On Plaister of Paris, 93 



a due mixture of the acid with the calcareous earth, 

 may turn into wholesome food, what of itself might be 

 injurious to vegetation. But as I chiefly relate agricul- 

 tural facts, I leave the discussion of such points to phi- 

 losophers and chemists. It is enough for us if we know 

 effects. Causes are often hidden, among the arcana of 

 nature. Nothing has evidenced a greater diversity of 

 opinion, among the most eminent men, than the ques- 

 tion. " What is the food of plants ?" Nor do they 

 agree about the nature of the air contained in vegetables. 

 Some assert it is mere atmospheric air, changed, or lia- 

 ble so to be, mX.o fixed, by ebullition — phlogistic, by fer- 

 mentation — or dephlogisticated by the sun, the light 

 whereof operates a change in this air, not produced in 



air, by the soil ; and this vital air, being mixed with the 

 carbon, becomes carbonic acid, and enters the plant through 

 its roots. Jngenhausz* s doctrine, in another place, (an old 

 opinion) is that the earth, of itself, does nothing towards 

 the support of plants, their food being chiefly acquired 

 from the air; " the principal business of feeding being carried 

 on by the leaves in the atmosphere." The seed is said to con- 

 tain the carbonic acid sufiicient to forward the plant, till it 

 is enabled to acquire fresh supplies from the air, and through 

 the earth, which contains this acid in great plenty. I give 

 but a faint, and perhaps inaccurate, recital of these theories, 

 merely to shew the variety of opinions among men trulv 

 eminent on both sides ; and because those here mentioned 

 are some of the most modern. Farmers should not overlook 

 • theories; but they should depend only on careful and ju- 

 dicious experiment and practice. 



