46 Mr. T. Wedgwood's Experiments and Observations 
After all, it remains entirely problematical, in what manner 
heat operates to produce light from bodies: the air does not 
seem to have any concern in its production, as bodies are 
equally luminous in almost all kinds of air, and when immersed 
in liquids. The phosphorism of sugar is probably of a different 
kind from that of the earthy class ; for, though so soft and friable 
a substance, it produces its light very copiously upon gentle 
attrition. 
In speaking of the attrition of bodies upon the stone wheel, 
I have said that they became red hot about the touching part ; 
I should not have made use of this expression if the luminous 
sparks, which issued from them, had not kindled gunpowder 
and inflammable air, and thus proved that the part from which 
they came was raised to a temperature, at least equal to what 
is usually termed a red heat ; for till the temperature of the 
part touching the wheel had been thus ascertained, I attributed 
the whole of the light emitted to the common phosphorism of 
bodies. If the velocity of the wheel be much increased, the 
touching part of the body applied, emits a bright white light, 
much more vivid than any which powders ever give out on the 
heater, and, probably, the temperature of the luminous part is 
equal to what is usually called a white heat. 
Having thus made incombustible bodies red hot without the 
aid of fire, I once conceived that all the light which they emit 
when heated to redness in the Are, proceeded entirely from their 
great phosphorism ; for I could not suppose that they absorbed 
light from the burning fuel and emitted it again, at the same 
time, and during a continuance of the same circumstances. It 
appeared, however, equally inexplicable, why a stone put into 
