94 Mr. D. Forbes's Chemical Examination of, 



uncertain at present, it would be only guesswork to attempt 

 deducing a formula; but it will be at once seen from tbe above 

 analysis, tbat the mineral from Helle is identical with the Tyrite 

 from Hampemyr on the Island of Tromoe, described in the New 

 Edinburgh Philosophical Journal, No. 1, 1855. 



With regard to the mineral Fergusonite, from Kikertaurvak 

 near Cape Farewell in Greenland, our information at present rests 

 upon Haidinger's crystallographic examination (Ediub. Roy. 

 Soc. Trans, vol. x. p. 27-A), Berzelius's blowpipe reactions, and 

 Hartwall's chemical analysis, both in the Kongl. Vetenskabs 

 Academiens Handlingar for 1828, p. 167, to which INI. Kenngott 

 has not brought forward any addition; and not having seen 

 any specimen of that mineral, I must refer entirely to these 

 authorities. 



The comparison of the two minerals instituted by M. Kenngott 

 rests upon — 



1. Their crystallographic identity. But the crystals of Tyrite 

 hitherto found are so extremely uneven and irregular, as before 

 mentioned, that no dependence can be placed upon measurements 

 of their angles by a hand-goniometer; and quite as many dis- 

 cordant results could be adduced to meet those cited by M. Kenn- 

 gott. There seems, however, much probability that the crystal- 

 line form will prove the same, but this is not so certain as to set 

 the matter at rest. 



2. Resemblance in physical properties. These present several 

 points of unquestionable agreement; the colour, glance, and 

 streak must be pretty nearly the same. The hardness of Tyrite 

 is probably but very little higher, all specimens 1 have tried 

 scratching orthoclase, and being about 6-5 ; Fergusonite being 

 stated as 5-5 to 6-0. The specific gravity of Tyrite is, however, 

 decidedly lower than that of Fergusonite, which is stated as 5-838 

 (Allan), and 5 800 (Turner) ; whereas the mean of a number of 

 determinations of the specific gravity of specimens of Tyrite, both 

 from Helle and Hampemyr, was 5-33, and only one specimen 

 was found to be above 5-36. All the specimens were perfectly 

 pure, any decomposed or weathered parts being carefully removed 

 if present. 



3. Similarity of blowpipe reactions. As the action of fluxes 

 on these minerals depends chiefly on the presence of uranium, 

 they both react with borax and microcosmic salt similar to most 

 minerals containing this metal. 



According to Berzelius and Hartwall, Fergusonite only evolves 

 a trace of water when heated in a tube, whereas Tyrite decrepi- 

 tates often very strongly and evolves considerable water. 



4. Chemical composition and constitution are best seen from 

 the following tabulated statement : — 



