on the Production of Light from Bodies. 39 
brightly red, even in day-light, at the touching part ; if the 
wheel revolve at a quicker rate, the touching part emits a pure 
white light. In both cases, glowing sparks are continually 
emitted, some of which are not extinguished before they have 
passed twelve or fourteen inches through the air ; they explode 
gunpowder and inflammable air, and bum the skin ; their 
brightness is not sensibly increased by passing into pure air. 
The corner of an angular piece of window glass being applied 
to the wheel in motion, a full eighth of an inch of the glass 
above the point of contact becomes, apparently, red hot, and 
retains the redness for a second or two of time after its remo- 
val from the wheel ; during the attrition, large red sparks are 
continually emitted, and a mixture of softened glass, and the 
sand of the stone wheel, is collected about the touching point. 
Quartz, transparent agate, rock crystal, and window glass, 
give nearly the same flashing light, when rubbed against 
the stone wheel, or in the ordinary manner, excepting the 
tinge of red in the former, which it receives from the light of 
the grit : the transparent agate becomes red hot for a little 
way about the part in contact with the wheel, and is thus de- 
prived of its transparency, as it would be if made red hot in a 
common fire ; porcelain is heated to redness by the same treat- 
ment. The red sparks which are emitted by all these bodies 
during their attrition, are heated particles about the magni- 
tude of grains of fine sand, broken off by the friction. 
Bodies give out their light the instant they are rubbed upon 
each other, and cease to be luminous when the attrition is dis- 
continued. Colourless, transparent, and semitransparent bo- 
dies emit a flashing light, their whole masses being, for a mo- 
ment, illuminated ; ' opake bodies give little more than a 
