198 HELMHOLTZ 



at the moment it is shot off. Thus the crossbow concen 

 trates into an extremely short time the entire work which 

 the arm had communicated in the operation of stretching; 

 the clock, on the contrary, spreads it over one or several 

 days. In both cases no work is produced which my arm did 

 not originally impart to the instrument, it is only expended 

 more conveniently. 



The case is somewhat different if by any other natural 

 process I can place an elastic body in a state of tension with- 

 out having to exert my arm. This is possible and is most 

 easily observed in the case of gases. 



If, for instance, I discharge a firearm loaded with gun- 

 powder, the greater part of the mass of the powder is con- 

 verted into gases at a very high temperature, which have a 

 powerful tendency to expand, and can only be retained in the 

 narrow space in which they are formed, by the exercise of 

 the most powerful pressure. In expanding with enormous 

 force they propel the bullet, and impart to it a great velocity, 

 which we have already seen is a form of work. 



In this case, then, I have gained work which my arm has 

 not performed. Something, however, has been lost the 

 gunpowder, that is to say, whose constituents have changed 

 into other chemical compounds, from which they cannot, 

 without further ado, be restored to their original condition. 

 Here, then, a chemical change has taken place, under the 

 influence of which work has been gained. 



Elastic forces are produced in gases by the aid of heat, on 

 a far greater scale. 



Let us take, as the most simple instance, atmospheric air. 

 In FIG. 96 an apparatus is represented such as Regnault 

 used for measuring the expansive force of heated gases. If 

 no great accuracy is required in the measurement, the ap- 

 paratus may be arranged more simply. At C is a glass globe 

 filled with dry air, which is placed in a metal vessel, in 

 which it can be heated by steam. It is connected with the 

 U-shaped tube, S s, which contains a liquid, and the limbs 

 of which communicate with each other when the stop-cock 

 R is closed. If the liquid is in equilibrium in the tube S s 

 when the globe is cold, it rises in the leg s, and ultimately 

 overflows when the globe is heated. If, on the contrary, 



