CHAPTER IV. 



EXPANSION OF GASES. 



Expansion of Gases depends on Pressure Changes Volume Expansion at Constant 

 Pressure Gay-Lussac's Method Regnault's Experiments Increase of Pressure 

 with Constant Volume Gas Thermometry Regnault's Normal Air Thermometer 

 Hydrogen Thermometer Bottomley's Air Thermometer Callendar's Com- 

 pensated Air Thermometer. 



The Expansion of Gases. Since the volume of gas is very much 

 affected by alteration of the pressure to which it is subjected, the volume 

 expansion with a given rise of temperature may vary widely in different 

 cases through different changes in pressure, and may even be prevented 

 altogether, if the containing vessel is one which preserves a constant 

 capacity. If, for example, a flask containing a small quantity n 



of liquid is closed by a cork through which passes an open 

 tube, dipping under the liquid (Fig. 32), a rise of tempera- 

 ture is accompanied by an expansion of air, which drives the 

 liquid up the tube. But the height to which the liquid rises, 

 and, therefore, the expansion of the air, will differ with 

 different liquids. With mercury it will be much smaller 

 than with water, for the gas inside the flask is exposed to 

 the atmospheric pressure + that due to the column of liquid 

 in the tube. The greater increase of pressure, due to the 

 column of heavier liquid, lessens the expansion of the air. 



We see from this, that the rise of temperature may have 

 two effects : increase of volume and increase of pressure ; 

 and that the effects may differ for different arrangements. 

 It is usual to study the two effects separately, investiga- 

 ting the change of volume when the pressure is kept constant, and the 

 change of pressure when the volume is kept constant. 



It may be observed that if Boyle's Law were exactly true, the two 

 effects for a given rise of temperatnre would be equal. For, let a 

 volume V of a gas expand, by a given temperature-rise, to V, the 

 pressure remaining at its original value, P. Now bring back the volume 

 to V, increasing the pressure to P'. Then the pressure P would increase 

 to P', with the given temperature-rise, at constant volume V. But by 

 Boyle's Law 



FIG. 32. 



and 



V 



v : 



F 

 P 



Or the volume changes in the same ratio in the one case as the pressure 

 in the other. 



