SUGGESTIONS FOR AN EXPERIMENT TO DEMON- 
STRATE THE POLARIZED STATE OF THE GAS IN 
CROOKES’S LAYER. 
BY 
GEORGE FRANCIS FITZGERALD, M.A., F.T.C.D. 
[Read 21st January, 1878. ] 
I DESIRE to apologise to the Society for bringing before them only 
a proposed instead of a performed experiment, but my excuse is 
that it will probably be some time before Iam .able to perform 
the experiment myself, and as I desire to get credit for having 
at least proposed it, I take this opportunity of publishing it, and 
of giving my reasons for supposing it likely to be successful, by 
showing that the quantities involved are quite within the reach 
of our present methods of observing them. 
I would first notice that, according to both Clausius and Max- 
well’s theories of the conduction of heat in gases, the existence 
of a force like Crookes’s depends essentially upon the distribution 
of the velocities among the molecules, it being easily seen from 
either of their investigations (as is also evident from many other 
obvious considerations) that it is quite possible to imagine such 
a distribution of velocities among the molecules as that, though 
heat be propagated through the medium, yet the pressure in all 
directions shall be the same. Hence, any independent method of 
demonstrating a polarized state of the gas is of considerable im- 
portance ; and the experiment I am about to propose is for the 
purpose of doing so. 
When any homogeneous transparent substance is in a state of 
stress, its refractive index for light, polarized in certain planes, is 
different from that for light polarized in other planes, and conse- 
quently a ray of plane polarized light, when passed though such 
a medium in certain directions, emerges elliptically polarized. 
If the gas in a Crookes’s Layer be in the state of stress that 
theory indicates, it ought to behave similarly, and a plane 
polarized beam when transmitted along it should emerge ellipti- 
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