36 Experiments upon Gunpowder. 



can be attributed, and that is the motion and friction of 

 the internal parts of the metal among themselves, occa- 

 sioned by the sudden and violent effort of the powder 

 upon the inside of the bore, and to this cause I imagine 

 the heat is principally, if not almost intirely, owing. It 

 is well known that a very great degree of heat may be 

 generated in any hard and dense body in a short time 

 by friction, and in a still shorter time by collision. 

 " For if two dense, hard, elastic bodies be struck against 

 each other with great force and velocity, all the parts of 

 such bodies will every moment be closely compressed, 

 and, being rigid, will react with equal force. Hence, a 

 quick and powerful contraction and expansion will arise 

 in every part, resembling that swift kind of vibration 

 observed in stretched strings ; how great these vibra- 

 tions are may be learnt from the instance of a bell 

 when struck with a single blow, by which the whole 

 bulk, however vast, will for a long time expand and 

 contract itself in infinite ellipses. And when the attri- 

 tion above described is produced, with what force and 

 velocity are all the particles of the rubbed body com- 

 pressed, shaken, and loosened to their very intimate 

 substance ! " * And in proportion to the swiftness of 

 this vibration, and the violence of the attrition and 

 friction, will be the heat that is produced. 



A piece of iron that would sustain the pressure of 

 any weight, however large, without being warmed, may 

 be made quite hot by the blow of a hammer ; and even 

 soft and un-elastic bodies may be warmed by percussion, 

 provided the velocity with which their parts are made to 

 give way to the blow is sufficiently rapid. If a leaden 

 bullet be laid upon an anvil, or on any other hard body, 



* Vide Shaw's Translation of Boerhave's Chemistry, vol. i. p. 249. 



