216 FLUORESCENCE OF THE URANYL SALTS. 



DIAMMONIUM URANYL NITRATE. 

 a form (NH 4 )2U0 2 (N03)42H 2 O. 



This phase crystallizes from slightly acid solutions of uranyl nitrate con- 

 taining a large excess of ammonium nitrate. Its solubility increases very 

 rapidly with rising temperature, until at about 60 C. the uranyl nitrate dis- 

 solves out, leaving a residue of ammonium nitrate. This salt crystallizes 

 very readily in large sulphur-yellow perfect crystals of 5 or 10 grams from a 

 volume of solution that only gives 1 or 2 grams of monoammonium salt and 

 crystallizes best if the room temperature is 10 to 15. The fluorescence is 

 very faint at 20 C., but increases rapidly below 0, becoming stronger than 

 that of the monoammonium salt at liquid-air temperatures. 



System monoclinic axial; ratio a : b : c = 0.8419: 1 : 0.5594; j8 9455'. The 

 habitus resembles that of a cube with octahedron, the forms being c (001), 

 a (100), b (010), and o (111). The faces p (110) were also occasionally observed, 

 and possibly (211). 



No cleavage was observed, thus increasing the resemblance to sulphur 

 crystals. However, on heating rapidly, the crystals fill with cracks, and con- 

 sequently seeded crystals often have a large single crack more or less parallel 

 to a (100). 



Etch figures produced by resolution in a crystal in the mother-liquor were 

 found once, the distinct forms being on b (010), with two rounded faces meet- 

 ing in a line in the bottom as though a lens had been pressed in. The bottom 

 edges were all parallel and about halfway between the edges of b (010) and 

 o (111), i. e., parallel to (102). 



The specific gravity was found to be 2.777. 



When these crystals are heated slowly they break down at about 140, 

 giving a pasty mass full of bubbles, which clears up slightly at 220, but does 

 not give a clear solution below 240. On cooling, bright green crystals of the 

 monoammonium salt form in a background of white ammonium nitrate. 



Crystals of this phase placed over sulphuric acid with a pressure of water- 

 vapor of 4 mm. started to lose water, turning to a whitish powder, and con- 

 tinued to do so slowly at 5 mm., although then they do not start. Placed 

 over sulphuric acid with a vapor-tension of 11 mm., they deliquesce rapidly, 

 soon going completely into solution. 



The refractive index was observed in two different directions, using natural 

 faces. First, between 111 and Til giving the light vibrating in the ac plane 

 nearly parallel to the edge olllrolll, with an index of 1.546, and that at 

 right angles to the ac plane as 1.639; between the faces b 010 and o 111, 

 giving for light appearing on b to vibrate nearly parallel to edge between 6 (010) 

 and o' ill as 1.508, and at right angles to that 1.619. 



The composition was determined by ignition to U 3 08, which gave 47.68 

 and 47.62 per cent against theoretical 47.58. 



URANYL CHLORIDE. 



Neither the monohydrate U0 2 C1 2 H 2 O described by de Coninck 1 as being 

 formed by evaporating the solution prepared by precipitating the sulphate 

 solution nor the trihydrate, which, according to Myh'us and Dietz, 2 forms from 



1 de Coninck, Comptes Rendus, 148, 1769. 1909. 



2 Mylius and Dietz, Ber. d. d. Ch. Ges., 34, 2774. 1901. 



