2 77 
on the Production of Light and Heat. 
not be luminous at so low a temperature. I therefore took a 
circular piece of glass, about ~ of an inch thick, and having 
gilt one side of it, exposed the ungilt side to a stream of 
air passed through a red hot tube ; but did not perceive that 
the gold shone at all before the glass. This experiment, how- 
ever, is not decisive ; glass being so slow a conductor of heat, 
that its exterior surface might have been heated some time 
before the interior, and thus have deceived the eye. I could 
not meet with any glass sufficiently thin for this purpose, nor 
think of any other possible mode of trial. 
EXPERIMENT IX. 
Having often remarked, that the surfaces of red hot metals 
had an appearance different from what they present by re- 
flected light when cold, I had an idea that this peculiar ap- 
pearance might be derived from a transmission of the light 
through the superficial parts of the ignited body. To ascer- 
tain whether they acquired any degree of transparency by 
heat, I fixed a circular plate of fine gold, about ~ of an inch 
thick, on the end of a tube, which was perfectly closed by it ; 
then, having heated it to redness, and looking down into the 
tube, I pressed the outer surface of the gold against single 
grains of gunpowder : the red light of the gold looked whiter 
on every flash. To be satisfied that no light found admission 
through the sides of the tube (which were of thick earthen 
ware), I covered the exterior surface of the gold plate with a 
thick coat of clay luting, and again making it red hot, fired 
gunpowder with it as before, but no increase of light was 
now perceptible from the flash ; which proves, that the sides 
of the tube were impervious to the light. When this gold 
Oo 
MDCCXCII. 
