MOLECULAR CONSTITUTION OF BODIES. 429 



with this velocity, it would have the same kinetic energy as the gas actually 

 has, only it would be in a visible form and directly available for doing work. 



If in the same vessel there are different kinds of molecules, some of greater 

 mass than others, it appears from this investigation that their velocities will be 

 so distributed that the average kinetic energy of a molecule will be the same, 

 whether its mass be great or small. 



Diagram of Velocities. 



Here we have perhaps the most important application which has yet been 

 made of dynamical methods to chemical science. For, suppose that we have 

 two gases in the same vessel. The ultimate distribution of agitation among the 

 molecules is such that the average kinetic energy of an individual molecule is 

 the same in either gas. This ultimate state is also, as we know, a state of 

 equal temperature. Hence the condition that two gases shall have the same 

 temperature is that the average kinetic energy of a single molecule shall be the 

 same in the two gases. 



