ON SOME REMARKABLE INSTANCES OF CROOKES'S 

 LAYERS, OR COMPRESSED STRATA OP POLARIZED 

 GAS, AT ORDINARY ATMOSPHERIC TENSIONS. 



GEORGE JOHNSTONE STONEY, m.a., f.k.s. 

 [Read November 19th, 1877.] 



1. In a communication which I had the honour to lay before 

 the Royal Dublin Society at its last scientific meeting, I gave 

 some instances of Crookes's layers at ordinary atmospheric ten- 

 sions,* and among them described one which accounts for the 

 great mobility that may be imparted to a light powder by heating 

 it in a metal capsule. It is shown that in this case the powder 

 floats on a stratum of air which it compresses by its weight, at 

 the same time that it maintains the requisite polarized condition 

 of the layer by radiating away its own heat so freely as to keep 

 itself cooler than the capsule. 



2. In exactly the same way we may explain a very curious 

 phenomenon which has been recorded by travellers in Arabia, and 

 to which Professor Barrett has directed my attention. There is in 

 Arabia a mountain called Jebel Nagus, or Gong Mountain, which 

 produces sounds resembling the booming of the Nagus, or wooden 

 gong, used in Eastern churches instead of bells. The mountain 

 consists of a white friable sandstone, which produces to the south- 

 westward a great slope of very fine drift sand, and another smaller 

 one to the north. The large one is 115 metres high, 70 metres 

 wide at the base, and tapers towards the top. It is so steep, 

 being inclined to the horizon at an angle of nearly 30°, and con- 

 sists of such fine sand, that its surface can be easily set in motion 

 by scraping away a portion from its base or by disturbing it 



* 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 mobility of finely divided precipitates in heated capsules ; for he 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. 164, p. 526). I 

 have only become acquainted with this passage since writing the present paper, or, if I had 

 seen it before, I had forgotten it, otherwise I should have referred to it in my last paper. 



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