im 
On Plaister of Paris. 113 
sulphuric acid, in the plaister, finds sufficient or 
to set in motion, for the nutrition of plants.* Le 
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 acid, 
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 ishere supposed to be effect- 
ed by a small quantity in the plaister. Crude kmestone, 
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. 
