394 Prof. N. S. Maskelyne on New Cornish Minerals [1865. 



Cleavage parallel to I distinct ; parallel to 1 nearly equally so. 

 The plane 1 is brilliant ; 1 rather less so, as is the rarer plane 010; 

 the plane 1 1 sometimes exhibits hollows, the sides of which are parallel 

 to the cleavages. The specific gravity of the mineral is 3'48 to 3*50. Its 

 hardness less than 3. On looking through a section of one of these 

 microscopic crystals of Langite, ground parallel to the plane [ 1] in the 

 polarizing microscope, the plane of the optic axes is seen to be parallel to 

 100; but though coloured rings are visible, the axes lie beyond the field, 

 and the double refraction is weak. Probably, however, the first mean line 

 is the normal to 1, and it is negative. The symbol for its optical orien- 

 tation would be b c CT 



The crystals present dichroism : 



1st. As seen through 1 (along axis b c) : 



C (plane of polarization || to 1 0) greenish blue. 

 b (plane of polarization [| to 1 0) blue. 



2nd. As seen through 100 (along axis a) : 



C (plane of polarization [| to I) darker bluish green. 

 0. (plane of polarization || to 1 0) lighter bluish green. 



It is a fact worthy of remark that Langite is geometrically isomorphous 

 with Leadhillite. 



Langite is insoluble in water, but readily soluble in acids and ammonia. 

 When submitted to the action of heat, it loses its blue colour, turning at 

 first bright green. As the heat is increased, it passes gradually through 

 various darker hues of this colour, till it becomes of a dull olive-green, and 

 ultimately black. Water is given off the whole of the time, which in the 

 later stages of the change has an acid reaction. Before the blowpipe, it 

 gives off water and acid fumes, colours the flame green, and becomes reduced 

 to metallic copper with carbonate of soda on charcoal. The chemical 

 composition of Langite is represented by the empirical formula, 



4Cu"05H' 2 OS0 4 . 



which may be written as 



The copper was determined in one case (i.) by precipitation on the interior 

 of a platinum crucible, by means of a cell of a Grove's battery, a method 

 that seems, however, to give the value of the metal in excess ; in other 

 cases (ii., iii., and iv.) by means of the volumetric method, wherein the 

 iodine set free on precipitation of copper by an iodide is determined by 

 means of starch and hyposulphite of soda. The sulphuric acid was de- 

 termined in the usual way. The water in two cases (i. and ii.), by mixing 

 the powdered and dried mineral with previously ignited carbonate of 

 barium and heating the mixture in a combustion-tube in a current of dry 



