SUGGESTIONS FOR AN EXPERIMENT TO DEMON*. 

 STRATE THE POLARIZED STATE OF THE GAS IN 

 CROOKES'S LAYER. 



iiY 



GEORGE FRANCIS FITZGERALD, M.A., F.f .CD. 



[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 I am 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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