104 0?t 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 hi a state of activity, prepares it to enter the plants, and be- 

 come their ^ood,in combination with whatever else is neces- 

 sary, to their existence and growth. 



Why the plaister is applied, with greater profit on light^ 

 than on claijy or other wet soils, it is difficult to account. It 

 may, possibly, be owing to the d fferent materials it finds to 

 operate upon. Most claiis 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 sulphuric acid, when poured on 

 iro?i or zinc, by decomposition of water, produces the hydro- 

 genoiis gas, or infammable^ and not Jixed, air, or carbonic 

 acid. Chaptal, Page 61. Clays, and all wet soils have so 

 much redundant moisture, that (by means o-- the sulphuric 

 acid in the plaister, operating on the iron they contain) they 

 afford an overcharge of hydrogene. The hydrogene, accord- 

 ing to Chaptal, page 97, fixes itseli in vegetables, though 

 it is otherwise in 7netals, &c. while the oxijfren^ or vital air^ 

 necessary to be combined with it, as a nutrition to plants, 

 is disengaged and escapes. Thus the inflammable air, being 

 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 ingredent in the ^ood. But when 

 in great quantities, and of itself, it may be destructive. In light 

 soils, the v/ater 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 oi infammable, ox fxed air, than 

 is necessary for the salutary supply of the plant. A shorter 



