G. Jounstone Stoney On Polarization Stress in Gases. 43 
the same time the flow of heat decreases to zero ; for while p tends to zero as the 
exhaustion proceeds, the polarization does not tend to infinity, but to a limit, viz. : 
Vy — Vs 
Vers 
peratures of the pistons. Now when gas is polarized with this kind of polarization 
within a tube the sides of which reflect the molecules, we can find limits between 
which its thermal and mechanical properties must lie. 
9. Before proceeding to determine these limits it will be well to guard ourselves 
against making mistakes by passing under review the orders of the several 
magnitudes with which we are dealing in this inquiry. No accwrate measures 
appear yet to have been made of the thickness of the chinks of air or vapour on 
which spheroidal drops rest. But from approximate measures, some of which 
were made by Mr. Fitzgerald, and some by myself, I think it may be inferred that 
this thickness is somewhere about the thickness of a sheet of paper (i.e., about a 
fourth-metre or the tenth of a millimetre), when a spheroidal drop of the density 
of water, at a temperature of 10° centigrade, and 4 or 6 millims in diameter, 
floats over a surface of liquid which is about 10° warmer. We further know in 
this case that the Crookes’s pressure, as it supports the weight of this drop, must 
be about the two-thousandth part of an atmosphere. These determinations are very 
rude, but they at all events tell us what kind of magnitude we are dealing with, 
and therefore suffice for our present purpose. They show that we shall not be far 
wrong in assuming definitively that the phenomenon presented by experiment which 
we have to explain is that the stress across a stratum of air will be gogo part of 
the stress at right angles to that direction, if this stratum occupies the space 
between a heater and cooler at temperatures of 10° and 20° C, if, moreover, this 
interval is a fourth-metre (a metre divided by 10‘), and if the atmosphere has free 
access to the stratum of air at its edges. Let us now imagine a reflecting tube, 
such as is described above, to be placed across this stratum. It will therefore be a 
fourth-metre long, and we may assign to it any width we please. Let us take 
a width equal to the diameter of the smallest object that can be seen with a 
microscope, which is about 2°5 seventh-metres, or the 100,060th part of an inch. 
We have now to compare the dimensions of this tube with the number and 
motions of the molecules included within it. The number of molecules ina cubic 
millimetre of atmospheric air is about a unit-eighteen (10°). (See Phil. Mag., 
August, 1868.) Whence the average interval between them 1s about a ninth- 
metre. This is the 100,000th part of the length of our tiny tube and the 
250th part of its breadth. Hence the tube will contain a vast number of 
molecules, some such number as five thousand millions. Again, the average 
striking distance (i.¢., the average length of path between the encounters) of 
the molecules is about the 1,500th part of the length of the tube, or the fourth 
part of its breadth. There is, therefore, abundant room within the tube, small as 
where v, and v, are the velocities corresponding to T, and T,, the tem- 
