits are of vegetable origin and 

 were formed at the surface. The properties of free nitrogen are well 

 known ; it is a colourless, odourless gas, possessing little chemical 

 activity, taking part in very few of the changes occurring in the at- 

 mosphere. Indeed its most remarkable characteristic is its general 

 chemical inertness. It accompanies oxygen in all the multitudinous 

 processes in which the latter takes such an active part and, in most in- 

 stances, escapes unchanged. Only by very extreme means can it be 

 caused to combine with other substances and usually heat is absorbed 

 by the act of union. 



The compounds of nitrogen, unlike the element itself, are extremely 

 active chemically and many of them are of great importance. 



All organisms, whether plant or animal, require nitrogenous com- 



