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



the passage of boat under the laws of the conduction of heat 

 in gases. Now these facts also follow as consequences from 

 the theory advanced by the author, and therefore become con- 

 firmations of it. 



4. This theory seeks to account for Crookes's force by show- 

 ing that a layer of gas placed between a heater and cooler is 

 in a polarized condition of such a kind that the stresses within 

 the gas are different in different directions. Gas is polarized 

 whenever the molecules within a spherical element of volume 

 are moving towards different quarters with numbers or velo- 

 cities that are not distributed alike in all directions, the velo- 

 cities being measured from the centre of mass of these mole- 

 cules. This definition excludes the case of mere wind, which 

 is to be regarded as unpolarized gas travelling forward in a 

 certain direction ; but it includes the case of gas across which 

 heat is making its way, which is the case with which we have 

 here to deal. 



5. Let us recur to the simple instance of a heater and cooler 

 with extensive flat parallel surfaces maintained at constant 

 temperatures, and with gas' between them freed from the 

 action of gravity, and which has had time to adjust itself to 

 its position. Gas so circumstanced will become stationary 

 in the ordinary sense of the word ; i. e., though in active 

 molecular motion, it will have no currents like convection- 

 currents or wind passing through it*. We have now to show 

 that the stress across such a layer will be greater than the 

 stress sideways. 



6. Imagine a unit of suface marked out on either heater 

 or cooler, and let perpendiculars to the surface be raised from 

 the boundary of this enclosure. These will trace out a straight 

 tube extending between the heater and cooler, and closed at 

 the ends by equal patches of the heater and cooler. These we 

 may call the pistons of the tube. The molecules which strike 

 the pistons are returned by them, and with altered velocity 

 whenever the pistons are at different temperatures ; but mole- 

 cules pass without hindrance through the sides of the tube. 

 Now it is evident that, if the molecules passing through an 

 element of the side of the tube are considered, those pass- 

 ing out in a unit of time will be an exact counterpart of 

 those passing in, in such a sense that the state of the gas 

 would not be disturbed by making the sides of the tube im- 



* There will be currents close to the boundary of the heater and 

 cooler ; but these are secondary phenomena caused by, and in no 

 degree the cause of, Crookes's stress. They will not be appreciable 

 within the layer at any considerable distance from the edge ; and they 

 may be avoided by giving to the opposed surfaces of the heater and 

 cooler thp form of concentric spheres. 



