294 ON THE CONSERVATION OF FORCE. 



winding up the clock, and are uncoiled by the working of the 

 clock. To coil up the spring we consume the force of the arm; 

 this has to overcome the resisting elastic force of the spring as 

 we wind it up, just as in the clock we have to overcome the force 

 of gravity which the weight exerts. The coiled spring can, 

 however, perform work; it gradually expends this acquired 

 capability in driving the clockwork. 



If I stretch a crossbow and afterwards let it go, the stretched 

 string moves the arrow ; it imparts to it force in the form of 

 velocity. To stretch the cord my arm must work for a few 

 seconds ; -this work is imparted to the arrow at the moment it 

 is shot off. Thus the crossbow concentrates 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 pro- 

 cess I can place an elastic body in a state of tension without 

 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 gunpowder 

 the greater part of the mass of the powder is converted into 

 gases at a very high temperature, which have a powerful ten- 

 dency 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 pel-formed. Something, however, has been lost the gun- 

 powder, 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. 



