458 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



These initial observations already proved clearly that the metal 

 armatures with which the first two thermometers were furnished, 

 and the similar metallic sides of the three receiving vessels, were 

 not sensibly cooled by radiation ; for otherwise the thermometers 

 furnished with polished metallic armatures would have marked 

 a higher temperature than that of the thermometer whose arma- 

 ture was blackened. But this conclusion became still more evi- 

 dent on taking away the covers of the two latter vessels, and 

 leaving one only of the two thermometers with polished armature 

 in the same condition as before. The blackened thermometer 

 now began to descend rapidly ; ten minutes afterwards it had 

 attained its lowest point, and marked 3°'4 less than the thermo- 

 meter contained in the other uncovered vessel ; and this latter 

 indicated the same temperature as the thermometer in the closed 

 vessel. Hence the immobility of the metallic thennometer in 

 the open vessel, and the identity of its temperature with that of 

 the metallic thermometer in the closed vessel, incontestably prove, 

 — 1st, that the cooling of the blackened thermometer is owing to 

 radiation, and not to the contact of the external air; 2nd, that 

 the cold produced by the radiation of the metallic thermometer 

 is nothing, or at least so feeble as to escape direct observation. 



The first conclusion is in perfect accordance with what we 



know of the great radiating power of lamp-black ; but the second 



is opposed to the views hitherto entertained as to the relative 



emissive powers of lamp-black and metallic surfaces. In fact, 



if we denote by 100 the calorific radiation of lamp-black, gold, 



silver, tin, and brass will have their radiating power expressed 



by 12, according to the experiments of Leslie, inserted in all 



treatises on physics. The degree of cold acquired by the black 



thermometer in consequence of its being freely exposed to the 



sky being 3'*4, the uncovered metallic thermometer ought to be 



12 

 cooled — — - (3°'4), i. e. 0°'41, a very appreciable quantity on an 



in.strument whose scale was divided to fifths of a degree ; never- 

 theless the experiment indicated no clearly appreciable variation 

 in the column of Lhe uncovered metallic thermometer. 



On the other hand, if metallic surfaces are not endowed with 

 that capacity of calorific emission usually attributed to them by 

 experimenters, it certainly could not hence be inferred that they 

 are absolutely deprived of all radiating power. 



I have therefore endeavoured to repeat with the greatest pos- 



