ON DEW, &c. 245 



moisture, by bodies in contact with it, dew is 

 never observed upon any plants, that are ele- 

 vated a few feet above the ground. 3. If a 

 plant has become, by radiating its heat to the 

 heavens, so cold, as to be enabled to bring the 

 air in contact with it below the point of reple- 

 tion with moisture, that which forms upon it, 

 from its own transpiration, will not then, in- 

 deed, evaporate. But other moisture will, at 

 the same time, be communicated to it by the 

 atmosphere ; and when the difference in the 

 copiousness of these two sources is considered, 

 it may, I think, be safely concluded, that almost 

 the whole of the dew, which will afterwards 

 form on the plant, must be derived from the 

 air ; more especially when the coldness of a 

 clear night, and the general inactivity of plants 

 in the absence of light, both lessening their 

 transpiration, are taken into account. 



An experiment, however, has been appealed 

 to in proof, that the dew of plants actually does 

 originate from fluid transpired by them ; that 

 namely, in which a plant, shut up in an air-tight 

 case, becomes covered with moisture. But this 

 experiment, if attentively examined, will be 

 found to have little weight. First ; the in- 

 closed plant, being exempt from the cold, which 

 its own radiation would have produced in its na- 

 tural situation, on a dewy night, will transpire 



