ON THE CONSERVATION OF FORCE. 



337 



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. 44 an apparatus is represented such as 



FIG. 44. 



Kegnault used for measuring the expansive force of heated 

 gases. If no great accuracy is required in the measure- 



