THEORY OF VEGETATION. 35 



principal food, and accordingly recommended, as 

 the great desideratum in agriculture, an oil compost. 

 Lord Kaimes attempted to revive the expiring creed 

 of Lord Bacon ; but finding, from Kale's statics, that 

 one third of the weight of a green pea was made up 

 of carbonic acid, he added air to the watery aliment 

 of the English philosopher, but entirely rejected 

 oil and earth, as too gross to enter the mouths of 

 plants, and salt as too acrid to afford them nourish- 

 ment. Quackery, which at one time or other has 

 made its way into all arts and sciences, could not 

 easily be excluded from agriculture. Hence it was 

 that the Abbe de Valemont's prolific liquor, and De 

 Hare's and De Vallier's powders, &c., &c., wereJoe- 

 Iteved to be all that was necessary to vegetanon, 

 and found the more advocates as they promised 

 much and cost little. But before the march of 

 modern chymistry quackery could not long main- 

 tain itself; and from the labours of Bennet, Priest- 

 ly, Saussure, Ingenhouz, Sennebier, Schaeder, Chap- 

 tal, Davy, &c., &c., few doubts remain on this im- 

 portant subject. These will be presented in the 

 course of the following inquiry. 



I. Of earths, and their relation to vegetation. 



Of six or eight substances which chymists have 

 denominated earths, four are widely and abundantly 

 diffused, and form the crust of our globe. These 

 are silica, alumina, lime, and magnesia. The first is 

 the basis of quartz, sand, and gravel ; the second of 

 clay ; the third of bones, river and marine shells, 

 alabaster, marble, limestone, and chalk ; and the 

 fourth of that medicinal article known by the name 

 of calcined magnesia. In a pure or insulated state,* 

 these earths are wholly unproductive ; but, when de- 

 composed and mixed,! and to this mixture is added 



* See Gisbert's experiments on pure earths and their mixtures. 

 See also Davy's Elements, p. 156. 



f In this respect nature has been neither negligent nor nig- 

 gardly, if (as Fourcroy asserts) the purest sand be a mixture of 



