132 M. Melloni on the Theory of Dew, 



Thermoscopic apparatus, similar to those I have described, 

 having the protecting armatures covered with varnish, pkim- 

 bago, glue, saw-dust, sand, earth, and leaves of plants, have 

 invariably shewn a very sensible sinking of temperature be- 

 fore becoming moistened with dew ; the interval has some- 

 times been many hours ; often, also, there has been a lower- 

 ing of temperature without precipitation of vapour at any 

 period of the night. This latter phenomenon is produced the 

 more frequently as the thermometers were placed at a greater 

 distance from the ground. By operating, therefore, at a cer- 

 tain elevation, we may suppress or retard, at pleasure, the 

 deposition of dew on bodies, and perfectly convince ourselves 

 that it always follows, never precedes, the production of cold. 

 Witlj regard to the polished metallic armatures of my ther- 

 mometers, I have never seen them covered with condensed 

 aqueous vapour in very humid nights, when there was not 

 a trace of mist in the atmosphere. 



It thus appears that dew, properly so called, always re- 

 quires a certain coldness in the body on which it gathers, 

 and metals exposed to a clear sky are not covered with it, 

 because they cool only in a very trifling degree. But may 

 there not be other influences which prevent the precipitation 



that we ought to ascribe the historical error contained in the Notes presented 

 by these gentlemen to the Academy. According to them, the relation hitherto 

 admitted between the radiating powers of metals and of lamp-blacky would 

 result, not only from Leslie's experiments, but also from mine, and from that 

 of Petit and Dulong. It is true that Dulong and Petit have found numbers 

 differing but little from the relations ascribed by Leslie to the radiating powers 

 of metals and lamp-black. With regard to myself, I am not occupied with 

 researches of this nature. The only questions on calorific radiation, considered 

 as to its origin, which appeared to me to require further illustration by means 

 of the experiment so as to deserve for an instant the attention of natural philo- 

 sophers, were, the above mentioned action of the asperities of the surface of a 

 warm body, and the action of colour, both solved in the negative ; then the 

 influence which the thickness of the superficial bed, from which the interior 

 radiations issued, exercises on the intensity of the radiation ; an influence which 

 appears to me suflRcient to account for the enormous difference which exists 

 between the emissive power of metals and that of other bodies. With regard 

 to the value of these two powers, and their numerical relation as resulting from 

 my experiments, any one may readily satisfy himself that these were never 

 taken into consideration in my memoirs. 



