PROPERTIES OF DIFFERENT EARTHS. 33 



are often moist in the morning; the return of the sun and 

 the heat of the day evaporate this liquid, to be deposited 

 again at sunset, and during the night ; thus by an alternate 

 movement, determined by the changes in the temperature 

 of the atmosphere at different periods of the twenty-four 

 hours, water is constantly applied to plants, to preserve 

 them from the excess of heat, which would wither, and 

 dry up their organs. 



The aqueous vapors suspended in the air begin to be 

 condensed and precipitated at sunset, and with them is 

 deposited the greatest part of the emanations which have 

 arisen from the earth during the day ; these exhalations, 

 though beneficial to vegetation, are almost always injurious 

 to man, and it is not without reason that he fears and 

 shuns the night damps. In southern climates, where the 

 heat of the sun is more intense, and rains less frequ nt 

 than in northern, vegetation is supported by the dews, 

 which are very abundant. In order that the dews of night 

 may produce their best effects upon vegetation, it is neces- 

 sary that the soil should unite certain qualities, which it 

 does not always possess. 



When the soil is hard and compact, and forms by the ' 

 action of the air an impenetrable crust, the dew is deposit- 

 ed upon its surface, and evaporated by the rays of the 

 sun, without having moistened the roots of the plants, or 

 softened the earth around them ; so that, of the organs that 

 serve to convey nourishment to the plants, the leaves are 

 the only ones benefited by the dew, while the roots, which 

 are the principal vehicles of nutriment when the plant is 

 fully developed, are not in any degree benefited by it. 

 It is necessary, in such cases, that the soil should be soft- 

 ened, lightened, and divided, so that the air may convey \ 

 the water with which it is charged, to the roots of the / 

 plants, and to every part of the earth surrounding them, 

 to a certain depth ; then the plant can imbibe, through all 

 its pores, the reviving moisture ; and that which is received 

 by its roots is more lasting than that which it absorbs in 

 any other way, because the roots being sheltered from the 

 direct rays of the sun, evaporation takes place less rapidly, 

 and the moisture is retained, whilst the leaves are speedily 

 dried by the heat. Besides, that earth which is most 

 easily affected by the dews, yields most readily to the 

 action of roots, whether it be to fix the plant firmly by 

 their extension, or to draw from the soil its nutritive prop- / 

 erties. -^ 



