550 MELLONl ON THE NOCTURNAL COOLING OF BODIES. 



formation of dew to the exhalation of the soil and the mysterious 

 appearance of a stratum of cold air at the earth's surface. This 

 refutation, however useful in Italy, where this theory of dew is 

 still taught in certain schools placed under the protection of the 

 Austrian government, appears to be superfluous for those per- 

 sons who, after reading attentively the preceding pages, can no 

 longer retain the shadow of a doubt as to the true cause of the 

 phenomenon. We shall merely call the reader's attention to 

 the two subjects treated of in the Third Part of M. Melloni's 

 memoir, namely, the experiment by means of which the dew is 

 forced to deposit itself on certain portions of a metallic surface, 

 whilst the other parts are maintained in their habitual state of 

 dryness ; and the theory of that sort of dew (serein) or excess- 

 ively fine rain, which falls sometimes on fine summer evenings 

 when the sky is clear and free from clouds. 



The experiment, which is described in detail in the first of 

 the two letters of M. Melloni to M. Arago, consists in par- 

 tially varnishing one of the two surfaces of a very thin tin 

 plate ; afterwards covering a portion of the varnished surface 

 with a sort of small detached roof of polished metal, and ex- 

 posing this arrangement of two plates to the free air, so 

 that the side which is varnished and partially sheltered under 

 the metallic roof, may be turned towards the sky, and the 

 surface which is entirely polished towards the ground. The 

 dew is deposited in large quantity on the surface uncovered by 

 varnish, and from thence in diminishing proportions, on the ad- 

 jacent parts. The roof of polished metal remains entirely dry 

 and brilliant, as also the central portions of both sides of the 

 subjacent disc ; the rest of the surface looking towards the 

 ground is, on the other hand, quite covered with dew. 



Some experiments of Wells had shown that dew does not fall 

 from the sky ; others, that it does not rise from the ground. 

 The experiment of M. Melloni proves these two things at the 

 same time; for there ai'e portions of metal moistened by the 

 dew, and others perfectly dry and brilliant above and below the 

 system of two plates, that is, on the side looking towards the 

 sky and on the side turned towards the ground. It proves, be- 

 sides, incontestably, that metals cooled by the juxtaposition of 

 a radiating substance, condense the elastic vapour of the atmo- 

 sphere as well as the leaves of plants, and consequently, that 

 the ordinary want of dew on the polished surfaces of these bodies. 



