HUMUS.

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saline substances which they naturally contain, andwhich not being subject to decay, are gradually washedout by the rains during the decomposition of the organicsubstances, and are ready to be absorbed by the roots ofgrowing plants.

315. The decay of the vegetable remains in the soilnever proceeds so far that the whole of the elementswhich they contain are converted into carbonic acid andwater. The first change which they undergo, whenexposed to air and moisture in circumstances favourableto decay, is a diminution of the quantity of oxygen andhydrogen, and, likewise, though in smaller relative pro-portion, of the carbon which they contain.

316. The result of this is a change of colour, some-what similar to that produced by slightly burning orsingeing them by heat: they become more or less brown,according to the degree to which this change has pro-ceeded. Whilst in this state, they constitute what iscalled humus, the name by which chemists distinguishall kinds of vegetable matter when in a state of decay,and more especially woody fibre, which indeed usuallyconstitutes a very large proportion of the bulk of plants.

317. This process of decomposition proceeds slowlyfor a considerable time, the relative proportion of theoxygen and hydrogen to the carbon gradually decreasing,carbonic acid and water being all the time formed, until,at last, the humus ceases to undergo any further change,and then it is termed mould by chemists.

318. In order to understand this process, which is one