On Plaister of Paris. 1 13 



sulphuric acidy in the plaister, finds sufficient materials 

 to set in motion, for the nutrition of plants.* 



4. Mere matter of opinion is no proof: but it seems 

 most likely that the acid (vitrolic salt) in the plaister, is 

 most soluble in water, though the earth be also solu- 

 ble — and, (if this be the criterion,) that it is the «cfflf, 

 more than the calcareous earth, which produces the 

 effect. But I still conceive this effect is produced by 

 the acid operating on other substances, and not imme^ 

 diately on the plant. If it be the calcareous earth, why 

 do not other calcareous earths operate in a similar de- 

 gree ? It would take an immense proportion of other 

 calcareous earths, to do what is here supposed to be effect- 

 ed by a small quantity in the plaister. Crude limestone, 

 pulverized, has a considerable effect on grass grounds. 

 But it takes so much of it, as to preclude, by the ex- 

 pense of obtaining, the profit of using it. Broken or 

 powdered oyster shells, (which contain calcareous mat- 

 ter as well as salt) are excellent for grass grounds. But 

 their powers bear no reasonable proportion to those 

 of the plaister. Limestone gravel is pulverized, in some 



^ Since these observations were made, I have tried a great 

 variety of experiments, with lime and ashes respectively, in 

 large and small quantities, on land. I have, in no instance, 

 found, that the English theory could be justified. On the con- 

 trary, the plaister has frequently drawn forth the powers of 

 both the lime and ashes, and corrected them when too much, 

 or assisted, when too little, of either of the latter, had been 

 spread on the field. 



R. P. 

 September, 1810. 



