228 FLUORESCENCE OF THE URANYL SALTS. 



and cadmium salts being reported. They are characterized by a temperature 

 dimorphism being above a given temperature, trigonal, and below that mono- 

 clinic twins and pseudotrigonal. They all have the same type formula de- 

 rived from the monovalent uraniacetic acid HU0 2 (C2H 3 O 2 ) 3 3H 2 0, which 

 takes on in this case 1 atom each of monovalent metal (sodium in all cases 

 studied) and 1 bivalent metal. These formula may also be written NaOH-fR 

 (OH) 2 +3UO 2 (OH) 2 + 9HC 2 H 3 2 , which would explain the exact relation 

 between the number of acetic radicals and molecules of water of crystallization. 

 This series was studied by Erb 1 and by Wyrouboff . 2 The temperature change 

 was studied by Schwarz. 3 This group is characterized by the pseudotrigonal 

 appearance of a large basal pinacoid, six-sided, due to rhombohedron faces 

 which are uneven, due to twinning. They are readily distinguished from 

 uranyl acetate or the metallic acetate or uranyl double acetates by this char- 

 acteristic shape. They do, however, resemble the sodium uranyl acetate 

 considerably if it has uneven faces, due to the varying composition of the 

 solution as it crystallizes. Under the crossed nicols, however, there is no 

 question, the very characteristic twinning surfaces showing unmistakably. 



SODIUM MAGNESIUM URANYL ACETATE. 

 NaMg[(UO2(C2H 3 O 2 )3 3H 2 O 2 ] 3 . 



These crystals, according to Wyrouboff, 4 are simply monoclinic if grown 

 below 15 C., but as the temperature rises the twins become more numerous, 

 until at 50 the whole crystal becomes trigonal. This process then reverses 

 on cooling. Erb 5 describes them as sulphur-yellow crystals which weather 

 readily. The axial ratio has not been determined closely because of the 

 twinning. These crystals were grown better by slow evaporation than by slow 

 or rapid cooling of supersaturated solutions. 



SODIUM ZINC URANYL ACETATE. 

 NaZn[(UO 2 (C 2 H 3 O2) 3H 2 O] 3 . 



These crystals were produced by Erb, who did not obtain sufficient measure- 

 ments to calculate the axial ratio. The temperature of conversion was found 

 by Schwarz to be 95. These crystals were made by adding to zinc oxide an 

 excess of acetic acid, sodium uranyl acetate, and uranyl acetate in calculated 

 proportions. These crystals were identified by the twins, as shown by 

 polarization. They were found to weather fairly rapidly, even in moist 



summer weather. 



SODIUM CADMIUM URANYL ACETATE. 



NaCa[UO 2 (C 2 H 3 O 2 ) 3 3H 2 O] 3 . 



These crystals, prepared and measured by Wyrouboff, 6 have the axial ratio 

 a: b: c = 0.5162: 1:0.9798; /3 = 909'; plane of the axes normal to b (010) 

 and nearly parallel to a (100) ; positive bisectrix parallel to 6, with large axial 

 angle. The change to a uniaxial figure does not take place until nearly 200, 

 at which temperature the crystals effloresce. 



1 Erb, N. Jahr. of Min. B. B., 6, 121-147. 1888-89. 



2 Wyrouboff, Bull. Soc. fran. Min. 24, 93-104. 1901. 



3 Schwarz, Beitr. Z. Kenntn. d. umkehrbaren Umwandlungen polymorpher Korper. Preisschr 



d. Univ. Gottingen. 1892. 



4 Wyrouboff, I. c., 104. 



5 Erb, I. c., 126. 



6 Wyrouboff. 1. c.. 103. 



