and the Mode of its Communication. 59 



As the conducting power of lead, with respect to heat, 

 is much greater than that of any other metal, I con- 

 ceived that, if the radiation of a body were any way 

 connected with its conducting power, the cooling of the 

 water contained in the leaden vessel would necessarily 

 be either more or less rapid than in a vessel constructed 

 of any other metal. 



The result of this experiment, as also the results of 

 several others similar to it, showed that heat is given 

 off with the same facility, or with the same celerity, 

 from the surfaces of all the metals. ' 



Is not this owing to their being all equally wanting 

 in transparency ? And does not this afford us a strong 

 presumption that heat is in all cases excited and com- 

 municated by means of radiations, or undulations, as I 

 should rather choose to call them ? 



I am sensible, however, that there is another and 

 most important question to be decided before these 

 points can be determined ; and that is, whether bodies 

 are cooled in consequence of the rays they emit or by 

 those they receive. 



The celebrated experiment of Professor Pictet, which 

 has often been repeated, appears to me to have put the 

 fact beyond all doubt, that rays, or emanations, which, 

 like light, may be concentrated by concave mirrors, 

 proceed from cold bodies ; and that these rays, when so 

 concentrated, are capable of affecting, in a manner per- 

 fectly sensible, a delicate air thermometer. 



One of the objects I had principally in view, in con- 

 triving the before-described instrument, which I have 

 called a thermoscope, was to investigate the nature and 

 properties of those emanations, and to find out, if pos- 

 sible, whether they are not of the same nature as those 



