BACTERIA IN NATURAL PROCESSES. 125 



an important part in this nitrogen fixation, and 

 that bacteria must therefore be regarded as the 

 agents which have furnished us the nitrogen 

 stored in the coal. 



But in a later stage of coal formation bacteria 

 have contributed more directly to the formation of 

 coal. Coal is not simply accumulated vegetation. 

 The coal of our coal beds is very different in its 

 chemical composition from the wood of the trees. 

 It contains a much higher percentage of carbon 

 and a lower percentage of hydrogen and oxygen 

 than ordinary vegetable substances. The conver- 

 sion of the vegetation of the carboniferous ages 

 into coal was accompanied by a gradual loss of 

 hydrogen and a consequent increase in the per- 

 centage of carbon. It is this change that has 

 added to the density of the substance and makes 

 the greater value of coal as fuel. There is little 

 doubt now as to the method by which this woody 

 material of the past has been converted into coal. 

 The same process appears to be going on in a 

 similar manner to-day in the peat beds of various 

 northern countries. The fallen vegetation, trees, 

 trunks, branches, and leaves, accumulate in 

 masses, and, when the conditions of moisture and 

 temperature are right, begin to undergo a fer- 

 mentation. Ordinarily this action of bacteria, 

 as already noticed, produces an almost complete 

 though slow oxidation of the carbon, and results 

 in the total decay of the vegetable matter. But 

 if the vegetable mass be covered by water and 

 mud under proper conditions of. moisture and tem- 

 perature, a different kind of fermentation arises 

 which does not produce such complete decay. 

 The covering of water prevents the access of 

 oxygen to the fermenting mass, an oxidation of 



