514 



Mr. H. C. Sorby on Jargonium. 



[June 17, 



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 modifica- 

 tions of jargonia, and there seems good reason to believe that this differ- 

 ence in physical 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 examina- 

 tion with the spectroscope, no one would have suspected any difference be- 

 tween 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 specific 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 jargonium, are of a very marked character, 

 and quite unlike those due to any other element in similar conditions. 



The alteration produced in jargons by heat is, to some slight extent, 

 analogous to what occurs on heating carbonate of lime in the state of 

 arragonite ; but, instead of changing into an opaque mass of mmute crys- 

 tals of another form of the carbonate (calcite), which has a less specific 

 gravity, is less hard, and does not give a different spectrum, they are still 

 as simple and transparent crystals as at first ; the specific gravity and 

 hardness are increased, and the spectrum is entirely changed. Iodide of 

 mercury is an excellent illustration of an alteration in the spectrum, due 

 to a change in crystalline form produced by heat ; but still the facts 

 differ most materially from those described, and there are only two modi- 

 fications — the yellow 7 and the scarlet. The existence of three crystalline 

 modifications is similar to what occurs in titanic acid. Anatase, Brookite, 

 and rutile have distinct crystalline forms, but they do not differ much in 

 specific gravity, and their spectra present no characteristic differences. 

 On the whole, the different states of carbon (charcoal, graphite, and dia- 

 mond) are perhaps the best illustration of the existence of three different 

 conditions in the same substance, since they differ materially in specific 

 gravity and optical characters, one being black, the other having a metallic 

 lustre, and the third being transparent and colourless ; hut these are varia- 

 tions of the element itself, and not, as in the case of jargonium, modifica- 

 tions of its compounds. So far as I am aware, there is indeed no substance 

 which shows strictly comparable facts. 



