IV.—ON THE MECHANICAL THEORY OF CROOKES’S, OR 
POLARIZATION, STRESS IN GASES. 
BY 
G. JOHNSTONE STONEY, m.a., F.rs., &e., 
Secretary to the Society. 
[Read February 18, 1878.] 
INTRODUCTION. 
Two papers will be found in the first volume of the fifth series of the Philosophical 
Magazine (March and April, 1876), in which I endeavoured to explain the force 
that Mr. Crookes had detected within vacuum chambers, by pointing out that when 
heat passes across the residual gas, the molecules of the gas that tend respectively 
towards the heater and towards the cooler must interpenetrate one another in a 
greater degree than they would if the gas were in its ordinary or unpolarized 
condition, and that this behaviour will render the stresses within the gas unequal, 
causing the stress to be greatest in the direction in which the augmented inter- 
penetration takes place. 
When writing the foregoing papers, and afterwards when writing a paper on the 
transfer of heat which accompanies the phenomenon, I was under the mistaken 
impression that the flow of heat between a heater and cooler in fixed positions and 
at constant temperatures, will become greater if the number of gaseous molecules 
that intervene is reduced below the number required for the transfer of heat by the 
laws of “ conduction*”; and for this supposed increased flow of heat I suggested the 
name penetration. It has recently been pointed out by Dr. Schuster, “‘ Nature,” 
vol. 17, p. 148, that experiments have been made which show that the flow of heat. 
diminishes instead of increasing when the limit for “conduction” is passed. It thus 
appears that what I have called penetration is always feebler than conduction, and 
is to be sought in the figures representing De la Provostaye and Desains’ experi- 
ments in those portions of the curves which slope steeply downwards. Accordingly 
my paper on penetration (Memoir 2 of the present volume), and especially that part 
of it in which I apply the theory to experiment, requires considerable modification, 
and some of the statements I made in my earlier papers on Crookes’s force need 
amendment. Although the corrections that are required do not affect any material 
part of the theory of unequal stresses within polarized gas, it has appeared desirable 
* Tt is known that gases feebly conduct heat by diffusion, and that the amount of heat which passes in 
this way between a heater and cooler is independent of the density of the intervening gas, provided that’ 
the density of the gas does not fall below a certain limit. The question that presented itself was, as to 
what happens below that limit. 
H 
