ON SOME REMARKABLE INSTANCES OF CROOKES’S 
LAYERS, OR COMPRESSED STRATA OF POLARIZED 
GAS, AT ORDINARY ATMOSPHERIC TENSIONS. 
BY 
GEORGE JOHNSTONE’STONEY, ™.a., F.R.s. 
[Read November 19th, 1877.] 
1, Ina 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 powdér 
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 me 
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 80°, 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. 
