302 



ON THE CONSERVATION OF FORCE. 



and which is far greater than the heating which takes place 

 when the hands lie gently on each other. Uncivilised 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. 



So long as it was only a question of the friction of solids, in 

 which particles from the surface become detached and com- 

 pressed, it might be supposed that some changes in structure of 



FIG. 47. 



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 structure, or of 

 the liberation of latent heat. The first decisive experiment of 

 this kind was made by Sir Humphry Davy in the commence- 

 ment of the present century. In a cooled space he made two 

 pieces of ice rub against each other, and thereby caused them 

 to melt. The latent heat which the newly formed water must 



