Heat and Energy. The Kinetic Theory 205 



of a Boyle's tube as used in Experiment 36 and, by 

 inclining the tube, cause some of the air from the closed 

 limb to escape through the mercury. Immerse the 

 lower part of the tube in a deep vessel containing 

 water, so that the top of the closed limb is under water. 

 Bring the tube to a vertical position, and mark the 

 level of the liquid in each limb. This may readily be 

 done by scratching the surface of the glass with a sharp 

 file. Gently heat the vessel, and when the mercury 

 begins to rise in the open limb, pour in more mercury 

 in quantity sufficient to bring the level of that in the 

 closed limb back to its original position. It will be 

 found that, as long as the temperature continues to 

 rise, constant supplies of fresh mercury will be required. 

 From this it is evident that the gas, when prevented 

 from expanding, exerts greater pressure as its tempera- 

 ture rises. 



This increase of pressure is well shown if a bladder 

 be inflated with air as fully as possible at the ordinary 

 temperature and then be placed in front of a good fire. 

 The increase of pressure brought about by this treatment 

 is generally sufficient to burst the bladder. 



When considered from the point of view of the 

 Kinetic Theory, as set out earlier in the chapter, the 

 increase of pressure noted is explained thus : The mole- 

 cules move more rapidly as a result of the rise of 

 temperature, and consequently impinge more frequently 

 upon the walls of the containing vessel. It is the 

 continual impact of the molecules which causes the 

 gas to exert pressure, and when the frequency and 

 force of impact of the striking molecule both receive 

 augmentation as a result of heating the gas, the pressure 

 of the gas shows a corresponding increment. 



