Cl
obtaining it in sufficient abundance. Clay and sand are thechief ingredients of the soil. Clay is the lightest of the earths,being only twice the weight of the same bulk of water. It takesin water slowly at first, but continues to absorb more till it be-comes a soft pliant mass, intensely cohesive. It is said to holddouble its weight of water, without suffering any to drop. Whenit is exposed to a dry air, the water quickly exhales from thesurface, but is rigorously retained in the internal parts. As ithardens, it becomes full of clefts on the surface, still coheringin hard masses. Clay, indeed, is not found, in nature, in apure state, being always mixed with a large proportion of sand,besides other extraneous substances : and it is sometimes foundto contain a considerable proportion of fine sand, intimatelymixed, without its cohesion being sensibly weakened, or its cha-racter differing materially from what is above stated. Theweight of sand is to that of water as two and two thirds is to one.The particles of sand have no mutual cohesion, only resting onone another, and leaving interstices between them. When wa-ter comes in contact with a mass of dry sand, the former passesthrough the interstices of the latter, mounting upwards, andquickly pervading the whole. Sand has been found to hold a-bout a fourth part of its weight of water without any dropping.It sometimes occurs almost unmixed, and is then very unfertile.Clay opposes great resistance to the roots of vegetables, check-ing and discouraging their progress. These roots pass directlythrough the interstices of sand ; and meeting with no resistance,send forth few ramifications, and consequently form few mouthsfor collecting vegetable aliment.
The different proportions of these primitive earths, so oppo-site in their qualities, so far as they are concerned in the forma-tion of the soil, constitute the principal difference of the varioussoils which occur. But there is another kind of earth whichgreatly contributes to the fertility of the soil in which it is con-tained. Animal and vegetable bodies are compound substancescomposed of various simples, combined, by the vital power, inan order contrary to the laws of physical affinity by which deadmatter is governed. When life ceases, this combination is dis-solved, and the component principles return to their originalstate. The earths form a part, though not a considerable partof all animal and vegetable bodies, and when deserted by theirmore volatile companions, are by fortuitous causes, or by ma-nipulations, added to the soil. Perhaps, indeed, this residue isnot altogether pure earth, but mixed with some undissolvedcharcoal, this substance being a very considerable componentpart of all animal and vegetable bodies ; and this, its black co-lour would indicate. Whatever it is, its character differs fromthat of the primitive earths. It neither coheres violently, norlies open and porous. It tends to weaken tire cohesion of dense