16 AMERICAN AGRICULTURE. 



The plowing of clay lands for spring crops should be done 

 in the autumn if practicable, by which their adhesiveness is 

 temporarily destroyed, the earth is finely pulverized by the 

 frost, and they are left in the finest condition for early spring 

 sowing, and without additional working. If plowed in the 

 spring, it should be done when they are neither too wet or 

 dry ; if the former, the earth subsequently bakes, and for a long 

 time it is almost impenetrable to the hoe or the teeth of the 

 harrow ; if too dry, they are so compact as to be turned over 

 only with great effort, and then in solid lumps. The action 

 of the atmosphere, will pulverize these masses of baked earth 

 after a time, but not sufficiently early for the convenience or 

 advantage of such crops as are intended immediately to fol- 

 low the plowing. 



No soils are so tenacious of the manures which may be 

 incorporated with them as the clays. They form an inti. 

 mate combination, both mechanical and chemical,* and hold 

 them securely against waste from drainage or evaporation for 

 an indefinite time, till the growing crops demand them. 

 They also greedily seize upon and hoard up all such fertil- 

 izing principles as are conveyed to them by the air and 

 rains. We may mention as an example of their efficiency 



* By mechanical in the sense above used, is understood the external relation of 

 bodies, which is nearly equivalent in its meaning in this connexion, to artificial. 

 Thus the clay envelopes the manures, and from its impervious nature, it shields 

 it from escape either by drainage or evaporation, and almost as efieclually as if 

 it were enclosed in an carthern vessel. 



By chemical is meant, its internal or constitutional character. Thus clay not 

 only absorbs the gases which are brought into contact with it from manures, from 

 moisture and from air, as a sponge absorbs water , but it also forms new combina- 

 tions with them, which change the original nature of these elementary principles, 

 and from light evanescent gases, they becone component parts of solid bodies, in 

 which condition they are retained till exhausted by the growing vegetation. 



These terms arc important, and should be clearly understood. For the sake of 

 aiding the young student, we will give some further examples. If we take apiece 

 of crystalized marble, compact uncrystalized limestone, and chalk, we shall have 

 three substances exactly alike in their chemical character; for they are all combi- 

 nations of carbonic acid and lime associated together in precisely the same propor- 

 tions Hut in their external arrangements, as they appear in n recent fracture to 

 the eye and touch, that is in their mechanical arrangements, they are all totally 

 dissimilar. 



Again ff we take the pure lime (quick lime) that is obtained from each of the 

 foregoing by subjecting them to an intense heat, by which the carbonic acid is 

 expelled, and pour upon it nearly one third of its weight of water, great heat is 

 developed and the lime both mechanically absorbs, and chemically combines with 

 it, forming a new compound, or salt, which is a hydrate of lime. 



If s*nd (mostly silex) be added to the lime with water, and mechanically mixed 

 or stirred together and allowed to remain for a sufficient time, they will combine 

 ihtmicaUy, forming silicate of lime, (the common morlar of stone masons.) 



Sand (silex) stirred in with clay (an impure alumina) is mechanically mixed; 

 if then subject to a strong heat as in making brick, they become chemically 

 united, forming silicate of alumina, inseparable by any human means short of the 

 chemist's crucible. If we divide or separate a stick by splitting or cutting, it is a 

 mechanical; and it by burning or charring, it is a chemical change. Thug every 

 alteration either in nature or art is referable to one of the above conditions or 

 changes. 



