80 LIFE AND DEATH. 



§ 4. Thermal Energy. 



In the elements of physics it is nowadays taught 

 that mechanical work may be transformed into heat, 

 and reciprocally that heat may be transformed into 

 mechanical work. Friction, impact, pressure, and 

 expansion destroy or annihilate the mechanical energy 

 communicated to a body or to the organs of a 

 machine. With the disappearance of motion we 

 note the appearance of heat. Examples abound. 

 The tyre of a wheel is heated by the friction of the 

 road. Portions of steel are warmed by the impact 

 with stone, as in the old flint and steel. Two pieces 

 of ice were melted by Davy, who rubbed them one 

 against the other, the external temperature being 

 below zero. The boiling of a mass of water caused 

 by a drill was noticed by Rumford in 1790, during 

 the manufacture of bronze cannon. Metal, beaten on 

 an anvil, is heated. A leaden ball flattened against a 

 resisting obstacle shows increase of temperature carried 

 to the point of fusion. Finally, and symbolically, we 

 have the origin of fire in the fable of Prometheus, by 

 rubbing together the pieces of wood which the Hindoos 

 called pramantJia. Correlation is constant between 

 the thermal and mechanical phenomena, a correlation 

 that becomes evident as soon as observers have ceased 

 to restrict themselves to the determination in isolation 

 of the one fact or the other. There is never any real 

 destruction of heat and motion in the true sense of the 

 word ; what disappears in one form appears again in 

 another; just as if something indestructible were ap- 

 pearing in a series of successive disguises. This 

 impression is translated into words when we speak 



