SECT. XXV. ABSORPTION OF CALORIC DEW. 221 



cases they transmit them and undergo no change of 

 temperature. 



All substances may be considered to radiate caloric, 

 whatever their temperature may be, though with dif- 

 ferent intensities, according to their nature, the state of 

 their surfaces, and the temperature of the medium into 

 which they are brought. But eveiy surface absorbs as 

 well as radiates caloric ; and the power of absorption 

 is always equal to that of radiation ; for under the same 

 circumstances, matter which becomes soon warm also 

 cools rapidly. There is a constant tendency to an 

 equal diffusion of caloric, since every body in nature is 

 giving and receiving it at the same instant : each will be 

 of uniform temperature when the quantities of caloric 

 given and received during the same time are equal, 

 that is, when a perfect compensation takes place be- 

 tween each and all the rest. Our sensations only 

 measure comparative degrees of heat: when a body, 

 such as ice, appears to be cold, it imparts fewer calorific 

 rays than it receives ^ and when a substance seems to 

 be warm, for example, a fire, it gives more caloric 

 than it takes. The phenomena of dew and hoar-frost 

 are owing to this inequality of exchange ; the caloric 

 radiated during the night by substances on the surface 

 of the earth into a clear expanse of sky is lost, and no 

 return is made from the blue vault, so that their tem- 

 perature sinks below that of the air, whence they 

 abstract a part of that caloric which holds the atmos- 

 pheric humidity in solution, and a deposition of dew 

 takes place. If the radiation be great, the dew is 

 frozen and becomes hoar-frost, which is the ice of dew. 

 Cloudy weather is unfavorable to the formation of dew, 

 by preventing the free radiation of caloric ; and actual 

 contact is requisite for its deposition, since it .is never 

 suspended in the air like fog. Plants derive a great 

 part of their nourishment from this source ; and as each 

 possesses a power of radiation peculiar to itself, they 

 are capable of procuring a sufficient supply for their 

 wants. The action of the chemical rays imparts to all 

 substances more or less the power of condensing vapor 

 on tlwse parts on which they fall, and must therefore 

 have a considerable influence on the deposition of dew. 



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