FORMATION OF DEW. 31 



some refrigerating cause upon the air. After sunset, most 

 bodies, by projecting or radiating heat into free space, become 

 colder than the neighbouring air, and as soon as their refri- 

 geration has attained a certain point, they must naturally, in 

 consequence of the physical law above mentioned, get covered 

 with dew. The best radiators of heat part, of course, most 

 easily with their caloric, and for this reason grass, leaves, or 

 plants in general, get much sooner and more plentifully covered 

 with dew than slower radiators, such as stones, the soil, or 

 metals, which frequently still remain dry when the meadows 

 are already covered with plentiful moisture. Hence we can 

 understand why, in summer, every serene night (for clouds, 

 by reflecting or throwing back again upon the terrestrial 

 surface the caloric which would else have been dissipated into 

 space, prevent its rapid refrigeration) is accompanied with a 

 copious formation of dew ; why it is more abundant in autumn 

 and spring than at any other season, as then very cold and 

 starlight nights frequently follow upon warm days, and why 

 it is most copious in the torrid zone, as in those sultry regions 

 the air is more saturated with moisture than anywhere else, and 

 the comparatively cold nights are almost constantly serene and 

 calm. 



Had naked stones been as good radiators of heat as leaves or 

 herbs, then the latter would vainly have thirsted for refresh- 

 ment, while the former would have been bathed in useless 

 moisture. 



Had the dew been plentiful during violent winds, the plants 

 must frequently have suffered, or been frozen to death in con- 

 sequence of the rapid evaporation of their moisture. 



Where the sun has most power, where during the day he most 

 thoroughly dries up the soil, there also the cool night is most 

 prodigal of dew. 



Thus the history of dew gives us many opportunities of 

 admiring the wisdom that has presided over its formation and 

 distribution. 



While dew is merely produced on the surface of solid bodies, 

 the refrigeration of large volumes of air forms clouds or fogs, 

 consisting of vast numbers of minute globules, or vesicles 

 of moisture, which float like soap-bubbles in the atmosphere, 

 until, their quantity increasing with the cold, or in consequence 



