; ns Te re 7 yi . SA! 3 ae 
» s = 4 
ry > - : 
wy 
104 On Plaister of Paris. 
stances in which the fixed air, or carbonic acid, is found. It re- 
leases this, from the bodies impregnated with it ; and, by put- 
ting in a state of activity, prepares it to enter the plants, and be- 
come their food,in combination with whatever else i is neces- 
sary, to their existence and growth. oY 
Why the plaister is applied, with greater profit on Hight, 
than on clay, or other wet soils, it is dificult to account. It 
may, possibly, be owing to the different materials it finds to 
operate upon. Most clays have a great proportion of iron, 
in their composition.—Chaptal, page 214, [Philadelphia edi- 
tion.] They are so retentive of moisture, that they are gene- 
rally cold, wet, and sour. The sez/phuric acid, when poured on 
iron or zinc, by decomposition oi water, produces the hydro- 
genous gas, or inflammable, and not jixed, air, or carbonic 
acid. Chaptal, Page 61. Clays, and all wet soils haye so 
much redundant moisture, that (by means of the suldphuric 
acid in the plaister, operating on the iron they contain) they 
afford an overcharge 0! hydrogene. The hydrogene, accord- 
ing to Chaptal, page 97, fixes itself in vegetables, though 
it is otherwise in metals, &c. while the oxygen, or vital air, 
necessary to be combined with it, as a nutrition to plants, 
is disengaged and escapes. Thus the inflammable air, bemg 
left alone, and in too great a quantity, either does nothing, 
or injures vegetation. This infammable air may, in small 
portions, in combination with oxygen, or carbonic acid, be, 
as Chaptal asserts it is, an ingredient in the food. But when 
in great quantities, and of itself, it may be destructive. In light 
soils, the water and moisture are soon drained away. But 
the plaister counteracts the percolating,or porous, qualities 
of these soils, by attracting, arresting and retaining as much 
moisture (and perhaps no greater quantity) as will answer 
all beneficial purposes. The superfluous water or moisture 
passes off. And thus the operative principle in the plaister 
produces no more, either of inflammable, or fixed air, than 
is necessary for the salutary supply of the plant. A shorter 
