Mr. H. C. Sorby on Jargonium. 



67 



ever both occur in the same mineral. In the case of jargonium, 

 however, the three types of spectra are all met with in crystalline 

 modifications of apparently the same compound. 



The most characteristic test for jargonia is the spectrum of the 

 borax blowpipe beads, seen with the spectrum-microscope, which 

 enables us to detect it in zircons containing less than one per cent. 

 As much of the earth or natural silicate as will completely dissolve 

 should be melted in circular loops of platinum wire, about J of an inch 

 in diameter, with a mixture of borax and boric acid, and a very strong 

 heat kept up till crystals begin to be deposited, owing to loss of the 

 solvent by volatilization. On removing the beads from the flame 

 they remain clear, and show a few acicular crystals, but give no 

 absorption-bands in the spectrum. On reheating to a temperature 

 just below very dull redness, they turn white, and so very opaque 

 that no ordinary light will pass through them. When, however, a 

 small and very bright image of the sun is formed in their centre, by 

 means of an almost hemispherical condensing lens of \ inch diameter, 

 and a cap placed over the object-glass, with a round hole less than 

 the beads nearly in the focus, so as to prevent the passage of extra- 

 neous light, they are seen to be illuminated by transmitted light of 

 about the same brilliancy as that of a bright cloud, so as to give 

 an excellent spectrum, without being at all dazzling. In the case of 

 beads containing jargonia, the spectrum differs completely according 

 to the temperature at which the included crystals have been depo- 

 sited. As already mentioned, a clear glassy bead gives no absorp- 

 tion-bands ; and when the crystals are deposited at as low a tempe- 

 rature 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 spec- 

 trum 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 coincides with the 

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



Red end. 



Blue end. 



Fig. 1. 



Fig. 2. 



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 



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