36 CHYMISTRY APPLIED TO AGRICULTURE. 



It is of the utmost importance in the science of agri- 

 culture, that the comparative powers of the various soils 

 for absorbing atmospheric moisture, and the degrees of 

 force with which they retain it, should be ascertained. 

 The means necessary to be employed in ascertaining these 

 capacities of soils, are in the power of every cultivator ; 

 he has only to dry thoroughly the same weight of each 

 soil in a state of equal division, and to weigh them night 

 and morning for several days, and he will be able to form 

 an estimate of the quantity of moisture which each has 

 imbibed during the night. In order to obtain these results 

 with exactness, it is necessary that the assays should be 

 made upon equal weights of earth, in an equal state of 

 division, equally dried, and spread in layers of an equal 

 degree of thickness. 



From the statements which I have made, it is easy to 

 be perceived, that air and water are two powerful agents 

 in promoting vegetation ; they act upon it directly, by 

 furnishing from their own decomposition nutritive princi- 

 ples ; and they act as auxiliaries, by serving as vehicles 

 for the conveyance into the organs of plants of such sub- 

 stances as are necessary for their support. 



But though the plant is furnished with aliment through 

 these agents, it is heat alone, that, by animating the vege- 

 table organs, enables it to elaborate within itself the 

 nourishment which it receives. The effect of temperature 

 is perceptible not only in plants, but in many classes of 

 animals ; nearly all insects are benumbed by the cold, 

 and reanimated by the heat. 



All soils are not equally capable of receiving and retain- 

 ing heat. The white earths are warmed with difficulty ; 

 when pipe clay or aluminous marl predominates in a soil, it 

 is nearly always damp, and retains but little heat. White 

 chalky soils require much heat to warm them ; but they 

 part with their heat less quickly than the first, whilst col- 

 ored earths absorb heat, in proportion as the depth of 

 their hue increases from brown to black. 



Davy has remarked that a black soil containing nearly 

 ■^ of vegetable matter, when exposed to the sun, acquired 

 in one hour an elevation of temperature which raised the 

 thermometer from 65° to 88°, whilst under the same cir- 

 cumstances, a soil whose basis was chalk, raised it only 

 to 69 degrees. When the black earth was carried into 

 the shade at the temperature of 62°, the thermometer fell 



