68 Royal Society: — 



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 

 described 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 absorp- 

 tion-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 examined 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 remark- 

 able 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 absorption-bands are developed; and, as 

 a general rule, the increase varies simply and directly as the amount 

 of jargonia which passes from one state into the other. Zircons in 

 their natural condition from various localities contain a very variable 

 absolute and relative amount of these two modifications of jargonia, 

 and there seems good reason to believe that this difference in phy- 

 sical state may materially assist us in determining the temperature 

 at which certain rocks have been formed. I have also met with 

 one example of the third form of spectrum. A brown-red zircon 

 from Ceylon was so dark in one part as to be quite opaque, and 

 therefore I do not know what the original spectrum might have 

 been. On heating it to redness, the whole became a clear pale green ; 

 and, without examination with the spectroscope, no one would have 

 suspected any difference between the different portions. That which 

 was originally a pale brown-red then showed the same spectrum as 

 that usually developed by heat, whilst that which was originally very 

 dark showed an entirely different spectrum, corresponding exactly 

 with that of the borate deposited in blowpipe beads at a medium 

 temperature. It also corresponds in general character, but not in 

 detail, with that of the blue spinels from Ceylon, which must, I think, 

 contain a small quantity of jargonia. That part of the zircon which 

 gave this spectrum appears to have had the same remarkably low spe- 

 cific gravity of about 4*0 both before and after ignition, as though 

 the volume of the silicate of jargonia in this state were even greater 

 than in that which gives no bands. All these spectra due to jargo- 

 nium are of a very marked character, and quite unlike those due to 

 any other element in similar conditions. 



