123-125] Temperature 109 



We define the temperature T of this gas to be such that each fraction is 

 equal to RT, where R is an absolute constant, as yet undetermined. We 

 accordingly have the equations 



nm (265). 



This temperature, as we shall see later, is an " absolute " temperature, the 

 zero of the scale being at about 273 C. 



Equalisation of Temperature. 

 125. If gases of types a, /3 ... are mixed, we have from equation (161), 



m a u = . . . = . . . sr , 



expressing of course that the average kinetic energy of a single molecule is 

 the same for each kind of gas. In terms of the temperature, this becomes 



shewing that when two gases are mixed, they assume the same temperature. 



In this connection an observation of Maxwell's is of importance. He 



says*: 



"The theorem that the average kinetic energy of a single molecule is 

 the same for molecules of different gases is not sufficient to establish the 

 condition of equilibrium of temperature between gases of different kinds 

 such as oxygen and nitrogen, because when the gases are mixed we have no 

 means of ascertaining the temperature of the oxygen or of the nitrogen 

 separately. We can only ascertain the temperature of the mixture by 

 putting a thermometer into it. 



"We cannot legitimately assert that the temperatures of the oxygen and 

 of the nitrogen must be equal because they are in contact with each other, 

 for the only way in which we can conceive the oxygen or the nitrogen as 

 existing in the mixture is by picturing the medium as a system of molecules, 

 and as soon as we begin to see the molecules distinctly, heat becomes resolved 

 into motion. 



" But since our investigation f is equally applicable to a system of any 

 kind, provided only it satisfies the equations of dynamics, we may suppose it 

 to consist of pure oxygen and pure nitrogen separated by a solid diaphragm, 

 the solid diaphragm consisting of molecules capable of motion, but acting on 

 each other with forces which are sufficient to prevent any molecule from 



* Collected Works, n. p. 727. 



t Maxwell here refers to the investigation which has heen given in 88 et seq. of the present 

 book. 



