782 EXPANSION OF GASES. 



height. Suppose, for example, that the barometer 

 reads 755 mm when the temperature is 16 ; then the cor- 



Gases are not only more expansible than solids and 

 liquids, but their expansion is also quite regular ; more- 

 over, while the expansion of solids and liquids is dif- 

 ferent for each different substance, the coefficient of 

 expansion is the same for all permanent gases, viz. 

 0*003665, or 2T3-. This must not be understood to 

 imply that a quantity of gas at any temperature will 

 expand by ^rg- of its volume when its temperature 

 rises by 1 ; it means that the gas under consideration 

 will expand for a rise of temperature of 1 by 2+3 f 

 that volume which it would occupy at 0. Thus 273 CC of 

 air at expand for each degree by which the tempera- 

 ture is raised ^^-3 of 273 CC ; that is, l cc , and the volume 

 therefore becomes 274 CC at 1. At 100 it becomes 

 373 CC , at 101 it is 374 CC ; it follows that when air at 

 100 is heated 1 it expands 2-7-3 of the volume it had 

 at 0, and ^f^- of the volume it occupies at 100. 



When air or any other gas is heated in a closed vessel, 

 so as not to be allowed to expand more than the vessel 

 itself (an expansion which is in all cases very small 

 in comparison), it presses against the sides of the 

 vessel with a force proportional to the expansion which 

 would have taken place if the enclosed gas had freely 

 expanded, the pressure of the atmosphere remaining 

 the same throughout ; in other words, the pressure on 

 the sides of the vessel is the same as if the gas, after 

 being allowed to expand freely while the external pres- 

 sure remained constant, were afterwards compressed 



