442 Koenig — Mohawkite, Stibio- domeykite, Domeykite, 



nickel-cobalt arsenide is here presented. Against such a view 

 speaks the physical condition of the substance in die first place 

 and also that a ratio of (Ni, Co) 3 As has not been observed so 

 far. The question presented itself at once as to whether any 

 of the domeykite occurrences in Houghton Co. contain nickel 

 and cobalt. Winkler gives this metal in the Zwickau occur- 

 rence at 0-44 per cent. F. A. Genth does not mention it, and 

 even from the Michipicoten Island location where niccolite 

 accompanies it, the nickel is absent in the domeykite. I have 

 examined every accessible specimen qualitatively. Neither 

 nickel nor cobalt could be found by Plattner's test ; but never- 

 theless it is very probable that all domeykite contains both 

 metals in traces, as my careful analysis of the mineral from 

 the Sheldon-Columbia location, both old and recent, demon- 

 strates. From 0'5 gram of substance just sufficient cobalt 

 oxide was obtained to give color to 50 mg. of borax glass and 

 the nickel was proved by reducing the bead with tin on char- 

 coal. Another matter engaged my attention at once, namely 

 the discrepancy in the specific gravities given by the several 

 authors and recorded on page 44, Dana Min., 6th edit. These 

 densities vary between 6*70 and 7*547 against my determina- 

 tion of 8'07 for the Mohawkite. The collection of the Michi- 

 gan College of Mines contains one fine specimen of domeykite 

 from the Sheldon-Columbia mine. The material is very uni- 

 form, a few specks of calcite the only visible gangue. Of this 

 3-8559 grams were selected. The specific gravity at 21° C. 

 was found = 7'9Jf,86. The whole material was then dissolved, 

 no residue was left and in a part of the liquid calcium was 

 looked for, but only a trace was found. Hence we have every 

 reason to take this specific gravity as that belonging to domey- 

 kite ; the figures in the literature must be wrong. It is true 

 that the analysis of this substance does not agree with the 

 theoretical composition exactly, namely : 



S "xf" "A'< 7 fn2 1 Spec. gr. found 7-94.86 



(Fe,Ni,Co) 0-06 t [ "calculated 8-1020 



As ... 26-14 ) 



100-20 



Now if we assume that the elements in this combination pos- 

 sess the same specific gravity as they do in the free state, we 

 may calculate the specific gravity. If specific gravity of 

 Cu = 8-96 and of As = 5*63 then the above composition must 

 weigh 8-96x74-00 + 5-63X26-14 = 8-1020. The specific grav- 

 ity of the theoretical Cu 3 As will be = 8-017. For Mohawkite 

 the calculated specific gravity will be, taking Co = 8-96 and 

 Ni = 9*5 (the highest number on record) : 



