SECT. XXVII. DEW EVAPORATION. 269 



rapidly. There is a constant tendency to an equal diffusion of 

 heat, since every body in nature is giving and receiving it at the 

 same instant ; each will be of uniform temperature when the 

 quantities of heat given and received during the same time are 

 equal that is, Avhen a perfect compensation takes place between 

 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 sub- 

 stance seems to be warm for example, a fire it gives more 

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

 owing to this inequality of exchange ; the heat radiated during 

 the night by substances on the surface of the earth, into a clear 

 expanse of sky, is lost to us, and no return is made from the blue 

 vault, so that their temperature sinks below that of the air, 

 whence they abstract a part of that heat 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 unfavourable 

 to the formation of dew, by preventing the free radiation of heat ; 

 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 suffi- 

 cient supply for their wants. The action of the chemical rays 

 imparts to all substances more or less the power of condensing 

 vapour on those parts on which they fall, and must therefore 

 have a considerable influence on the deposition of dew. There 

 may be a low degree of humidity in the air which may yet 

 contain a great quantity of aqueous vapour, for vapour while it 

 exists as gas is dry. The temperature at which the atmosphere 

 can contain no more vapour without precipitation is called the 

 dew point, and is measured by the hygrometer. In foretelling 

 the changes of weather it is scarcely inferior to the barometer. 



Steam is formed throughout the whole mass of a boiling 

 liquid, whereas evaporation takes place only at the free surface 

 of liquids, and that under the ordinary temperature and pressure 

 of the atmosphere. There is a constant evaporation from the 

 land and water all over the earth. The rapidity of the formation 

 does not depend altogether on the 'dry ness of the air ; according 

 to Dr. Dalton's experiments, it depends also on the difference 



