ON PLANTS. 137 



The second mode which nature employs to water plants 

 is the dew. I hope you recoftect the very ingenious 

 theory of Dr. Wells on that subject. 



Emily. I fear but imperfectly. 



Mrs. B. I advise you to look it over;* at present I 

 shall only say that it is founded on Professor Prevost's 

 Theory of Radiant Heat. In proportion as a body radi- 

 ates, its temperature must necessarily be lowered, unless 

 it be supplied with heat from some foreign source : dur- 

 ing the day the sun affords this supply very amply, but 

 after sunset the earth, as well as every object upon its 

 surface cools by radiation. The atmosphere, which ra- 

 diates much less than the solid earth, preserves its tem- 

 perature longer ; but the stratum of air which is immedi- 

 ately in contact with the ground is cooled by it, and de- 

 posits upon it that portion of vapor which the diminution 

 of its temperature prevents it from longer holding in so- 

 lution. This precipitation is the dew, which you per- 

 ceive on the grass, after sunset. 



Emily. Since it proceeds from the cooling of the sur- 

 face of the earth, why is it not equally precipitated on 

 gravel walks and pavement ? 



Mrs. B. Because the stones of which these are com- 

 posed are not good radiators, and therefore preserve 

 their temperature longer ; and if they do not cool quick- 

 er than the air with which they are in contact, no depo- 

 sition of dew will take place. Minerals, and especially 

 metals, are bad radiators ; they require no dew : Nature 

 reserves this mode of watering for the vegetable creation: 

 to plants she gives the power of abundant radiation, both 

 to enable them to throw off the heat with which they 

 have been oppressed during the day, and to call down 

 those refreshing showers of dew which restore their vigor. 

 One knows not which most to admire, the wise provision 

 which is thus made for the benefit of the vegetable king- 

 dom, or the simplicity of the means by which it is accom- 

 plished. 



* See Conversations on Chemistry, Con. vii , 



762. What is the second mode which nature employs to water plants! 

 763. On what is the theory of dew founded! 764. How does Mrs. 

 B. explain the theory of dew as caused by radiation of heat! 765. 

 Why is there not dew on gravel walks and pavements, if it is occasioned 

 by radiation! 766. What is said of minerals, metah, and vegetables 

 in relation to this subject! 



12* 



