From the view here taken of the known component parts ofthe soil, we are naturally led to inquire, in what manner thismixture of substances promotes the production of vegetables: inthe solution of which problem, we must resort a little to vegeta-ble physiology. The seeds of plants do not freely germinate,unless they be covered from the light, and from the witheringair; and a regulated warmth is required during this and the suc-ceeding process of vegetation. For these purposes, the soil, asabove described, is admirably adapted. Its particles, separatedby culture, form a light and easy covering, excluding the lightand drought, and admitting the rays of the sun; which it absorbs,retains, and slowly conducts, diffusing a mild heat. As theroots extend, they require a medium through which they can pe-netrate, and at the same time so compact as to embrace themclosely around, and oppose such gentle resistance as to obligethem to ramify more copiously, and form a great number ofmouths for the reception of nourishment. The different ingre-dients above mentioned are well adapted to this purpose. Sandtempers clay, separating its particles, and weakening their cohe-sion : clay fills the interstices of sand, and preserves it in somedegree of compactness. Hence it is, that soils in which sandand clay are mixed in proportions considerably different, may beSometimes equally fertile. Again, the red oxid of iron bindsparticles which might otherwise lie too loose ; and the orangeoxid probably, when not in excess, may frequently tend to keepasunder particles which might lie ton dense without it. Vege-table mould, when it abounds, contributes still more to that hap-py medium, which is the perfection of the soil. The great elas-ticity of air, much of w'hich we have seen is compressed in thesoil, continually actuated by alternate vicissitudes of temperature*must produce a kind of oscillatory motion, which may probablybe favourable to vegetation. But growing vegetables, daily in-creasing in size, must obtain matter of which that increase isformed, which it does not appear they can derive from the soilas above described. The earths form only a very inconsiderablepart of vegetables ; and it is extremely doubtful if any be drawn,from the soil in which they vegetate. The almost imperceptiblemouths of the roots, by which plants receive their nourishment*seem to be very ill adapted to admit particles of gross earth ; andthere are instances on record, of plants growing to a great size*m boxes filled with earth, and that earth suffering only a scarce-ly perceptible diminution of bulk and weight. There is still lessreason to think that vegetables draw the other substances ofwhich they are principally composed from the soil, the composi-tion of all vegetables, as modern analysis has fully ascertained*being charcoal united with two aeriform fluids, inflammable andvital air, in a triple combination. In the nomenclature now used,this charcoal is called carbon ; inflammable air, hydrogen ; and
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