452 PHYSIOLOGICAL PHYSICS. [Chap, xxxiv. 



Mayer, a physician of Heilbronn, in 1842, and 

 experimentally demonstrated by Joule, of Manchester, 

 in 1843. 



According to the theory, mechanical energy is 

 capable of being converted into heat, and heat into 

 mechanical energy, the amount of one being directly 

 determined by the amount of the other. The conver- 

 sion of mechanical energy into heat is exhibited in 

 many well-known occurrences. The production of 

 heat by friction is one of the commonest illustrations. 

 Thus, the rapid twirling of a pointed stick on a piece 

 of hard wood is the method pursued by savages for 

 obtaining fire, while everyone knows that the heat 

 developed by the friction of the various parts of a 

 machine is one great cause of loss of power. The 

 hammering of a mass of metal on an anvil involves 

 the expenditure of a large amount of energy, but the 

 hammered metal is found to be much warmer at the 

 end of the process than it was at the beginning. When 

 a falling stone is suddenly stopped by concussion with 

 something which destroys its motion, the energy of 

 the falling body is not destroyed, for a large amount 

 of heat is developed by the shock, and an amount 

 proportional to the mass of the body and the distance 

 through which it has fallen. Similarly, the energy of 

 a rifle bullet which hits a target is not lost, but may 

 produce sufficient heat to fuse the ball. The produc- 

 tion of heat by various other forms of mechanical 

 work is easily shown experimentally. If air be com- 

 pressed in a metallic box heat is generated. This 

 can be shown by using the thermopile, whose con- 

 struction and mode of use are explained in chapter 

 xiv. If a thermopile be connected with a galvano- 

 meter in the way represented in Fig. 77, and if, by 

 means of a pair of bellows, a current of air be driven 

 against one set of junctions of the pile the needle of 

 the galvanometer will be deflected in a direction that 



