ON THE INTERACTION OF NATURAL FORCES, 163 



chemical forces, all of which nevertheless stand in mani- 

 fold relation to mechanical processes. There is hardly a 

 natural process to be found which is not accompanied by 

 mechanical actions, or from which mechanical work may 

 not be derived. 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 different manner, namely, by the combustion 

 of the powder. Gunpowder 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 pressure. In this case, 

 therefore, it is the heat communicated which generates 

 the mechanical force. The heat thus necessary for tiie 

 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 con- 

 stituent 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, 



