TEMPERATURE AND THE PROPERTIES OF GASES 29 



hence there would be no motion. If we could suppose this 

 equation to hold until that condition were reached, the state 

 of no motion, beyond which it would logically seem impossible 

 to go, would be reached at a temperature / = i/a. 



From the comparatively rough measurements mentioned 

 above, a was found to have a mean value of 2rs> and hence, with 

 the limitations stated above, 273 would be the temperature 

 below zero centigrade at which all motion would cease and 

 matter would be quiescent. This point has been called the 

 absolute zero, and although the value given to it now is not 

 exactly — 273 C, it is sufficiently near this for any difference 

 to be considered in the light of a correction. It was also 

 shown by Lord Kelvin that the value of the absolute zero 

 could be obtained from a study of the cycle of a perfect engine, 

 that the thermodynamic temperature which enters into this is 

 very nearly 273 + f C, and that its zero value is identical 

 with the temperature at which motion would cease with a 

 perfect gas. In consequence of the great importance of this 

 work, it is common to call temperatures on the absolute scale 

 temperatures Kelvin, so that zero C. = 273 K. 1 



As indicated above, this ideal gas state is found to exist 

 to a very near approximation when the density of real gases 

 is very small, and it is assumed that it would apply strictly 

 at exceedingly small densities near to zero density. The 

 coefficient of expansion a, found under these conditions, will 

 then be the inverse of the absolute temperature, and this is the 

 principal means by which an estimate is arrived at of the real 

 value of this temperature. 



If the molecules of a gas have no attractions for one another, 

 no work will be done on allowing the gas to expand into a 

 vacuum. It was at first thought that air and other similar 

 gases conformed with this, but the experiments of Joule and 

 Kelvin showed that real gases were in general either heated 

 or cooled when allowed to expand in this way, excepting under 

 certain definite conditions of temperature and initial pressure 

 which vary for each gas, and at which there is no change. A per- 

 fect gas would, under all conditions, be in the condition so that 



« *(&)-- (f) E -« 



E = Total Energy 



1 As will be explained later, the best value at present is 273'oc). 



