42 Mr. T. Wedgwood's Experiments and Observations 
has no dependence on the light produced by attrition, as if is 
often very strong when no light is emitted. 
Rock crystal, quartz, feldspat, white biscuit earthen w r are, 
and probably all such hard bodies, produce this smell under 
water. 
Quartz stones, violently rubbed upon one another for a few 
minutes in a cup of water, communicate this smell, and a pecu- 
liar taste, to the water. The taste is probably derived from an 
impalpable powder, which floats in the water for many days. 
Derbyshire black marble, and the stinking blue fluor, give 
out, on attrition, a strong smell peculiar to themselves, both 
in air and water ; they lose this property by being once made 
red hot. 
Quartz produces the smell equally strong in fixed, pure, and 
common, air. 
Having now stated all the facts relative to phosphorescent 
bodies which I have as yet been able to discover, I shall beg 
leave to offer a few reflections, tending to shew, that heat is the 
probable cause of the light produced from bodies by attrition. 
The powders of all earthy bodies emit light when heated a 
little under redness. Now, when two bodies are rubbed upon 
each other, it is probable that heat is always generated on 
their surfaces : may not then the light which they yield upon 
attrition be attributed to a sudden heating of particles in their 
surfaces? for these particles will be affected in the same way 
as if they had been equally heated by any other means ; they 
will therefore give the same light as if they had been laid upon 
the heater of an equal temperature. 
The shining sparks which hard bodies send out during 
