356 



THKRMOD YNAMICS. 



broken up, so to speak, and distributed among the mole- 

 cules, producing that form of molecular motion that w % e call 

 heat. The same transformation was 

 illustrated in the kindling of a fire by 

 the "flint and steel " of a century ago. 

 It may be experimentally illustrated 

 by the "air-syringe." 



(a.) The air-syringe consists of a cylinder 

 of metal or glass and an accurately fitting 

 piston. By suddenly driving in the piston, 

 the air is compressed and heat developed. 

 A bit of gun cotton previously placed in 

 the cylinder may thus be ignited. If the 

 cylinder be made of glass, and a bit of ordi- 

 nary cotton dipped in sulphuric ether be 

 used, repeated flashes of light may be pro- 

 duced by successive combustions of ether 

 vapor. The fumes of one combustion 

 must be blown away before the next com- 

 bustion is attempted. 



564. Heat from Friction. 



Common matches are ignited and cold 

 hands warmed by the heat developed 

 by friction. It is said that some savages kindle fires 

 by skilfully rubbing together well-chosen pieces of wood. 

 In the case of the axles of railway cars and ordinary car- 

 riages, this conversion of mechanical energy into heat is 

 not so difficult as its prevention. Lubricants are used to 

 diminish the friction and prevent the waste of energy due 

 to the undesirable transformation. A railway train is 

 really stopped by the conversion of its motion into heat. 

 When this has to be done quickly, the change is hastened 

 by increasing the friction by means of the brakes. Ex- 

 amples of this change are matters of every day experience. 



FIG. 264. 



