A. M. Mayer — Researches on the Rontgen Rays. 469 



mination and grain throughout, and exactly like the photo- 

 graph obtained by the X-rays passing through a similar glass 

 disc with nothing on its surface. The herapathites used in the 

 experiments were so thin that they did not appreciably screen 

 the X-rays, whether the axes of the superposed crystals were 

 parallel or crossed. But the action of the rays on the square 

 of blotting paper proved even more conclusively that the 

 X-rays cannot be polarized in this manner, for where this paper 

 covered the photographic plate nothing was visible, except by 

 careful scrutiny and with a favorable illumination, and then a 

 mere ghost of the paper was detected, but with no traces what- 

 ever of the herapathites. 



These experiments confirm in a convincing manner what 

 Rontgen found by his experiments, viz : that the X-rays can- 

 not be polarized. At least, they cannot be polarized by pass- 

 ing through herapathite, which is by far the most powerful 

 polarizing substance known. It is unlikely that polarization 

 will be detected by using any doubly refracting substance 

 which transmits two beams polarized in planes at right 

 angles to each other ; for if polarization exist, the thickness 

 of the substance required to get a measurable departure 

 of the two rays so screens the X-rays that a very small 

 fraction of them (at least by calcite) will be transmitted ; also, 

 Rontgen is of the opinion that if the X-rays be refrangi- 

 ble, the index of refraction is nearly unity even in such a 

 highly refracting substance as ebonite, which has an index 

 of about 1'6.* It is therefore reasonable to suppose that 

 the difference in the refraction of the ordinary and extraordi- 

 nary beams will be too small to be measurable in the faint 

 shadowgraphs obtained by traversing doubly refracting sub- 

 stances. 



It remains to be decided whether the X-rays can or cannot 

 be polarized by reflection, Professor Rood having recently 

 proved conclusively that they are reflected from platinum. 



2. The density of herapathite. 



Herapath gives 1-89, at 60° F., as the density of the 

 very remarkable substance he discovered. f As its density 

 is interesting to have in connection with the experiments 

 described above, I made two determinations of it by weigh- 

 ing about *3 grs. of the substance in water, and also by 

 the displacement it gave of the water in a specific gravity 

 flask. The mean of these two measures was 1*6. As this 



* On the Physical Properties of Vulcanite, by A. M. Mayer, this Journal, Jan. 

 1891. 



\ Phil. Mag., May, 1855, p. 369. 



Am. Jour. Sol— Fourth Series, Vol. I, No. 6.— June, 1896. 

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