HERMANN VON HELMHOLTZ 281 



pulsion 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 pro- 

 pels the bullet is obtained in a totally different manner, namely, by 

 the combustion of the powder. Gunpowder is transformed by com- 

 bustion for the most part into gaseous products, which endeavor to 

 occupy a much larger space than that previously taken 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 ex- 

 pand, sets the machine in motion. Here, also, we do not condense the 

 steam by means of an external mechanical force, but by communicat- 

 ing 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 de- 

 veloped under strong pressure. In this case, therefore, it is the heat 

 communicated which generates the mechanical force. The heat thus 

 necessary for the machine we might obtain in many ways ; the ordi- 

 nary 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 constituents of the com- 

 bustible 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 which 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 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 bul- 

 let. 



While now the steam engine develops 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 hammer- 



