1869.] 



Mr. H. C. Sorby on Jargonium. 



513 



been deposited. As already mentioned, a clear glassy bead gives no ab- 

 sorption-bands ; and when the crystals are deposited at as low a tempera- 

 ture as possible, much below dull redness, and only just high enough to 

 soften the borax, there may be scarcely any trace of bands ; but, if a clear 

 bead be quickly raised to a temperature very little below dull redness, it 

 suddenly becomes opaque, and shows a spectrum with a number of narrow 

 black absorption-bands (fig. 1). The most distinct is in the green, then 

 one in the red, and one in the blue ; and there are three fainter, one in 

 the orange, and two in the green. On raising the temperature to bright 

 redness all these bands vanish, and four others appear, none of which coin- 

 cides with the former (fig. 2). Three are situated in the red and orange, and 



Red end. Blue end. 



Fig. 1. 

 Fig. 2. 





X, 



1 



i 







1 





1 



11 





one in the green, so as to give a spectrum of very different general cha- 

 racter. In this state the bead is a pale straw-colour, and not, as before, 

 almost white. In the case of nearly pure jargonia, the bead should not 

 be more than of an inch thick, or else it would be too opaque. 

 Pure zirconia treated in the same manner gives no bands whatever in any 

 condition ; the bead is quite white, and sufficiently transparent when two 

 or three times as thick as just named. 



It might be thought that the three different spectra thus briefly de- 

 scribed were due to different compounds, if it were not that there is a 

 similar series in the case of the natural crystalline silicate. Some of the 

 jargons of Ceylon have a specific gravity very little inferior to that of pure 

 zircons (4*70), and contain very little jargonia ; but those of low gravity 

 (4'20 or thereabouts) contain perhaps nearly 10 per cent., in a form 

 which gives scarcely any trace of absorption-bands. On keeping such a 

 specimen at a bright red heat for some time, the specific gravity increases 

 from about 4*20 to 4*60. Judging from the imperfect data now known, 

 this indicates that the volume of the silicate of jargonia is reduced to 

 about one-half ; the hardness becomes somewhat greater, and, when exa- 

 mined with the spectrum-microscope, the spectrum is found to be entirely 

 changed. Instead of a mere trace of bands, a spectrum is seen with 

 thirteen narrow black lines and a broader band, more remarkable than that 

 of any clear transparent substance with which I am acquainted. No such 

 changes occur in the case of zircons free from jargonia, like those from 

 Miask, Siberia ; there is no increase in the specific gravity, and no ab- 

 sorption-bands are developed, and, as a general rule, the increase varies 



