496 Prof. Helmholtz on the Interaction of Natural Forces. 



Here the question of a perpetual motion remained open ; the 

 decision of this question marks the progress of modern physics, 

 regarding which I promised to address you. 



In the case of the air-gun, the work to be accomplished in the 

 propulsion of the ball was given by the arm of the man who 

 pumped in the air. In ordinary firearms, the condensed mass 

 of air which propels the bullet is obtained in a totally differ- 

 ent manner, namely by the combustion of the powder. Gun- 

 powder is transformed by combustion for the most part into 

 gaseous products, which endeavour to occupy a much greater 

 space than that previously taken up by the volume of the powder. 

 Thus you see, that, by the use of gunpowder, the work which 

 the human arm must accomplish in the case of the air-gun is 

 spared. 



In the mightiest of our machines, the steam-engine, it is a 

 strongly compressed aeriform body, water vapour, which, by its 

 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 pressui'e. 

 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 it is named in chemistry, a strong affinity for the consti- 

 tuents 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 suffi- 

 ciently heated, the carbon unites itself with great violence to the 

 oxygen of the atmosphere and forms a peculiar gas, carbonic 

 acid, the same which we see foaming from beer and champagne. 

 By this combination light and heat are generated : heat is gene- 

 rally developed by any combination of two bodies of strong affi- 

 nity 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 condjustiou 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 mechanical 

 forces. A skilful blacksmith can render an iron wedge red-hot 

 by hammering. The axles of our carriages must be protected 



