218 APPENDIX A. 



We also know that always in the collision of 

 bodies there occurs a change of temperature, an 

 elevation of temperature. We cannot, as did M. 

 Berthollet, attribute the heat set free in this case 

 to the reduction of the volume of the body; for 

 when this reduction has reached its limit the liber- 

 ation of heat would cease. Now this does not oc- 

 cur. 



It is sufficient that the body change form by per- 

 cussion, without change of volume, to produce dis- 

 engagement of heat. 



If, for example, we take a cube of lead and strike 

 it successively on each of its faces, there will always 

 be heat liberated, without sensible diminution in 

 this disengagement, so long as the blows are con- 

 tinued with equal force. This does not occur when 

 medals are struck. In this case the metal cannot 

 change form after the first blows of the die, and 

 the effect of the collision is not conveyed to the 

 medal, but to the threads of the screw which are 

 strained, and to its supports. 



It would seem, then, that heat set free should 

 be attributed to the friction of the molecules of 

 the metal, which change place relatively to each 

 other, that is, the heat is set free just where the 

 moving force is expended. 



A similar remark will apply in regard to the col- 



