on the Production of Light from Bodies. 45 
at that temperature, which could not be effected by sprinkling 
their powders on a red hot heater, as the light of the powder 
would be mixed with that of the heater. In some cases of 
attrition, bodies are raised to a temperature beyond visible red 
heat. The corner of an angular piece of window glass being 
applied to the circumference of a revolving wheel of fine grit, 
part of its mass is worn away ; but a larger portion, lying just 
above the abraded part, is heated to redness. Now, as all the 
heat which is there collected, and a great deal more, which is 
carried away in the abraded part, and conducted off by the air, 
and by the glass lying up to the red hot portion, has once oc- 
cupied a smaller space in the part worn away ; it follows, that 
the abraded portion, or aggregate of heated surfaces, has been 
heated to a degree exceeding redness, by all the heat remaining 
in the red hot part, and by the quantity of heat conducted offby 
the air and the adjacent glass ; and, consequently, that each sur- 
face has been heated by the attrition to a degree as much ex- 
ceeding redness. 
I am aware that this reasoning is founded, in part, on the sup- 
position that the heat is generated on the surface or outermost 
coat of the body : some of it may undoubtedly proceed from 
an agitation of parts under the surface ; but the emission of 
red hot sparks at the instant of attrition, proves that a great 
heat is generated on the surface ; and, as the friction, or apparent 
heating cause, is so much greater there, that the parts are for- 
cibly broken and disjoined, whilst just beneath there is no per- 
ceptible alteration in the body, we may venture to conclude 
that the heat generated beneath the surface is but inconsider- 
able. 
