396 F. T. Allen, F F. Wright and J. K. Clement— 



tained by observing in convergent polarized light on a section 

 after 010 the direction in which the dark hyperbolae of the 

 interference figure emerge from the field on revolving the 

 stage, this direction being that of the acute bisectrix.* The 

 result was, furthermore, confirmed on a section normal to an 

 optic axis. An attempt was made to measure the optic axial 

 angle on such a section by the graphic method recently 

 described by F. Becke,f but the fine intercalated twinning 

 lamellae affected the sharpness of the figure to such an extent 

 that valid numerical results were out of the question. The 

 black axial bar of the figure, however, is only slightly curved 

 in the diagonal position and indicates a large axial angle. 



The disturbing influence of the twinning lamellae was fur- 

 thermore felt in some unsuccessful attempts to etch the prism 

 faces with hydrochloric acid. The fine lamellae apparently 

 destroyed the continuity of any larger etch figures which 

 might otherwise have formed. 



The specific gravity at 25° was determined on preparations 

 from the melt by the method of Day and Allen :£ 



Specific gravity of the Monoclinic Pyroxene, H 2 at 25° = 1. 

 Preparation I. Preparation II. 



3-191 3*194 



3-192 3-191 



Average, 3*192 



Melting Point of Magnesium Metasilicate. — The melting 

 point was determined by the Frankenheim method, using a 

 control element, as described by Allen, White and Wright.§ 



The results are contained in Table III. 



Table III. 

 Element E. Element K. 



1521-3° 1521-3° Both elements in same porcelain tube 

 1519*7 1522-2 Both elements in same porcelain tube 



1521- 



Mean, 1521°. 



After every experiment, elements E and K were compared 

 with the standard element, H, which was protected from con- 

 tamination and deterioration due to absorption of iridium 

 vapor, by a tube of pure platinum. The heating coils of the 

 furnace, which we have used heretofore, have been wound with 

 platinum wire containing 10 per cent of iridium. It has been 

 found that at temperatures of 1200° or ~ more, the iridium 



* Wright, F. E., this Journal (4), xvii, p. 385, 1904. 



\ Becke, F., Tscherm. Min. petr. Mitth., xxiv, 32-49, 1905. 



% This Journal (4), xix, 93, 1905. § Loc. cit. 



