the mass becomes more and more solid and condensed, its woody character becomes less and less distinct, and there is a gradual loss of the hydrogen and the oxygen.  Doubtless there is a loss of carbon also, for there is an evolution of marsh gas which contains carbon.  But, in this slow fermentation taking place under the water in peat bogs and marshes the carbon loss is relatively small; the woody material does not become completely oxidized, as it does in free operations of decay.  The loss of hydrogen and oxygen from the mass is greater than that of carbon, and the percentage of carbon therefore increases.  This is not the ordinary kind of fermentation that goes on in vegetable accumulations.  It requires special conditions and possibly special kinds of fermenting organisms.  Peat is not formed in all climates.  In warm regions, or where the woody matter is freely exposed to the air, the fermentation of vegetable matter is more complete, and it is entirely destroyed by oxidation.  It is only in colder regions and when covered with water that the destruction of the organic matter stops short of decay.  But such incomplete fermentation is still going on in many parts of the world, and by its means vegetable accumulations are being converted into peat.

This formation of peat appears to be a first step in the formation of denser coal.  By a continuation of the same processes the mass becomes still more dense and solid.  As we pass from the top to the bottom of such an accumulation of peat, we find it becoming denser and denser, and at the bottom it is commonly of a hard consistence, brownish in colour, and with only slight traces of the original woody structure.  Such material is called lignite.  It contains a higher percentage of carbon than peat, but a lower percentage than coal, and is plainly a step in coal formation.  But the process goes on, the hydrogen and oxygen loss continuing until there is finally produced true coal.

If this is the correct understanding of the formation of coal, we see that we have plainly a process in which bacterial life has had a large and important share.  We are, of course, densely ignorant of the exact processes going on.  We know nothing positively as to the kind of microorganisms which produce this slow, peculiar fermentation.  As yet, the fermentation going on in the formation of the peat has not been studied by the bacteriologists, and we do not know from direct experiment that it is a matter of bacterial action.  It has been commonly regarded as simply a slow chemical change, but its general similarity to other fermentative processes is so great that we can have little hesitation in attributing it to micro-organisms, and doubtless to some forms of plants allied to bacteria.  There is no reason for doubting that bacteria existed in the geological ages with essentially the same powers as they now possess, and to some forms of bacteria which grow in the absence of oxygen can we probably attribute the