THE KINETIC THEORY OF MATTER. 



141 



This formula is in accordance with Graham's experiments. Below are 

 given some of his results (Meyer, Kinetic Theory, p. 84) : 



If the gas flows out, not into a vacuum but into a space containing 

 another gas, the rate of efflux is still nearly the same so long as the 

 pressure of the outside gas is small, say less than half that of the inside 

 gas. The stream of issuing gas is to be regarded as all moving in the same 

 direction, sweeping the external gas away, and its velocity will depend 

 chiefly on the velocity of the molecules within the vessel. 



But if the pressure outside is nearly equal to that inside, the external 

 molecules will collide with those issuing, and the time of efflux will be 

 lengthened out. Still, with the two gases issuing under the same differ- 

 ence of pressure, the masses moving up to a very short orifice will be 



given by - and the same fraction of each will issue if the external gas 



is at the same pressure in each case, and still the ratio of the times of 

 efflux will be as 



Thermal Transpiration. If we have the same gas, but at two 

 different temperatures, 6 l and $ 2 , on the two sides of a small orifice, the 

 masses moving up to the orifice on the two sides will be proportional to 

 /DjVj and /3 2 V 2 respectively. If the pressures are equal to begin with, 



and 

 whence 



-VI 



or if # 2 ># p p 1 V 1 >/) 2 V 2 , and more gas moves in from the cold side than 

 moves out from the hot side and the pressure of the hot gas increases, 

 the increase will go on until there is a balance between the two streams 



or until' p l ~V l = p 2 V 2 



or putting V 2 = 



P 

 until 



