402 Mr, (J. J. Stoney on Polarization Stress in Gases. 



duction"*; and for this supposed increased flow of heat I 

 suggested the name penetration. It has recently been pointed 

 out by Dr. Schuster (' Nature/ vol. xvii. p. 143) that experi- 

 ments have been made which show that the flow of heat dimi- 

 nishes 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's experi- 

 ments, in those portions of the curves which slope steeply 

 downwards. Accordingly my paper on Penetration (Phil. 

 Mag. December 1877), and especially that part of it in which 

 I apply the theory to experiment, requires considerable mo- 

 dification, 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 to resume the subject and present the 

 theory freed from the error that has been pointed out. In 

 doing this I have taken the opportunity of introducing the 

 conception of the reflecting tube, which greatly facilitates the 

 inquiry into the mechanical effect of the interpenetration ; and 

 I have also availed myself of the admirable method of treating 

 the problem, described by Mr. George F. Fitzgerald in 

 ' Nature,' vol. xvii. p. 200, to obtain a complete expression for 

 the stress, and to show that my theory is not at variance 

 with results established by Professor Clausius, as has been 

 asserted by Professor Osborne Eevnolds in ' Nature/ vol. xvii. 

 p. 122. 



Part I. — Treatment of the Problem by General Mechanical 

 Considerations. 



1. If a drop of water or other volatile liquid is allowed 

 to fall into a smooth and sufficiently hot metal dish, it con- 

 tinues a liquid drop instead of spreading out or flashing off 

 into vapour, and it exhibits an appearance of great mobility. 

 The drop is then in what has been called the " spheroidal 

 state." Now, when a drop of liquid is so situated a chink 

 may be observed between it and the hot surface beneath ; so 

 that the drop does not rest directly upon the metal, but is in 

 reality floating upon a layer of vapour. We further learn 



* It 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. 



