upon Gun-powder , &c. 265 
iron, there is no doubt but they would hand fire as well as iron 
guns ; and if heel, or any other metal, either fimple or com- 
pounded, fhould upon trial be found to anfwer for that pur- 
pofe better than iron, it might be ufed inhead of it; and 
even if gold was made ufe of for lining the vent, I imagine it 
might be done in fuch a manner as that the expence would not 
be very confiderable, at the fame time that the thicknefs of the 
gold fhould be fufficient to withhand the force of the flame for 
a very great length of time. 
But to return to the heat acquired by guns in firing. It 
being pretty evident that it is not all communicated by the 
flame, there is but one other caufe to which it can be attri- 
buted, and that is the motion and friftion of the internal parts 
of the metal among themfelves, occafoned by the hidden and 
violent effort of the powder upon the inflde of the bore, and 
to this caufe I imagine the heat is principally if not almoh 
intirely owing. It is well known,, that a very great degree of 
heat may be generated in any hard and denfe body in a fhort 
fpace of time by fridtion, and in a hill fhorter time by colli- 
lion. 44 For if two denfe hard elahic bodies be hruck againft 
44 each other with great force and velocity, all the parts of 
46 fuch. bodies will every moment be clofely compreffed, and 
44 being rigid will re-a6l with equal force. Hence a quick and 
44 powerful contradlion.and. expanfon will arife in every part, 
44 refembling that fwift kind of vibrations obferved inhreiched; 
44 firings ; how great thefe vibrations are may be learnt hom. 
44 the inhance of a bell, when hruck with a ■ Angle blow, by 
44 which the whole bulk, however vah, will for a long time- 
44 expand, and contrah itfelf in infinite ellipfes. And when: 
44 the attrition above defcribed is produced, with what force and 
44 velocity are all the particles of the rubbed body comprefled, 
2 44 fhaken,, 
