G. Jonnxstone Stoney On Polarization Stress in Gases. Al 
3. We may express these facts in a very convenient form for our present purpose 
if the heater and cooler are extensive flat parallel surfaces at fixed temperatures. 
Conceive two exactly similar patches on the heater and cooler directly opposite to 
each other, and each occupying a unit of surface, and consider that portion of space 
which lies between these. Then the observations show that there is one definite 
quantity of the gas to be left in the volume so marked out, if we wish to produce 
the strongest Crookes’s stress. And further, by comparing Mr. Crookes’s experiments 
on the mechanical action, with those of De la Provostaye and Desains on the flow 
of heat, we learn another important fact, viz. : that the maximum stress occurs when 
the quantity of gas is too little to admit of the passage of heat 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 confirmations of it. 
4, This theory seeks to account for Crookes’s force by showing 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 velocities that are not distributed alike in all 
directions, the velocites being measured from the centre of mass of these molecules. 
This definition excludes the case of mere wind, which is to be regarded as un- 
polarized 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,'2.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 surface marked out on either heater or cooler and let per- 
pendiculars 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 molecules 
pass without hindrance through the sides of the tube. Nowit is evident that if 
the molecules passing through an element of the side of the tube are considered, 
those passing out in a unit of time will be an exact counterpart of those passing 
* There will be currents close to the boundary of the heater and cooler, but these are secondary phe- 
nomena 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 opposite 
surfaces of the heater and cooler the form of concentric spheres. 
H 2 
