104 New Outlines of Chemical Philosophy. 
Explanation. It has been shown in a former paper, that the 
electric machine creates nothing, but by friction only it collects 
the two elements which produce caloric from the earth and 
the atmosphere. Although the stone and the glass, in Mr, 
Wedgwood’s experiment, are both incombustible; yet it is almost 
a self-evident conclusion, that they collected the two elements of 
caloric from the air aud the earth by-friction in the same manner 
as the electric machine. 
If this explanation be not sufficiently conclusive, let two small 
pieces of plate-glass be rubbed together for a short time, and the 
will show electrical phenomena. The two pieces of glass being 
incombustible, the elements. which produce those phenomena 
must come from the earth and the atmosphere. This experi- 
ment is sufficient to show us how combustion was produced by 
the pressure of the glass upon the grit-stone when it was revolv- 
ing with great velocity. 
Experiment 7.‘ It is well known, that when flint and steel 
are smartly struck against each other, a spark always makes its 
appearance, which is capable of setting fre to tinder or to gun- 
powder. The spark in this case, as was long ago ascertained by 
Dr. Hooke, is asmall particle of the iron, ‘which is driven off, 
and catches fire during its passage through the air. This, there- 
fore, and all similar cases, belongs to the class of combustion. 
But light often makes its appearance when two bodies are struck 
against each other, when we are certain that no such thing as 
combustion can happen, because both the bodies are incombusti- 
ble. Thus, for instance, sparks are emitted when two quartz 
stones are struck smartly against each other, and light is emitted 
when they are rubbed against each other. Many other hard 
stones also emit sparks in the same circumstances. ' 
If they be often made to emit sparks above a sheet of white 
paper, there are found upon it a number of small black bodies, 
not very unlike the eggs of flies. These bodies are hard but 
friable, and when rubbed on the paper leave a black stain. 
When viewed with a microscope, they seem to have been melted. 
Muniatic acid changes their colour to a green, as it does that of 
lavas, These substances evidently produced the sparks by being 
heated red hot*. 
Explanation. It may be necessary to observe, in the first 
place, that if the flint be moved over the steel very slowly, no 
spark will be produced, nor .will any spark appear if the flint 
barely touches the steel, although the velocity of the flint be the 
greatest possible : consequently caloric cannot be produced by 
¢ollision or friction, but by velocity and pressure conjointly. 
* Thomson’s Chemistry, vol. i. p, 421. 
Secondly, 
