M. Melloni on the Theory/ of Dew. 135 



proves that dew is a pure consequence of nocturnal radia- 

 tion, which communicates to bodies possessed of great emis- 

 sive power, the degree of cold necessary for the condensation 

 of the elastic and invisible aqueous vapour diffused through 

 the atmosphere. 



Thus, all theso facts perfectly accord with the theory of 

 dew adopted by writers on natural philosophy and meteoro- 

 logy. I shall now mention other facts which cannot well 

 be explained by this theory, but which connect themselves 

 very happily with the principles of Wells, by means of consi- 

 derations which I shall have the honour to submit to the judg- 

 ment of the Academy. 



I now proceed to develop (says M. Melloni in his second 

 letter to M. Arago*) the proposition which I announced at 

 the conclusion of my former communication, namely, that al- 

 though we may regard the principles of Wells as to the origiii 

 of dew as unquestionably correct, it is impossible to give an 

 account of all the thermometrical and hygrometrical pheno- 

 nomena which takes place in the night, when the sky is clear 

 and the atmosphere tranquil, unless we take into considera- 

 tion a new circumstance hitherto entirely neglected, not- 

 withstanding its extreme importance in the nocturnal cooling 

 of bodies. But, in the first place, allow me to make a few 

 observations on two series of experiments which the advo- 

 cates of the rising of dew from the earth have put forward, 

 with the allegation that they are sufficient of themselves to 

 overturn all the theories of this meteor founded on calorific 

 radiation. 



They have taken a certain number of thermometers, equal, 

 as far as possible, in size and sensibility. Some of them have 

 been coated over with lamp-black, varnish, and china ink; 

 others were gilded, silverized, covered with leaves of plants, 

 of pewter, and of copper. Thus prepared, these instruments 

 were exposed in the fields during a calm and clear night, and 

 they indicated at first temperatures slightly different ; but, 

 after a certain interval of time, all of them at last nearly 



* Comptcs Remjus dcs Seances do rAcademie 4e9 Sciences, tome xxiv., No. 

 15, p. 641. 



