196 H, HelmhoUz on the Interaction of Natural Forces. 



boiler, we cliange tliis water into steam, wliich, in consequence 

 of tlie 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 attrac- 

 tion, or, as it is named in chemistry, a strong affinity for the con- 

 stituents 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, car- 

 bonic acid, the same which we see foaming from beer and cham- 

 pagne. 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, liglit 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 develops for us mechanical work 

 out of heat, we can conversely generate heat by mechanical 

 forces. A skillful 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. And lately 

 this property has been applied on a large scale. In some facto- 

 ries, where a surplus of water-power is at hand, this surplus is 

 applied to cause a strong iron plate to rotate swiftly upon an- 

 other, 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, which in its turn should be 

 able to keep the rubbing plates in motion? The perpetual mo- 

 tion w^ould 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. 



By a similar plan, however, a speculative American set some 

 time ago the industrial world of Europe in excitement. The 

 magneto-electric machines oflen made use of in the case of rheu- 



well known to the public. By imparting a 

 swift rotation to the magnet of such a machine we obtain power- 

 ful currents of electricity. If those be conducted through wa- 



ic disorders are 



r , 



