CHEMISTRY OF FLUORESCING URANYL SALTS. 219 



resemblance to gypsum, possessing the "fish-tail" twins and approximately 

 the same angles and appearing different only in the yellow absorption. The 

 crystals were so universally twinned that the interfacial angles could not be 

 determined certainly. The largest face was 6 (010), with the two prism faces 

 m (110) and /x (iTO), and, as determined by measurement, practically all the 

 end faces were d (Oil), r (031), s (031), although these usually appeared twice 

 on a crystal, and other faces indicated by the measurement were gill, x ( ill). 

 System triclinic. 



fc:m = 010:110 = 49 7' m: g = 110:lll =43 41' 



6:^ = 010:110 = 50 50' m: r = 110:031 =86 49' 



6:d = 010:011=4031' M : r = 110:031 =44 0' 



6:r = 010:031=67 48' fj.:x = 110: 111 =75 24' 

 b: s = 010:021 =57 41' 



The refractive index was determined through the faces 6 (010) and /z (ll0), 

 the more deviated ray vibrating at an angle of about 15 from the prism edge 

 toward the a axis. 



X720 1.618 1.692 

 X580 1.625 1.695 

 X500 1.634 1.714 



The index was also determined through the faces 6 (010) 'and d (Oil), the 

 less-deviated ray vibrating parallel to this prism edge. 



X580 1.614 1.691 



X580 1.622 1.698 



URANYL SULPHATE. 

 UO 2 SO-3H 2 O. 



This salt was prepared by Cragwall, presumably as the trihydrate. On 

 recrystallization of Kahlbaum material two forms appeared, a yellow opaque 

 needle-mass tending to replace the bright green fluorescent grains. The 

 yellow needles became more numerous on adding sulphuric acid, so uranic 

 oxide was added to saturation, which proved to be in excess on evaporation. 

 The bright green fluorescent crystals were difficult to keep, as they dried out 

 readily in the air. Microscopic examination of the Cragwall preparation 

 showed needles with parallel extinction and greater absorption and greater 

 index the long way of the crystals, while the angle of the optical axes was 

 very large and the sign positive. 



The acid sulphate, H 2 U0 2 (S0 4 ) 2 54 2 0, was reported by Wyrouboff. 1 



DOUBLE URANYL SULPHATES. 



The double sulphates differ from the previous double salts in the occurrence 

 of the sodium salt. The potassium, ammonium, rubidium, caesium, and 

 thallous salts were also prepared by Cragwall in the form of powders result- 

 ing from rapid precipitation by cooling. The potassium and rubidium salts 

 especially showed very strong fluorescence, the sodium and ammonium good 

 fluorescence, the caesium less, and the thallium practically none, and that was 

 not resolved. Rimbach 2 describes a dipotassium salt which was not pre- 

 pared and one of hydroxylamine also. 



1 Wyrouboff, Bull. Soc. fran. min. No. 32,351, 1909. 



2 Rimbach, /. c., 479. 



