As to coal, it is usually assumed that it requires about 8 parts of vegetable matter for one of bituminous coal. Much higher estimates are made by some, and an observation made by the writer at the Port Hudson bluff, Mississippi, in 1869, would seem to justify such estimates. The above figure, from a sketch made at the time, shows the proportions to which a pine log about eight inches in diameter had shrunk in drying into a small sheet of lignitized wood; the original trunk, projecting from a bed of sand some forty feet below the surface, being so porous and spongy that when wet it flattened somewhat by its own weight; it was connected with the little sheet of lignite by a spirally twisted, tapering stipe. Here evidently the proportion of lignite formed was a very minute one, doubtless because of the long leaching to which the trunk had been subjected. It thus seems impossible, as in the case of humus, to assign any definite proportion as between woody matter and coal formed from it. _Normal humification_ takes place only under the influence of moderate temperature. When the temperature is too low, bacterial and fungous growth are repressed or arrested; when too high, the fungous vegetation assumes a different phase, the result of which is the almost total oxidation of the organic matter, sometimes so accelerated as to initiate rapid combustion “fire-fanging” of dung; leaving in any case but a trifling organic residue of very high ash contents.[44] [44] A striking illustration of this is afforded by Naegeli’s experiment of enclosing several loaves of bread in a loosely closed tin-box. After eighteen months there remained only seventeen per cent of air-dry mouldy matter, totally destitute of starch. _Eremacausis._—In the absence of a sufficient degree of moisture to co-operate with the other agencies of humification, the final result in the soil is practically the same as in the “fire-fanging” of dung. The organic matter is almost wholly destroyed by direct oxidation (eremacausis) with or without the aid of minute organisms; leaving essentially only the ash behind to be reincorporated with the soil. This is to a very great extent the predominant process in the arid regions of the Globe; most of the soils formed in these climates being, therefore, very poor in humus-substances, and deriving it almost entirely from the decay of roots only.