ey oe 
94, On Plaister of Paris. 
plants in the shade.(q) Of what nature or species is the 
air contained in plaister; or whether this substance ope- 
rates by its powers of attracting or retaining moisture, 
and decomposing, preparing and communicating’ to 
plants, the air, the fittest for their nourishment; must be 
decided by others than practical farmers, to whom pro- 
fitable effects are more important, than the most learned 
and ingenious theories. (7) 
(q) Ingenhausz on vegetables page 184, 185, food of plants 
‘ce. Vital air, produced by vigorous plants in the sunshine, 
is of the greatest purity in itself. The air thrown out by 
them in the shade or in the dark, is of itself, unmixed with 
other air, the most active poison in destroying animal life. 
(r) Ingenhausz, page 12, Essay on the food of plants, 
&c. after observing, that ‘tall the most powerful manures have 
one common quality, viz. to contain, or to disengage, a great 
quantity of carbonic acid, proceeds to suppose, that animal 
and vegetable substances probably act as manures only, when 
in the act of decomposition by putrefaction, during which 
period a great quantity of carbonic acid, is produced. This 
putrefaction is promoted by almost all salts when mixed 
with those substances in moderate quantities, but zs checked 
by a large proportion of those salts,as Sir John Pringle 
found. It is thus with alkaline salts, with common salt, gyps, 
which last is a vitriolic salt, with an earthy basis. This notion 
may account for the benefit, which the Germans and the 
Americans derive from employing gyps,as a manure. The 
latter find it even worth their while to draw this ingredient 
Ceyps) from Europe.” ‘ According to. these notions, we 
may perhaps understand, why all those manures which un- 
dergo the quickest decomposition ought to be oftner applied © 
