1865.] of the Brochantite Group. 87 



normal inclinations giving the following angles, which are the averages of 

 many measurements : 



1 I 1~10=56 16 



10011 0=61 52 



001 201 = 51 46 

 conducting to the parametral ratios 



a :: c=l:0'5347:0-6346. 



The crystals are twinned after the manner of cerussite, the twin axis being 

 normal to the plane (110). 



1 1 (1 10) T 1 0=112 33 

 100 (110) 100=123 44 

 ! "10 (It 0) 110= 67 26 



Cleavages seem to exist parallel to 1 and 1 0. The planes 1 and 

 1 are very brilliant. The plane of the optic axes, as seen through a section 

 parallel to the plane 1, is parallel to 1 0. The normal to 1 would 

 seem to be the first mean line, and it is negative. The optical orientation 

 of the mineral is therefore 6, C, a. 

 The crystals are dichroic. 



1 . Seen along axis c, c, greenish blue. 



6, blue. 



2. Seen along axis a, c, darker greenish blue. 



0, lighter bluish green. 



The specific gravity of Langite is 3 48 to 3-50. Its hardness is under 3. 

 It will not abrade calcite. 



Before the blowpipe on charcoal it gives off water, and fumes and 

 becomes reduced to metallic copper. Insoluble in water, it is readily dis- 

 solved by acids and ammonia. Heated, it passes through (1) a bright 

 green, and (2) various tints of olive-green, till (3) it becomes black. 

 Water is given off the whole time, and finally it has a strongly acid 

 reaction. 



The first stage corresponds to the loss of one equivalent of water ; the 

 second reduces its composition to that of Brochantite ; at the third it loses 

 all its water. 



The chemical composition of Langite is represented by the formula 

 3Cu" H' ? O a +Cu" S0 4 +2H' 2 0, which requires the following numbers : 



Calculated Average 



percentage. found. 



4 equivalents of copper 126'72=52-00 52-55 



4 equivalents of oxygen 32' = 13-13 13'27 



1 equivalent of sulphuric anhydride 40- =16-41 16'42 



5 equivalents of water 45- = 18'46 18-317 



24372 100-00 100-56 



I have met with a small and old specimen of Connellite with a twin crystal 

 of Langite associated with it. 



H 2 



