of Compounds of Zirconia and the Oxides of Uranium. 457 



needle-shaped crystals begin to be deposited, when it must be allowed 

 to cool rapidly. It should then be transparent, with scattered crystals, 

 and the uranium all in the state of protoxide. On gently reheating 

 it, the bead ought to suddenly turn white and almost opaque ; and 

 care must be taken not to heat it any more than is just requisite to 

 cause the borate to crystallize out, or else the uranium will rapidly 

 pass into the state of peroxide. Such beads must be examined by 

 strong direct light from the sun, or from a lamp of very great bril- 

 liancy, condensed on them by means of an almost hemispherical lens 

 of about \ inch focal length ; and in addition to the means described 

 in my former paper, I have since found it very convenient to place 

 them over a hole in a black card, so as to entirely prevent the passage 

 of any light which has not penetrated through them, even when so 

 arranged in the focus of the microscope that the spectrum of their 

 thin edges may be examined, if the centre be too thick and opaque. 

 If thus properly prepared, the presence of more or less uranium will 

 be shown by the greater or less intensity of the absorption-bands of 

 the spectrum described and shown in fig. 1 of my former paper. 

 This test is so delicate that there is no difficulty in seeing the 

 darker band in the green in the case of zircons which contain no 

 more than -fa per cent, of uranic oxide ; and I find that very few 

 localities yield this mineral so free from it that it cannot be easily 

 detected. Those from Miask, Siberia, are the only specimens in 

 which I have not been able to recognize it. The jargons from Ceylon 

 contain an amount varying up to about 1 per cent., although in no 

 published analysis that I have seen is there any allusion to the 

 presence of even a trace. It has also been overlooked in several 

 other cases ; and it now becomes important, because it gives rise to 

 various well-defined spectra, which are so characteristic of the differ- 

 ent minerals, that they can be very conveniently identified, even when 

 cut and mounted as jewels, by means of the number and position of 

 the absorption-bands, as I intend to explain in a paper on the spectra 

 of minerals. 



On flaming the bead at a moderate red heat, the protoxide passes 

 into the peroxide, and the spectrum No. 6, given above, may be seen, 

 if sufficient oxide be present ; but considerably more is required than 

 in the case of the protoxide. I may here say that the examination 

 of better preparations has enabled me to detect another distinct band 

 in the extreme red, not shown in fig. 2 of my former paper, and also 

 an additional faint band in the blue, not shown in fig. 1. 



In applying this test to detect minute quantities of uranium in 

 other minerals, it is requisite to bear in mind that zirconia may play 

 the part of both an acid and a base, and that various oxides and acids 

 so combine with the zirconia or with the oxides of uranium as to 

 prevent the formation of the compounds which give rise to the 

 characteristic spectra. The zirconia appears to combine with some 

 rather than with the uranous oxide, and with others rather than 

 with the uranic, so that, if one spectrum cannot be obtained, the 

 other may ; and there are few, if any, cases when neither can be seen, 

 especially if care be taken to use excess of zirconia. If, however, the 



