104 Mr. D. Forbes on the Chemical Composition 



Arsenic acid . . . 44*05 1 



Protoxide of nickel . 19-711 2g . g5 V 73*00 

 Protoxide of cobalt . 924 J J 



Water 2698 



9998 



These results immediately identify themselves with the formula 

 (NiO + CoO) 2 As0 5 + 8HO, which, expressed in percentages, 

 would require 



Arsenic acid 43-891 7 o.^o 



Protoxide of nickel and cobalt. 28*63 j 

 Water 27*48 



100-00 



From the above, it will be seen that this mineral is analogous 

 to pharmacolite, 2CaO, As0 5 + 6HO, in which all the lime is 

 replaced by the protoxides of nickel and cobalt, but differs in 

 having eight instead of only six equivalents of water : in this 

 last respect it resembles the other tribasic arseniates of nickel, 

 cobalt, and zinc — Annabergite, erythrine, and Kottigite, 



It is remarkable that this mineral is colourless notwithstand- 

 ing its containing so large a proportion of the protoxides of 

 nickel and cobalt, the combinations of which are so generally 

 distinguished by their more or less prominent tints. 



Along with the above-mentioned mineral is frequently found 

 a sulphur-yellow compound in thin amorphous scales, or rather 

 crusts : on qualitative examination it yielded arsenic, nickel, and 

 cobalt, but was extremely difficult to obtain entirely free from 

 the preceding mineral, from which reason it has not as yet been 

 analysed. Possibly it may be the mineral from Johann-Geor- 

 genstadt examined by Bergmann*, with the following results : — 



Arsenic acid .... 50-53 



Protoxide of nickel . . 48*24 



Protoxide of cobalt . . 0*21 



Protoxide of copper „ . 0*57 



Sesquioxide of bismuth . 0*62 



100*17 



from which is deduced the formula 3NiO, AsO 5 . 



Bismuthic Silver. 



The locality of this mineral is the silver mine of San Antonio 



de Potrero Grande, about 35 miles to the south-east of Copiapo; 



the mine is situated in the beds of porphyry conglomerate and 



breccia of upper oolitic age, the interstices of one or more of 



* Journal fur Praktische Chemie, vol. lxxv. p. 239. 



