266 Mr. Thompson’s Experiments 
“ {haken, and loofened to their very intimate fubftance* ?” And 
in proportion to the fwiftnefs of this vibration, and the violence 
of the attrition and friction, will be the heat that is produced. 
A piece of iron that would fuftain the prefiure of any weight, 
however large, without being warmed, may be made quite hot 
by the blow of a hammer ; and even fott and un-elaftic bodies 
may be warmed by percuftion, provided the velocity with which 
their parts are made to give way to the blow is fufficiently rapid. 
If a leaden bullet is laid upon an anvil, or any other hard body, 
and in that fituation it is {truck with a fmart blow of a ham- 
mer, it will be found to be much heated ; but the fame bullet in 
the fame fituation may be much more flattened by preflure, or 
by the ftroke of a very heavy body moving with a fmall velo- 
city, without being fenfibly warmed. 
To generate heat therefore the adlion of the powder upon the 
infide of the piece mult not only be fufficient to {train the metal, 
and produce a motion in its parts, but this effedt muft be ex- 
tremely rapid ; and the heat will be much augmented, if the 
exertion of the force and the duration of its action are momen- 
taneous , for in that cafe, the fibres of the metal (if I may ufe 
the expreffion) that are violently ft retched, will return with 
their full force and velocity, and the fwift vibratory motion and 
attrition before defcribed will be produced. 
The heat generated in a piece by firing is therefore as the 
force by which the particles of the metal are ftrained and com- 
prefled, the fuddennels with which this force is exerted, and the 
• fhortnefs of the time of its adtion ; that is to fay, as the 
ftrength of the powder and the quantity of the charge, the 
quicknefs of its inflammation, and the velocity with which the 
generated fluid makes its efcape. 
* Vide shaw’s tranflatioa of boerhaave’s Chemifhy, vol. I. p. 249. 
Now 
