CHEMISTRY OF FLUORESCING URANYL SALTS. 209 



59.5 C. which is probably not as accurate. Lowenstein 1 gives the vapor- 

 pressure of the saturated solution as 18 mm. approximately at 25 and the 

 pressure of the equilibrium between hexahydrate and trihydrate as over 

 4 mm. The author found the two hydrates to be stable together at 5 mm. at 

 20. The result is that the crystals always effloresce and fall to a yellow 

 powder if left in the air, in the winter especially if the sun falls on them, and 

 may deliquesce in the summer. Lescouer 2 gives the vapor-pressure of the 

 solution at 6 as 12 mm. and for the trihydrate below 3 mm. The author 

 found that the hexahydrate dissolved in various concentrations of nitric acid 

 at 20 C. in the following ratio: 1.6 grams of hexahydrate in 1 gram of 10 per 

 cent HN0 3 , 1.15 grams in 20 per cent, 0.8 gram in 30 per cent, 0.65 gram in 

 40 per cent to 70 per cent HN0 3 . The values have not been determined 

 accurately above 40 per cent on account of the complications due to the 

 occasional formation of the trihydrate. 



These crystals were usually grown by evaporation in the room. For work 

 on the polarization they were grown in thin plates tabular on a or b by putting 

 small seed crystal in a solution of the depth desired for the thickness of the 

 crystal, in the position desired. 



URANYL NITRATE TRIHYDRATE. 

 UO 2 (NO 3 )23H 2 O. 



This hydrate is mentioned by Lescouer 3 and by Ditte 4 as being formed when 

 the hexahydrate is heated to boiling. Drenkman 5 and Schultz-Sellack 6 found 

 that on adding the hexahydrate to strong nitric acid and crystallizing by cool- 

 ing or evaporation the trihydrate was obtained. Lebeau 7 also obtained it 

 by heating the hexahydrate on the water-bath or by evaporating the nitric 

 acid solution in a dessicator over H 2 SO 4 or KOH. Marketos 8 mentions it as 

 formed directly from the hexahydrate over sulphuric acid in a dessicator, as 

 does also Forcrand. 9 As can be deduced from the vapor-pressure data of 

 Lowenstein and the author, this air-drying takes place as soon as the vapor- 

 pressure of the water in the atmosphere goes below 5 mm. The best crystals 

 are obtained by slow evaporation of the solution of the hexahydrate, dried on 

 the water-bath in concentrated nitric acid in a dissicator over sulphuric acid 

 and caustic potash or quicklime. 



The crystalline form was measured by G. Wyrouboff, 10 who obtained his 

 crystals by evaporating the neutral solution at 65. 



System triclinic; axial ratio, a : b : c= 1.7753: 1 : 1. 4104. 



a 8535'; /3_9412'; 7 8144'. 



Forms p (001), making pjates with h' (100), a' (101), and a* (201) on the 

 edges, and c* (111) and b* (111) oblique-angled ends. 



The specific gravity was found to be 3.345. No cleavage has been noticed, 

 although the crystals are likely to form with irregular cracks across or radiat- 



1 Lowenstein, Zeit. Anorg. Chem. 63, 105-107. 1909. 



2 Lescouer, Ann. Chim. Phys. (7), 7, 429. 1896. 

 ^ Lcscoucr loc* cit* 



4 Ditte, Ann. Chim. Phys. (5), 18, 337. 1879. Compt. Rend. 89, 643. 1879. 

 6 Drenkman, Jahrsber der Fortschritt Chem., 256. 1861. 



6 Schultz-Sellack, Jahrsber. Fort. Chem., 365. 1870; Zeit. fur Chem., 646. 1870. 



7 Lebeau, Bull. Soc. Chim. (4), 9, 299. 1911. 



8 Marketos, Comptes Rendus, 155, 210. 1912. 



9 Forcrand, Comptes Rendus, 156, 1044, 1207, 1954. 1913. 



10 Wyrouboff, Bull. Soc. fran. Mineral, 32. 340. 1909. 



