on the Production of Light from Bodies. 43 
attrition, prove, that particles in their surfaces are heated at 
least to more than 6 oo° of Fahrenheit, for the powders of hard 
bodies are not luminous on a heater much below redness. The 
heat generated by the attrition of soft friable bodies is pro- 
bably but little, as the minute particles in their surfaces are 
not much crushed by each other, but merely disjoined from 
the masses ; nor can this little be easily appreciated, for as the 
surfaces continually crumble away, the heated parts are carried 
off before they can warm the masses, and are themselves very 
quickly cooled by the surrounding air. But as many soft bodies 
emit a faint light on the heater at the low temperature of about 
400° of Fahrenheit, and as it is not a stronger light which they 
yield on attrition, little heat is required to render the particles 
in their surfaces luminous. It must be observed too, that, 
though the absolute quantity of heat generated by one rub be 
but inconsiderable, the effects of it may be very striking ; for 
just on the instant of attrition, its action is confined to the mi- 
nute colliding points of the surfaces, and will consequently 
operate upon them as much as a greater quantity would on 
larger points. 
The light emitted by bodies in attrition is of momentary du- 
ration, whereas a powder on the heater continues to emit light 
for some minutes : this difference is easily explained ; in the 
latter case, the particles are constantly heated ; in the former, 
they are instantly cooled by the subjacent mass to a tempe- 
rature in which bodies are not luminous, i. e. a little under 400° 
of Fahrenheit. 
When the uneven surfaces of bodies are rubbed upon each 
other, a flashing light is produced at frequent intervals, by the 
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