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. /^r^ 



(q) Ingenhausz on vegetables page 184, \%5^food of plants 

 8fc. Vital air, produced by vigorous plants in the sunshine, 

 is of the gr atest purity in itseli. 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 observ ng, that "all 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, zvhen 

 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 is checked 

 by a large proportion of those salts, as Sir John Pringle 

 found. It is thus with alkaline salts, with common s^Xt, gyps, 

 which last is a vitriolic salt, widi 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 fmd it even worth their while to draw this ingredient 

 C gyps J 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 



