of Compounds of Zirconia and the Oxides of Uranium. 455 



trum with five well-marked absorption-bands, all of which occur at 

 the red end, where no trace of bands exists in the case of ordinary- 

 salts, as will be seen on comparing it with Nos. 1, 2, and 3. I have 

 tried many experiments in order to ascertain whether any other 

 element besides zirconia will cause uranium to give similar abnormal 

 spectra ; but none show any thing of the kind, at all events in similar 

 conditions. A few have special characters, as described below, but 

 the majority exert little or no influence \. and even when the blow- 

 pipe-beads are crystalline, they show only the usual spectra of the 

 oxides of uranium. Moreover no such great change in the character 

 of the spectra of any other elements which give absorption-bands is 

 to be seen when they are combined with zirconia ; and, as far as my 

 present experience goes, it seems as if such very abnormal spectra 

 were met with only in the case of these remarkable compounds of 

 zirconia with the oxides of uranium. 



Such, then, being the facts, it appears to me that we are now in 

 a position to explain why certain zircons give three different spectra, 

 as described in my former paper. Some jargons (usually those of 

 a green tint) contain a little uranium so combined that the charac- 

 teristic spectrum is only faintly visible, whereas, after ignition, the 

 intensity of the absorption-bands is permanently increased to a vari- 

 able extent, occasionally only a little, but in some cases as much as 

 twenty-five times. This more powerful action on light is accompauied 

 by an increase in hardness and in specific gravity (sometimes as much 

 as from 4*20 to 4 - 60), as described in my former paper ; and I have 

 since found that these changes are approximately proportional to the 

 amount of uranic oxide in the various specimens, as shown by com- 

 paring the spectra of the blowpipe-beads. This change may partly 

 depend on the oxidization of the uranous oxide, since some speci- 

 mens slightly increase in weight when ignited ; but I think it cannot 

 be mainly due to that ; for sometimes there is no such increase, and 

 uranous oxide combined with zirconia gives rise, not to a spectrum 

 without bands, but to one with several of very marked character, as 

 described below. On the whole, since this abnormal type of spec- 

 trum is so characteristic of combination with zirconia, it appears to 

 me more probable that the effect of a high temperature is to cause 

 the uranic oxide to combine more specially with the zirconia, as 

 though the greater part existed naturally as a silicate, but after igni- 

 tion as a zirconiate. We may also apply the same explanation in 

 the case of zircons more or less strongly coloured by other oxides, 

 which become almost colourless when heated ; and thus this unex- 

 plained peculiarity of zircons may depend on the fact of zirconia being 

 able to play the part of both a base and an acid which, as compared 

 with silica, has an affinity for bases varying according to the tempe- 

 rature. 



The brown-red zircon from Ceylon, named at page 514 of my 

 former paper, kindly presented to me by Mr. E. L. Mitford, of 

 Rusthall, gives a spectrum precisely like that of the borax blowpipe- 

 beads crystallized after treatment in the deoxidizing flame, and there- 

 fore no doubt contains uranous oxide. This spectrum being given by 



