ON THE CONSEKVATION OF FORCE. 345 



the dry hands together to feel the heat produced by fric- 

 tion, and which is far greater than the heating which 

 takes place when the hands lie gently on each other. 

 Uncivilized people use the friction of two pieces of wood 

 to kindle a fire. With this view, a sharp spindle of hard 

 wood is made to revolve rapidly on a base of soft wood in 

 the manner represented in Fig. 47. 



Fig. 47. 



So long as it was only a question of the friction of 

 solids, in which particles from the surface become de- 

 tached and compressed, it might be supposed that some 

 changes in structure of the bodies rubbed might here 

 liberate latent heat, which would thus appear as heat of 

 friction. 



But heat can also be produced by the friction of liquids, 

 in which there could be no question of changes in struc- 

 ture, or of the liberation of latent heat. The first de- 

 cisive experiment of this kind was made by Sir Humphry 

 Davy in the commencement of the present century. In 



