s 8 o Mr. T. Wedgwood’s Experiments and Observations 
The light produced from bodies by attrition consists of a 
double light ; that which their powder would give out on the 
heater under redness ; and that which particles in their sur- 
faces give out by being made red hot. The sudden heating 
of a body to redness, by a single rub or blow, is a remarkable 
phaenomenon, and deserves to be investigated. One effect 
produced upon a body by attrition, is a compression or con- 
densation of the parts in its surface ; and it appears from ge- 
neral observation, that a condensation of the parts occasions a 
diminution of its capacity for heat. Iron may be made red 
hot by repeated blows of a hammer ; and I have found, that 
if red hot iron be forcibly struck by a heavy hammer, with a 
sharp edge to concentrate the action, the part so struck emits 
a white light for a sensible time, and is probably raised to a 
white heat : also, that my father's thermometer clay has its 
capacity for heat diminished one-third, by being burnt to 120° 
of his scale, and thus reduced to about one-half of its bulk ; 
and as it loses in weight little more than two grains on a pound, 
the diminution of capacity can only be attributed to its con- 
densation. Many other analagous instances might be ad- 
duced if necessary ;* but these will, perhaps, be deemed suffi- 
cient to render it probable, that the sudden ignition of the 
particles by attrition proceeds from the compression, and con- 
sequent diminution of the capacity for heat. 
I am not certain that the increase of brightness in the gold 
plate, experiment ix. must be attributed to its transparency : 
it may arise from the gold being suddenly heated to a white heat 
by the light of the explosion ;*or the force of the explosion 
* See Dr. Darwin’s excellent paper of Frigorific Experiments on the mechanical 
Expansion of Air, &c. Phil. Trans. Vol. LXXVII1. p. 43. 
