* On Plaister of Paris. 91 
small, it is indifferent. If it remains an inert ee! like 
sand, it is worthless. 
One might suppose, from this rude experiment, that 
plaister was good or bad, according to the quantity 
of phlogiston it contained.* When phlogiston was in 
fashion (for its existence is now denied) it was said 
to be a considerable ingredient in the food of plants. 
See Young’s experiments, in his Annals, on this point. 
It may be also, that the greater the proportion of cal- 
careous earth, which absorbs the vitriolic acid in its 
composition, the better the gypsum. Some have suppo- 
sed mineral acids to be imimical to vegetation; while 
others have asserted, that the vitrolic acid will greatly 
* Although some of the theoretical ideas I have hazarded, 
have since been useful in prosecuting practical experiments ; 
I should have omitted many of them, had I been enabled to 
' accomplish my design of condensing the whole of what is 
now known, into a short and regular system. I leave them 
as they are, though some speculations might be suppressed. 
I believe Mr. Young does not now value the experiments he 
exhibited, to prove phlogiston to be the food o; plants.— 
Practical farmers do best, when they content themselves with 
exemplary facts. Theorists, agricultural as well as others, 
frequently employ themselves in groping in the dark. Light 
is, however, often approached, through dark and devious 
passages. The phenomena of the gyps were so extraordi- 
nary, that all endeavours to develope its mysteries, seemed 
justifiable. 
R.- PR. 
September, 1810. 
