Coal, as is well known, has come from the accumulation of the luxuriant vegetable growth of the past geological ages.  It has therefore been directly furnished us by the vegetation of the green plants of the past, and, in general, it represents so much carbonic dioxide which these plants have extracted from the atmosphere.  But while the green plants have been the active agents in producing this assimilation, bacteria have played an important part in coal manufacture in two different directions.  The first appears to be in furnishing these plants with nitrogen.  Without a store of fixed nitrogen in the soil these carboniferous plants could not have grown.  This matter has already been considered.  We have no very absolute knowledge as to the agency of bacteria in furnishing nitrogen for this vegetation in past ages, but there is every reason to believe that in the past, as in the present, the chief source of organic nitrogen has been from the atmosphere and derived from the atmosphere through the agency of bacteria.  In the absence of any other known factor we may be pretty safe in the assumption that bacteria played 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 conversion of the vegetation of the carboniferous ages into coal was accompanied by a gradual loss of hydrogen and a consequent increase in the percentage 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 fermentation.  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 temperature, 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 the carbon is largely prevented, and the vegetable matter slowly changes its character.  Under the influence of this slow fermentation, aided, probably by pressure,