VI. THE POLARIZED SPECTRA OF THE DOUBLE CHLORIDES 



AND DOUBLE NITRATES. 



The polarization of the fluorescent light from crystals, first rioted 

 by Grailich in 1857, 1 has since been studied by Maskalyne, 2 von Lom- 

 mel, 3 E. Wiedemann, 4 Sohncke, 5 Schmidt, 6 H. Becquerel, 7 and Pochet- 

 tino. 8 



With the exception of the work of Becquerel on the ruby, in which 

 low temperatures were employed, the authors cited above dealt chiefly 

 with fluorescence of the usual type, consisting of broad bands. In 

 such cases the most that can be done is to determine the direction of 

 vibration and estimate the proportion of polarized light. 



The uranyl salts afford a much more favorable field for such investi- 

 gations. Well-formed crystals of certain of these salts show a marked 

 pleochroism. When viewed through a Nicol prism their color changes 

 from a yellow-green to a very pale yellowish-white when the plane of 

 the Nicol is turned through 90. In the case of the double chlorides of 

 uranyl (i. e., U0 2 C1 2 -2NH 4 C1+2H 2 0; UO 2 C1 2 -2KC1+2H 2 0; U0 2 C1 2 - 

 RbCl+2H 2 0; and U0 2 C1 2 2CsCl), these changes of color are connected 

 with striking and significant variations in the fluorescence and absorp- 

 tion spectra, and this is true also of certain of the double nitrates. 



These double chlorides, as has been shown in Chapter V, differ from 

 the other uranyl salts thus far studied in the greater degree of resolu- 

 tion exhibited by their spectra at +20. The further resolution 

 effected by cooling the crystal to the temperature of liquid air is in 

 general the same for all; i.e., the bands are resolved into doublets the 

 components of which in some cases, particularly noticeable in the 

 absorption spectra, show indications of further complexity. The 

 doublets, moreover, are polarized, 9 the planes of vibration of the com- 

 ponents being at right angles to one another, so that two entirely dis- 

 tinct spectra of fluorescence and absorption may be observed by the 

 use of a Nicol prism. 



For the study of these remarkable phenomena the apparatus depicted 

 in figure 70 was devised. 



Within the collimator of a spectroscope of constant deviation a 

 rhomb of calcite R was so mounted as to give two vertically displaced 

 images of the slit, and these by suitable adjustment of the length of the 

 slit could be rendered contiguous without overlapping. 



1 Grailich, Krystall-optische Untersuchungen, 8 G. C. Schmidt, Wiedemann's Annalen, LX, p. 



Wien, 1858. 740. 



2 Maskalyne, Proc. Royal Society, xxviu, 7 H. Becquerel, Comptes Rendus, CXLIV, p. 



p. 479. 671. 



J von Lommel, Wiedemann's Annalen, vni, 8 Pochettino, Nuovo Cimento (v), 18. 1909. 



p. 634. 9 Nichols and Howes, Proce. Nat. Acad. Sci. 

 4 E. Wiedemann, Wiedemann's Annalen, ix, p. i, p. 444, 1915; and more fully in Phys. 



158. Rev. vin, p. 364. 1916. 



6 Sohncke, Wiedemann's Annalen, LVIII, p. 417. 



102 



