364 
Book I. 
Emission 
of caloric 
by friction 
No oninge 
to conden 
sation, 
CALORIC EY 
whole of the phenomenon. We must either suppose 
that all the caloric is produced by mere condensation, 
which is not probable, or acknowledge that we cannot 
explain the phenomenon. This is almost the only in 
stance of the évolution of caloric and light, where the 
agency of oxygen cannot be demonstrated or even ren 
dered probable. The phenomenon, therefore, is of im 
portance, and may perhaps, when more accurately exa 
mined, give us some information concerning that un 
known step in combustion, which must be made before 
the theory can be completed. 
SECT. XI. 
OF FRICTION. 
CALORIC is not only evolved by percussion, but also 
by friction. Fires are often kindled by rubbing pieces 
of dry wood smartly against one another. It is well 
known that heavy loaded carts sometimes take fire by 
the friction between the axle-tree and the wheel. Now 
in what manner is the caloric evolved or accumulated 
by friction? Not by increasing the density of the bo 
dies rubbed against each other, as happens in cases of 
percussion ; for heat is produced by rubbing soft bodies 
against each other, the density of which therefore can 
not be increased by that means, as any one may con 
vince himself by rubbing his hand smartly against his 
coat. It is true, indeed, that heat is not produced by 
the friction of liquids, but then they are too yielding