EXPF.RIMENTS ON ALLANITt. 49 



principal facettes set on the larger lateral planes, with wliich 

 the centre one measures 125° and 55**. Of this specimen an 

 engraving is given in the annexed plate, fig, 2. 



Specific gravity, arcor<ling to my experiments, 3*533. 

 The specimen appears to be nearly, though not absolutely, 

 pure. This substaice, however, is so very much mixed "' * 



with mica, that no reliance cun be placed on any of the 

 trials which have been made. Count Bournon, surprised 

 at the low specific gravity noted by Mr. Allan, which was 

 3*480, broke down one of the specimens which had been 

 sent him, in order to procure the substance in the purest 

 state possible, and the result of four experiments was as 

 follows. 4*001 



3*797 

 3-654 

 3*119 

 In a subsequent experiment of Mr. Allan's, he found it 

 3-6()5. From these it appears, that the substance is not ia 

 a pure state. Its colour is so entirely the same with the 

 mica, with which it is accompanied, that it is only by mecha* 

 iiical attrition that they can be separated. 



Colour, brownish-black. 



External lustre, dull; internal, shining and resinour, ;; .cwV 



slightly inclining to metallic. ^^' 



Fracture, small conchoidal. 



Fragments, indeterminate, sharp-edgcd. 



Opuke. 



Semihard in a high degree. Does not scratch quartz or 

 felspar, but scratches hornblende ai^d crown glass. 



Brittle. 



Easily frangible. / 



Vowder, dark greenish-gray. 



Betore the blowpipe it froths, and melts imperfectly into 

 a broivn scoria. 



Gelatinises in nitric acid. Ia a strong red heat it loset 

 3*98 ;>tr cent of its weight. 



SecU 2. My first experiaients were made on the supposi- Experimsnu 

 tion, that the mineral was a variety of gadolinite, and were ^»*^<^*^fta.'" '*• 

 pretty much la the style of those previously made on that *^**"*^*""''"* 

 substance by Ekeberg, Klaproth, and Vauquelin. 



I. 100 grains of the mineral, previously reduced to a fine silex. 



Vol. XXIX,— May, 181 u h • p»wdtr 



