46 AGRICULTURE. 



that it makes part of the food of vegetables, is re- 

 ceived into the plant, and combined with it. The 

 last opinion we shall offer on this head is that of 

 the celebrated Chaptal. " Of all substances," he 

 says, " gypsum is that of whose action we know 

 the least. The prodigious effect it has on the whole 

 race of trefoils (clover), &e., cannot be explained by 

 any mechanical agency, the quantity applied being so 

 small ; nor by any stimulating power, since gypsum, 

 raw or roasted, has nearly the same effect ; nor by 

 any absorbent quality, as it only acts when applied to 

 the leaves. If permitted to conjecture its mode of 

 operation, we should say that its effects being great- 

 est when applied to the wet leaves of vegetables, it 

 may have the faculty of absorbing and giving out 

 water and carbonic acid, little by little, to the grow- 

 ing plant. It may also be considered as an aliment 

 in itself; an idea much supported by Mr. Davy's ex- 

 periments, which show that the ashes of clover yield 

 gypsum, though the clover be raised on soils not nat- 

 urally containing that substance." 



CHAPTER IV. 



OP THE ANALYSIS OF SOILS, AND OF THE AGRICULTU- 

 RAL RELATIONS BETWEEN SOILS AND PLANTS. 



WE have seen that the earths have a threefold 

 capacity ; that they receive and lodge the roots of 

 plants, and support their stems ; that they absorb 

 and hold air, water, and mucilage, aliments neces- 

 sary to vegetable life ; and that they even contrib- 

 ute a portion of themselves to these aliments. 

 But we have also seen that they are not equally 

 adapted to these offices ; that their parts, texture, 



