56 THE PRINCIPLES OF SCIENCE. 



stances, so that the total weights should be exactly equal 

 and the centres of oscillation at the same distance from 

 the points of suspension. Hence the resistance of the air 

 became approximately a matter of indifference ; for the 

 outward size and shape of the pendulums being exactly 

 the same, the absolute force of resistance would be the 

 same, so long as the pendulums vibrated with equal 

 velocity ; and the weights being equal the force would 

 diminish the velocity in like degree. Hence if any in- 

 equality were observed in the vibrations of the two pen- 

 dulums, it must arise from the only circumstance which 

 was different, namely the chemical character 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 s. 



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 surrounded entirely with 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 stop-cock was opened, so that the 

 air at once expanded and filled the two vessels uniformly. 

 The temperature of the water being again noted was 

 found to be almost entirely unchanged. The experiment 

 was then repeated in an exactly similar manner, except 

 that the strong vessels were placed in separate portions 

 of water. It was then discovered that cold was produced 

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

 exactly equal quantity of heat appeared in that to which 

 S ' Principia,' bk. III. Prop. vi. 



