The object of the author in this part of his investigation, is to shew that the portion of the food of plants which they receive from the soil, and which he endeavours to prove is chemically combined with it,—although to appearance generically the same in all soils,—is not composed, as some imagine, of one single proximate principle, the same in all circumstances, but consists of several principles varying in their respective proportions in different soils. And he farther attempts to establish the general proposition, that the varying proportion of these principles may be one great cause of the relative fitness of different kinds of soil for the cultivation or nourishment of different kinds of vegetables.