MELLONI ON THE NOCTURNAL COOLING OF BODIES. 551 



arises neither from a peculiar repulsive force, as Leslie supposed ; 

 nor from an electric action, as Saussure believed; nor from the heat 

 disengaged by the chemical reaction of the metal on the aqueous 

 vapour, as M. Fusinieri has latterly maintained ; but solely 

 from their extremely feeble emissive power, which does not pro- 

 duce a degree of cold sufficient to cause the condensation of the 

 elastic vapour contained in the surrounding medium. This ex- 

 periment, therefore, is a sort of collective proof, which embraces 

 in itself the fundamental principles of the theory of dew deve- 

 loped in the Second Part of the memoir. 



As regards the theory of " serein" we cannot do better than 

 give the translation of the passage in which it is pointed out. 



"Several authors," says M. Melloni, "attribute to the cold 

 resulting from the radiation of the air, the excessively fine rain 

 which sometimes falls in a clear sky, during the fine season, a 

 few moments after sunset. But, as no fact is yet known which 

 directly proves the emissive power of pure and transparent 

 elastic fluids, it appears to me more conformable to the prin- 

 ciples of natural philosophy, to attribute this species of rain to 

 the radiation and the subsequent condensation of a thin veil of 

 vesicular vapour distributed through the higher strata of the 

 atmosphere, in a manner so as not to cause any considerable 

 alteration in the azure tint of the sky. The beautiful phseno- 

 mena of colours which appear in the west when the solar rays 

 are quitting our hemisphere, have often afforded me the oppor- 

 tunity of observing isolated clouds, which had lasted for some 

 time, suddenly diminish in volume and intensity the instant they 

 ceased to be struck by the sun's rays, and soon become com- 

 pletely effaced Avithout leaving a trace of their former existence. 

 In meditating on the causes of these vanishings, it appeared to 

 me evident that the upper part of the cloud, no longer receiving, 

 after the sun has set, any compensation for the heat radiated 

 into space, is condensed into water, and the subjacent stratum 

 takes its place, undergoing the same changes, and so on in suc- 

 cession ; so that the whole cloud is soon reduced into liquid 

 drops which pass into the state of elastic fluids, in falling 

 through the space beneath*." 



* It is perliaps a process of tliis kind whicli lias helped to confirm an opinion 

 li( Id by the many, that the light of the full moon dissipates the clouds ; for then 

 the vesicular vapours are abandoned by the direct or diffused rays of the sun, 

 and begin to be cooled by vibrating their proper heat into space, at the very 

 moment the moon rises above the horizon. 1 would nevertheless observe, that 



