118 Mr. G. F. Fitza@ERALp, 
cally polarized. Now, the amount of elliptic polarization, we may 
expect, can be calculated in the following manner :—I have esti- 
mated by a series of unfortunately rather rough experiments that 
when a red hot ball is plunged into cold water to a depth of half 
a centimetre, the thickness of the Crookes’ Layer formed, is about ~ 
a quarter of a millimetre. This is, of course, only a rough ap- 
proximation, but it will give us results which determine with 
what order of quantity we are dealing. Hence, the excess of 
pressure in the vertical over that in the horizontal direction that 
I measured was half a gramme per sq. centimetre, which is about 
the ‘0005 of the atmospheric pressure. Now, I will make an as- 
sumption which is, however, only partially true, but as one object 
of the experiment is to determine to what extent it isso it is legiti- 
mate provisionally, and it is that we may treat the strain in the 
gas as due entirely to a difference of density in different directions. 
That we may do so to some extent, at least, is manifest, for, ac- 
cording to theory, the number of molecules moving in the direction 
of the strain is greater than the number moving in other di- 
rections. To what extent this is true could only be determined 
either by elaborate theoretical investigations into the state of the 
gas or else by experiments such as I am proposing. Assuming 
then the strain to be wholly due to a difference of density we 
ean proceed as follows :—The law connecting the refractive in- 
dices of a gas at different densities is > 7 ig a so that in air 

when p= 1:0002940, and in the case we are considering where 
oe. 1:0005 we have p’ == 10002941, As there are 100,000 
vibrations of light per five centimetres im vacuo there will be 
100029°40 when the density is », and 100029°41 when it is yp’, 
and consequently a difference of phase of ‘01 of a wave length 
will be introduced per five centimetres or a twentieth of a 
wave length in 25 centimetres, Now, as the intensity of light 
in the analyzer depends upon the square of the sine of the 
difference of phase, this will give the intensity as the square 
of the sine of 9°, which is 02 or 3th. Hence I conclude 
that if a ray of plane polarized light be transmitted through 25 . 
centimetres of a Crookes’s Layer between two surfaces, one of 
