220 INTERACTION OF NATURAL FORCES. 



liquids and gases, could only be sought after by bewildered 

 and ill-instructed people. But there are still other natural 

 forces which are not reckoned among the purely moving 

 forces, heat, electricity, magnetism, light, chemical forces, 

 all of which nevertheless stand in manifold relation to me- 

 chanical 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. 



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 endeavor to occupy a much 

 larger 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 eagine, 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 the machine we might obtain in many 

 ways ; the ordinary method is to procure it from the combus- 

 tion of coal. 



Combustion is a chemical process. A particular constitu- 

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



