attest 
& 
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, into fixed, by ebullition—phlogistic, by fer- _ hf 
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 sufficient 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 truly 
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. 
