Mr. G. J. Stoney on Polarization Stress in Gases. 405 



pervious to molecules, provided that they were made at the 

 same time perfect reflectors of 'molecules. By a reflector of 

 molecules is to be understood a surface endowed with the pro- 

 perty of throwing off any molecules that impinge upon it with 

 unabated speed, and at an angle of reflection which lies in the 

 same plane as the angle of incidence, and is equal to it. The 

 reflected molecules will affect the state of the gas within the 

 tube exactly in the same way as the molecules passing in from 

 outside had done before. We have now a portion of gas com- 

 pletely shut up inside a tube with sides that are perfect reflec- 

 tors of molecules, and closed at the ends by pistons that are 

 patches of the heater and cooler, and which therefore scatter 

 such molecules as reach them ; and we know that the beha- 

 viour of this gas will be the same as that of the corresponding 

 portion of the Crookes's layer. We may call such a tube 

 a unit reflecting tube. 



7. Let the pistons of such a tube be kept at the tempera- 

 tures T x and T 2 , and let gas be introduced into it. After a 

 brief period of adjustment the gas will become stationary; i. e., 

 if a plane forming a cross section of the tube be considered, 

 the molecular motions are such that the same number of mole- 

 cules pass forwards as backwards through this plane per 

 second. But they will pass it with unequal average veloci- 

 ties, because the vis viva of those crossing it towards the cooler 

 must exceed the vis viva of those crossing it towards the heater 

 by an amount bearing a known ratio to the quantity of heat 

 advancing. Hence the gas is polarized, the molecular motions 

 being swifter when they are directed forward or towards the 

 cooler, and slower when directed backwards. 



8. Suppose that Ave begin with dense gas and gradually 

 exhaust, and let us consider the succession of events that will 

 arise as the exhaustion proceeds, i. e. when n, the number of 

 molecules in the unit tube, is progressively diminished. It is 

 known that the flow of heat cannot conform to the laws of 

 " conduction " unless the number of molecules exceeds a cer- 

 tain limit which we may call N, — N depending upon the de- 

 scription of gas that is present, and upon the temperatures T x 

 and T 2 of the pistons which close the unit tube. We must 

 therefore divide the exhaustion into two periods, one lasting 

 while the number of molecules in the tube exceeds N, and 

 the other during the rest of the exhaustion. Throughout the 

 first period the flow of heat follows the known laws of con- 

 duction, and therefore remains constant. Hence, during this 

 part of the exhaustion the polarization of the gas (which may 



be measured by — , v being the average velocity at any point 



