146 ON THE INTERACTION OF NATURAL FORCES. 



effort to expand, sets the machine in motion. Here also we do 

 not condense the steam by means of an external mechanical 

 force, but by communicating heat to a mass of water in a closed 

 boiler, we change this water into steam, which, in consequence 

 of the limits of the space, is developed under strong pressure. 

 In this case, therefore, it is the heat communicated which gene- 

 rates the mechanical force. The heat thus necessary for the 

 machine we might obtain in many ways : the ordinary method 

 is to procure it from the combustion of coal. 



Combustion is a chemical process. A particular constituent 

 of our atmosphere, oxygen, possesses a strong force of attraction, 

 or, as is said in chemistry, a strong affinity for the constituents 

 of the combustible body, 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 combination 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 pro- 

 cesses and chemical forces which produce the astonishing work 

 of these machines. In like manner the combustion of gun- 

 powder 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 mechanical 

 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 sur- 

 plus 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. 



