on the Production of Light and Heat. 27 9 
EXPERIMENT XIII. 
A piece of red hot metal continues to shine for some time 
after its removal from the fire ; which proves, that constant 
accessions of light or heat are not necessary to the shining of 
ignited bodies. If the piece be strongly blown upon, it in- 
stantly ceases to shine ; for the cold air, continually applied, 
unites with the light as fast as it leaves the body, and which 
otherwise would have passed to the eye. 
I shall now close this paper with two or three miscellaneous 
observations. 
Red hot bodies, though ignited by white light, give out only 
the red rays. Perhaps the other more refrangible rays, from 
their greater attraction to matter, may be circulating as heat, 
whilst the red ones, having a less attraction, yield sooner to 
that force which propels the light of red hot bodies. If the 
intensity of the incident white light be much increased, so as 
to raise the body to a white heat, the more refrangible rays 
then come out with the others, constituting together a white 
light. 
The flash of a grain of gunpowder is a pure white light : 
but if the explosion be made within a thin, unglazed, earthen 
ware tube, close at both ends, all the light that pervades the 
sides of the tube is red : the other rays must, therefore, remain 
united with the matter of the tube, whilst the less attractive 
red ones are transmitted. Thus also, on looking at the sun 
through the thin bottom of an earthen ware tea-cup, only the 
red rays are transmitted, so that the others must be retained 
by the matter of the cup. 
It would perhaps be worth trying, whether a body can be 
made red hot by concentrated rays of other colours. 
O o 2 
