164 ON THE INTER ACTIOX OF NATURAL FORCES. 



which affinity, however, in most cases can only exert 

 itself at high temperatures. As soon as a portion of the 

 combustible body, for example the coal, is sufficiently 

 heated, the carbon unites itself with great violence to 

 the oxygen of the atmosphere and forms a peculiar gas, 

 carbonic acid, the same that we see foaming from beer 

 and champagne. By this combination light and heat are 

 generated ; heat is generally developed by any combina- 

 tion of two bodies of strong affinity for each other ; and 

 when the heat is intense enough, light appears. Hence 

 in the steam-engine it is chemical processes and chemical 

 forces which produce the astonishing work of these 

 machines. In like manner the combustion of gunpowder 

 is a chemical process, which in the barrel of the gun 

 communicates living force to the bullet. 



While now the steam-engine developes for us mechanical 

 work out of heat, we can conversely generate heat by me- 

 chanical forces. Each impact, each act of friction does it. 

 A skilful blacksmith can render an iron wedge red-hot by 

 hammering. The axles of our carriages must be protected 

 by careful greasing from ignition through friction. Even 

 lately this property has been applied on a large scale. In 

 some factories, where a surplus of water power is at hand, 

 this surplus is applied to cause a strong iron plate to rotate 

 rapidly upon another, so that they become strongly heated 

 by the friction. The heat so obtained warms the room, and 

 thus a stove without fuel is provided. Now could not 

 the heat generated by the plates be applied to a small 

 steam-engine, wliich in its turn should be able to keep 

 the rubbing plates in motion? The perpetual motion 

 would thus be at length found. This question might be 

 asked, and could not be decided by the older mathematico- 

 mechanical investigations. I will remark beforehand, 

 that the general law which I will lay before you answers 

 the question in the negative. 



