PROPERTIES OF GASES 87 



attached to the side of a vessel, the gas would tend to push 

 the piston out, provided the pressure of the gas on the inside 

 was greater than that of the atmosphere on the outside. 

 This is the case when a gas is compressed in a tank. The 

 gas may be transferred from place to place intact, and then 

 allowed to pass through pipes, to the place where its energy 

 is to be utilized. 



86. Expansion of Gases. Gases are said to be perfectly 

 elastic because they have no elastic limit and expand and con- 

 tract alike under the action of heat. That is to say, every sub- 

 stance when in the gaseous state and not near its point of 

 liquefaction has the same coefficient of expansion, this co- 

 efficient being TTIT of its volume for each degree Centigrade 

 or TsVi of its volume for each degree Fahrenheit.* 



Since a gas contracts ^j part of its volume when its 

 temperature is lowered 1 C, such a rate of contraction would 

 theoretically reduce its volume to zero at a temperature of 

 -273 C ( 459.4 F). Since all gases reach their liquefying 

 point before this low temperature is attained, however, no 

 such contraction exists. At the same time, it may be said 

 that if heat is considered as a motion of the molecules of a 

 substance, that motion is to be considered as having ceased 

 when the temperature has reached 273 C. 



This temperature of -273 C ( -459.4 F), therefore, is 

 called the absolute zero, and from it all temperatures should 

 properly be reckoned. Whenever a temperature is men- 

 tioned as being in degree absolute, either in the Centigrade 

 or the Fahrenheit scale, it is understood to be counted from 



* The relation between the Centigrade and Fahrenheit thermo- 

 meters is discussed in Chapter IX, 



