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rence, but likewise softness and penetrability; butneither they nor any other part of the soil must bein too great proportion ; and a soil is unproductiveif it consist entirely of impalpable matters.

Pure alumina or silica, pure carbonate of lime, orcarbonate of magnesia, are incapable of supportinghealthy vegetation.

No soil is fertile that contains as much as 19 partsout of 20 of any of the constituents that have beenmentioned. *

It will be asked, are the pure earths in the soilmerely active as mechanical or indirect chemicalagents, or do they actually afford food to the plant ?This is an important question ; and not difficult ofsolution.

. The earths consist, as I have before stated, ofmetals united to oxygene; and these metals havenot been decomposed; there is consequently noreason to suppose that the earths are convertibleinto the elements of organized compounds, intocarbon,'liydrogene, and azote.

Plants have been made to grow in given quan-tities of earth. They consume very small portionsonly; and what is lost may be accounted for by thequantities found in their ashes ; that is to say, it hasnot been converted into any new products.

The carbonic acid united to lime or magnesia, ifany stronger acid happens to be formed in the soilduring the fermentation of vegetable matter whichwill disengage it from the earths, may be decorn-

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