On Crookes's Layers at ordinary Atmospheric Tensions. 457 



In order to indicate the extent of the applicability of 

 Theorems I. and II., I take occasion to add that whereas in 

 the phenomena of frictional electricity the condition of grada- 

 tion of atomic density is produced by disturbance of the super- 

 ficial atoms of the electrified body, in the case of the phenomena 

 of galvanic force the same condition is a necessary accompani- 

 ment of the chemical action going on in the battery ; and in 

 the case of the magnet a gradation of interior atomic density is 

 maintained by the mutual counteraction of atomic repulsion 

 and molecular attraction, and with more or less persistence 

 according to the quality of the magnet and the degree of 

 magnetization. In all three classes of phenomena as the real 

 currents are produced by reason of the gradation of atomic 

 density, and as these currents are subject to the condition of 

 circulating, it follows, as a consequence of such motion, that 

 polarity is recognizable in electric, galvanic, and magnetic 

 phenomena. This theoretical inference is confirmed by ex- 

 periment. 



Cambridge, May 16, 1878. 



LXII. On some remarkable Instances of Crookes's Layers, or 

 Compressed Strata of Polarized Gas, at ordinary Atmospheric 

 "Tensions. By George Johnstone Stoney, M.A*, F.R.S* 



1. TN a recent communication I gave some instances f 



J- of Crookes's layers at ordinary atmospheric tensions, 



and among them described one which accounts for the great 



* Communicated by the Author, having been read before the Royal 

 Dublin Society, November 19, 1877. 



t Viz. those which support light powders in heated capsules, or liquids 

 in the " spheroidal " state, including the case of drops upon the surface of a 

 liquid (see Phil. Mag. for last December, p. 441 etseq.). The theory of 

 unequal stresses in polarized gas has thus fulfilled an anticipation which 

 Mr. Crookes entertained so long ago as 1873, that whatever theory would 

 account for the motion of radiometers would probably also explain the 

 spheroidal state of liquids, and the niobilit}' of finely divided precipitates 

 in heated capsules ; for be enumerates these among phenomena probably 

 due, at least in part, to the same " repulsive action of radiation " as is 

 manifested in radiometers. (See Philosophical Transactions, vol. clxiv. 

 p. 526.) I was not aware of this passage when writing the paper on Pe- 

 netration above quoted; otherwise I should have there referred to it. 



The heat which diffuses across a layer of gas passes under what are 

 called " the laws of conduction " if the number of gaseous molecules pre- 

 sent is sufficiently large. If fewer molecules are present, the heat passes 

 under other laws, which may be distinguished from the laws of conduc- 

 tion by calling them the laws of penetration. In writing the paper on 

 Penetration, I was under the misapprehension that the how of heat by 

 penetration between a heater and cooler of which the temperatures and 



