On Plaister of Paris. 91 



small, it is indifferent. If it remains an inert mass, 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 inimical 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. Toung does not now value the experiments he 

 exhibited, to prove phlogiston to be the food of plants. — 

 Practical iarmers do best, when they content themselves with 

 exemplary iacts. 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. P. 



September^ 1810. 



