Gases are said to be perfectly elastic because they have no elastic limit and expand and contract alike under the action of heat. That is to say, every substance when in the gaseous state and not near its point of liquefaction has the same coefficient of expansion, this coefficient being 1/273 of its volume for each degree Centigrade or 1/459.4 of its volume for each degree Fahrenheit.*

Since a gas contracts 1/273 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 mentioned 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 thermometers is discussed in Chapter IX (Heat And Expansion. 100. Generation And Movement Of Heat), the absolute zero, and therefore is equal to the observed temperature plus 273 or 459.4 as the case may be.

The lowest temperature which has thus far been attained is - 252° C. Dewar produced it by the evaporation of liquid hydrogen.