1HE CONSERVATION OF FORCE 20$ 



which heat performs, by assuming that the hypothetical 

 caloric endeavoured to expand like a gas; and from this 

 assumption he deduced in fact a remarkable law as 

 to the capacity of heat for work, which even now, though 

 with an essential alteration introduced by Clausius, is 

 among the bases of the modern mechanical theory of 

 heat, and the practical conclusions from which, so far 

 as they could at that time be compared with experiments, 

 have held good. 



But it was already known that whenever two bodies in 

 motion rubbed against each other, heat was developed anew, 

 and it could not be said whence it came. 



The fact is universally recognised; the axle of a carriage 

 which is badly greased and where the friction is great, 

 becomes hot so hot, indeed, that it may take fire; machine- 

 wheels with iron axles going at a great rate may become so 

 hot that they weld to their sockets. A powerful degree of 

 friction is not, indeed, necessary to disengage an appreci- 

 able degree of heat; thus, a lucifer-match, which by rubbing 

 is so heated that the phosphoric mass ignites, teaches this 

 fact. Nay, it is enough to rub the dry hands together to 

 feel the heat produced by friction, 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. 99. 



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

 in which particles from the surface become detached 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 structure, 

 or of the liberation of latent heat. The first decisive ex- 

 periment of this kind was made by Sir Humphry Davy in 

 the commencement 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 have here assimilated could not 



