[ i66 ] 



it, from whatever fource it may be derived, and 

 giving eafyaccefs to the roots and fibres which ex- 

 tend themfelves every way in queft of the fame. 



Chemical principles produce much the fame 

 effcdls, but in a way we cannot fo eafily, nor fo 

 perfe6lly, comprehend. The mechanical powers 

 are fubfervient to our wills, and we can continue 

 the ufe of them until the defired end is obtained; 

 but the efFeds of chemical operations are not fo 

 certain in this bufinefs, as a certain concurrence of 

 circumftances, not always in our power to procure, 

 is neceflary to produce the hoped-for fuccefs. 

 Much depends upon the temperature of the feafons, 

 the ftate of the air, and many other things not in 

 the power of man to forefee or to govern. Fer-^ 

 mentation feems to be the principal agent in pro- 

 jnoting vegetation by chemical powers. This 

 divides, attenuates, and fubtilizes, by means of an 

 internal motion of the parts, which we cannot 

 clearly conceive. Probably it is on this principle 

 that marl, chalk, fhells, and every kind of calcare- 

 ous earth, are fertilizers of land. Not by any 

 matter or fubftance inherent in them, as conftituent 

 parts of the fame; but as abforbent bodies, which 

 attrad much more powerfully the principles of ve- 

 getation than earth alone could do. 



We 



