SECT, xxv.] DEW. EAIN. COMBUSTION. 255 



caloric which holds the atmospheric 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 for- 

 mation 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 vapour on those parts on which 

 they fall, and must therefore have a considerable influence 

 on the deposition of dew. 



Rain is formed by the mixing of two masses of air of 

 different temperatures ; the colder part, by abstracting from 

 the other the heat which holds it in solution, occasions the 

 particles to approach each other and form drops of water, 

 which, becoming too heavy to be sustained by the atmo- 

 sphere, sink to the earth by gravitation in the form of rain. 

 The contact of two strata of air of different temperatures, 

 moving rapidly in opposite directions, occasions an abundant 

 precipitation of rain. When the masses of air differ very 

 much in temperature, and meet suddenly, hail is formed. 

 This happens frequently in hot plains near a ridge of moun- 

 tains, as in the south of France ; but no explanation has 

 hitherto been given of the cause of the severe hail-storms 

 which occasionally take place on extensive plains within the 

 tropics. 



An accumulation of caloric invariably produces light : with 

 the exception of the gases, all bodies which can endure the 

 requisite degree of heat without decomposition begin to 

 emit light at the same temperature ; but, when the quantity 

 of caloric is so great as to render the affinity of their com- 

 ponent particles less than their affinity for the oxygen of the 

 atmosphere, a chemical combination takes place with the 



