ACTION OP SALTS. 99 



ing first with the alkalies, and thus reproducing it- 

 self. It is again decomposed by the growing plant. 

 The same round of action continues. Suppose all 

 this had been witnessed on a worn out almost barren 

 field. It is concluded at once, that there is some 

 peculiar virtue in the salt applied, that it is of itself 

 food, or manure ; whereas the whole action is in 

 obedience to a general law applicable to all salts. 



151. Suppose plaster or gypsum has been applied ; 

 the effects of a bushel of plaster per acre, or even 

 the one four-hundredth part of one per cent, pro- 

 duces effects on alluvial land, which show its good 

 results, as far as eye can reach. It seems almost in- 

 credible, that so minute a portion of a mineral can 

 act at all, yet how beautifully is this result explained, 

 by the principle, that plants decompose, first, this 

 salt ; the lime, for plaster is a sulphate of lime, then 

 acts on geine, which is thus rendered soluble ; while 

 the acid, the oil of vitriol or sulphuric acid, immedi- 

 ately acts on silicates. If silicates of alkali exist in 

 the soil, we have now changed sulphate of lime for 

 an alkaline sulphate, and if silicate of lime is also 

 present, the potash or alkali, having been exhausted, 

 plaster of Paris is formed anew. So long as there is 

 in the soil organic matter, this action continues, and 

 will continue till the plant has gradually withdrawn 



