MELLONI ON THE NOCTURNAL COOLING OF BODIES. 471 



The emissive power of lamp-black, which has shown itself 

 here, as elsewhere, to be one of the most radiating bodies, has 

 been afterwards measured under many other circumstances ; for 

 to each apparatus used in the experiments were added always a 

 couple of free thermometers, one with metallic armature, the 

 other with blackened armature ; and the difference between these 

 two instruments never surpassed l°-8, whatever were the clear- 

 ness of the sky and the calm of the atmosphere. 



There are nevertheless cases where a body freely suspended 

 may fall 4° or 5° below the temperature of the surrounding air. 

 In fact, the temperature of a thermometer enveloped in wool or 

 cotton* is always more or less inferior to that of a blackened 

 thermometer. But this excess of cold by no means results from 

 a superiority of emissive power in wool or cotton above that of 

 lamp-black ; and to convince ourselves of this, it is sufficient to 

 compare the effects of different thermometers clothed with tufls 

 of cotton or wool more or less compact, and with tissues more 

 or less fine and hairy, of the same substances uncoloured. 



In the following table will be found collected the experimental 

 data necessary for making these comparisons : — 



• "Carton" in the Frencii — evidently misprint for "colon. 



