444 THE PKINOIPLES OF SCIENCE. [CHAP. 



Hence the resistance of the air became approximately a 

 matter of indifference ; for the outward size and shape of 

 the pendulums being the same, the absolute force of re- 

 sistance would be the same, so long as the pendulums 

 vibrated with equal velocity ; and the weights being equal 

 the resistance would diminish the velocity equally. Hence 

 if any inequality were observed in the vibrations of the two 

 pendulums, it must arise from the only circumstance which 

 was different, namely the chemical nature of the matter 

 within the boxes. No inequality being observed, the 

 chemical nature of substances can have no appreciable 

 influence upon the force of gravitation. 1 



A beautiful experiment was devised by Dr. Joule for 

 the purpose of showing that the gain or loss of heat by a 

 gas is connected, not with the mere change of its volume 

 and density, but with the energy received or given out by 

 the gas. Two strong vessels, connected by a tube and stop- 

 cock were placed in water after the air had been exhausted 

 from one vessel and condensed in the other to the extent 

 of twenty atmospheres. The whole apparatus having 

 been brought to a uniform temperature by agitating the 

 water, and the temperature having been exactly observed, 

 the stopcock was opened, so that the air at once expanded 

 and tilled the two vessels uniformly. The temperature of 

 the water being again noted was found to be almost un- 

 changed. The experiment was then repeated in an exactly 

 similar manner, except that the strong vessels were placed 

 in separate portions of the water. Now cold was produced 

 in the vessel from which the air rushed, and an almost 

 exactly equal quantity of heat appeared in that to which 

 it was conducted. Thus Dr. Joule clearly proved that 

 rarefaction produces as much heat as cold, and that only 

 when there is disappearance of mechanical energy will 

 there be production of heat. 2 What we have to notice, 

 however, is not so much the result of the experiment, as 

 the simple manner in which a single change in the appa- 

 ratus, the separation of the portions of water surrounding 

 the air vessels, is made to give indications of the utmost 

 significance. 



1 Principia, bk. iii. Prop. vi. 



2 Philosophical Magazine, 3rd Series, vol. xxvL p. 375. 



