134 FLUORESCENCE OF THE URANYL SALTS. 



II. THE DOUBLE NITRATES: INFLUENCE OF CRYSTAL FORM. 1 



There is good reason to think that crystal form has an important 

 bearing upon the structure and arrangement of fluorescence spectra. 

 The polarized spectra of the four double uranyl chlorides, described 

 in Chapter VI, are almost identical in arrangement and in the absolute 

 position, relative intensity, and resolution of their bands. These sub- 

 stances all crystallize in the triclinic system. Further evidence bearing 

 upon this subject will be found in section in of this chapter. 



Our object in the present section is to describe the fluorescence and 

 absorption of four double uranyl nitrates and to throw further light on 

 the role played by crystal structure. 



The two pairs of double nitrates studied are mono-ammonium uranyl 

 nitrate, NH 4 UO 2 (N0 3 ) 3 ; di-ammonium uranyl nitrate, (NH 4 )2U0 2 (NO 3 )4 

 2H 2 0; the mono-potassium uranyl nitrate, KU02 (N0 3 ) 3 ; and the 

 di-potassium uranyl nitrate, E^UC^CNOa)^ 



The crystallographic features of these four compounds may be 

 briefly specified as follows: 



(1) The mono-ammonium salt, which crystallizes from a solution of 

 the two component salts in concentrated nitric acid, was described by 

 Meyer and Wendel 2 and crystallographically by Steinmetz. 3 The 

 crystals are of the trigonal system, with an axial ratio of a : c = 1 : 

 1.0027. 



(2) The di-ammonium salt crystallizes from a slightly acid water solu- 

 tion of the two salts in which the ammonium nitrate is in excess of that 

 required for the mono-ammonium salt. This salt was at first thought 

 to be the a modification of ammonium uranyl nitrate made by Rim- 

 bach 4 and measured by Sachs, 5 but an examination of the spectrum of 

 the a modification so-called proved that it was simply uranyl nitrate 

 hexahydrate. The crystals analyzed by Rimbach were probably the 

 mono-ammonium form, as this sometimes forms in the same solution. 

 The crystals of the di-ammonium salt belong to the monoclinic system. 



(3) The mono-potassium salt crystallizes from nitric-acid solution 

 in the rhombic system, as described by Steinmetz, with axial ratio 

 a :b : c = 0.8541 : 1 : 0.6792. 



(4) The di-potassium salt crystallizes with reluctance; but when 

 seeded from an acid aqueous solution, it forms in beautiful, fluorescent 

 crystals of the monoclinic system. The axial angle is j8 = 90 and the 

 axial ratio a : b : c = 0.6394 : 1 : 0.6190. The composition is different from 

 that of the di-ammonium salt, since it lacks the water of crystallization. 



Both visual and photographic measurements of the spectra were 

 taken, and, since they agreed well, were averaged together. When 



1 Howes, H. L., and D. T. Wilber: Physical Review (2), ix, p. 125 (1917). 



2 Meyei and Wendel, Ber. d. d. Ch. Gea., vol. 36, 4055. 1903. 



3 Steinmetz, Groth's Chem. Krys., n, p. 150. 



4 Rimbach, Ber. d. d. Ch. Ges., vol. 37, 472. 1904. 

 6 Sachs, Zeitschr. f. Krys., vol. 38, 497. 1904. 



