M. Melloni on the Theory of Dew. 129 



at three successive sittings. I have sent you an extract, 

 with a request that you would communicate it to the Insti- 

 tute. You will see, my illustrious friend, that (here was 

 something to he done ; but the observers who have made such 

 severe attacks on Wells' principle were instigated by a spirit 

 so blindly hostile, that, far from seeking for any thing that 

 might be wanting, they wished to overturn and destroy all, 

 to reintroduce (who would believe it V) the old phantom of 

 the rising of dew from the earth ! 



After Wells' experiments, we may well admit with perfect 

 safety, that dew does not rise from the earth, that neither 

 does it fall from the sky, and that it is formed by the elastic 

 and invisible vapour diffused throughout the space which sur- 

 rounds bodies ; and it is thus that we have comprehended 

 all, by attributing, with the natural philosopher just named, 

 the precipitation of the aqueous vapour to the cold resulting 

 from the calorific radiation of bodies towards a clear sky. 

 According to this manner of viewing the subject, the' leaves 

 of vegetables, wood, glass, varnish, lamp-black, become 

 covered with dew, because they allow the heat to escape 

 readily, and are considerably cooled under a clear sky; 

 while metals remain dry on account of the difficulty they 

 have in vibrating their heat towards the higher regions of 

 the atmosphere. And, in truth, we observe a great differ- 

 ence between the indications of a thermoscopic apparatus, 

 when we present to it successively a vessel of polished metal 

 full of boiling water, and another vessel exactly similar, 

 having its sides covered with varnish or lamp-black, the se- 

 cond action being much more energetic than the first. The 

 inference is a just one ; but it must be confessed that it may 

 well be that it should not appear necessary or inevitable in 

 the eyes of every one. In fact, Benedict Prevost, and Saus- 

 sure before him, ascribed the want of dew on metals to an 

 electrical influence. Leslie explained this phenomenon by a 

 particular repulsion which metallic surfaces exert towards 

 aqueous vapour; and the actual partisans of the disputed 

 theory of the rising of dew, account for it by the heat and 

 electricity disengaged in the chemical action of metals on the 



VOL. XTilll. NO. liXXXV JULY 1847. I 



