112 Count Rumford’s Enquiry concerning the Nature of Heat , 
and silver, I now provided myself with two new instruments, 
the one made of lead, and the other covered with tinned sheet 
iron, improperly, in England, called tin. 
As th e conducting power of lead, with respect to heat, is much 
greater than that of any other metal, I conceived 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 communicated by means 
of radiations, or undulations , as I should rather choose to call 
them ? 
I am sensible, however, that there is another and most im- 
portant question to be decided, before these points can be deter- 
mined ; 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 perfectly sensible, a delicate air thermo- 
meter. 
One of the objects I had principally in view, in contriving the 
