Breislakite and Augite. 445 



been clearly ascertained, owing to the minuteness of the cry- 

 stals, and to the circumstance that usually both extremities of 

 the prisms are engaged in the sides of the little cavities in 

 which the substance occurs. A late examination of some speci- 

 mens from Capo di Bove, in a lava apparently free from the 

 ordinary augite crystals, enabled me, however, to detect a 

 crystal of the accompanying form ; the planes of which, though 

 extremely minute, were very brilliant, and sufficiently defined 

 to admit of measurement. This crystal was of the same golden 

 brown colour as the surrounding fibres, and in every way 

 similar to them in its general aspect, as well as in its comport- 

 ment before the blowpipe, as afterwards tried. It belonged 

 to the monoclinic system, and was a combination of the mon- 

 axial forms P, M and L; of the diaxial prismatic form D 

 and the diaxial pyramidal form E, or mE ; the 

 planes of the latter being mere lines, visible 

 from their brilliancy, but too small to reflect a^p 

 distinct image. 



P on M gave 106° 18'; P on L, 90°; P on 

 D, 100° 34'; D on D, 87° 10'. These values 

 sufficiently prove that the substance was an au- 

 gite*. 



The measured crystal fused very readily be- 

 fore the blowpipe into a shining bead of the same colour as 

 the unaltered mineral, and but very slightly attractable by the 

 magnet: several capillary crystals behaved exactly in the 

 same manner. With the usual blowpipe reagents I could not 

 obtain the least indications of copper ; nor have I been able 

 to do so with any specimen of breislakite that I have yet ex- 

 amined. The assertion, therefore, copied from one minera- 

 logical work into another, that this substance contains oxide 

 of copper, is certainly incorrect. In borax the breislakite 

 dissolves rather slowly, and still more so in microcosmic salt, 

 the little fibres retaining their form and colour, in the latter 

 reagent, for a considerable time — a behaviour resembling that 

 of most varieties of augite and hornblende. The tints im- 

 parted to the glasses are solely derived from iron. Silica is 

 also separated in the microcosmic salt bead ; and with carbo- 

 nate of soda, a faint reaction of manganese is obtained. 



The fibres are not apparently acted upon by boiling hydro- 

 chloric acid. The supernatant liquid, diluted, and tested with 

 sulphuretted hydrogen, does not furnish a precipitate, 



* In conformity with the French crystallographers, I have considered the 

 planes P as basal, in the usual acceptation of the term as applied to mono- 

 clinic forms. This view is supported not only by its simplicity, but by the 

 general symmetry of at least nine-tenths of the augite crystals. 



