188 Transactions of the Society. 



the formula, AI2 O3 . 3 SO3 + 18 H2 + 8 (Fea O3 . SO3), the 

 theoretical composition of which would be as follows : — 



Ferric oxide, Fe2 O3 49-47 per cent. 



Alumina, AI2 O3 3-98 „ 



Sulphuric acid, SO3 34-00 „ 



Water, H2O 12-52 „ 



Total .. .. 99-97 „ 



differing from the result of Dr. Eeinsch's essay, as published by 

 him in the 'Proceedings of the Eoyal Society' for 1881, No. 217. 



The most interesting facts connected with the cyprusite are, 

 however, revealed by a careful microscopic examination of the 

 mineral. It is then found to consist of a loose to compact aggregate 

 of very minute, translucent, very slightly coloured crystals, the 

 microscopic " projection " of which is generally more or less regu- 

 larly hexagonal. These crystals vary in diameter from 1/120 to 

 1/300 of a millimetre = 8 • 30 /* to 3 • 32 /^ = • 00032 of an inch 

 to -00013 of an inch. These are entirely soluble in hydrochloric 

 acid, but insoluble in water. 



Calcining converts these crystals into an opaque substance, 

 which generally retains the previous outline. 



Crystals are frequently found irregularly formed, as also occa- 

 sionally compound or twin-crystals. Under the polariscope the 

 micro-crystals, if examined dry or immersed in water, would, by a 

 superficial examination, be taken for isometric, an appearance 

 which is due to the hexagonal disks, all presenting this same face 

 towards the optical axis of the instrument, but if mounted in thick 

 balsam so as to lie in various positions to the observer, they are 

 found, small as they are, to be beautifully anisotropic, the hexagonal 

 sections alone remaining obscure under the crossed Nicols, so that 

 the crystalline system may be safely laid down as rhombohedral or 

 hexagonal. This determination is further supported by the fact 

 that some of the larger crystals seem to present apical modifications 

 which are multiples of three. 



It will be remembered that the ordinary alums, as also copperas, 

 crystallize in the monometric system, so that cyprusite seems to 

 constitute a really good and distinct mineral species. 



I may add that cyprusite is insoluble in water, and that 

 analysis has failed to detect in it either lime or magnesia. The 

 specific gravity is 1 • 8. Completely immersed in this bed of minute 

 crystals of cyprusite are to be found scattered numerous silicious 

 organic remains (non-polarizing) to the extent, on an average, of 

 about 16 "90 per cent, of the whole bulk, along with a very few 

 small grains of quartz sand, which last are readily distinguishable 

 under the micro-polariscope. 



The Microscope shows that these silicious organic remains. 



