Temperature of Sublimation. 865 



evidence to show that it is contained in group II. beyond 

 the inference that this molecule should be less stable than 

 the molecule RAs. Until more substances are examined the 

 placing of this molecule in group II. must be regarded as 

 provisional only. The two smaltites give a comparatively 

 small sublimate in group II. and the greater part of their 

 arsenic in group III.; somewhat in the manner of the chloan- 

 thite just referred to. In order that these substances should 

 be constituted as commonly written, CoAs 2 , there must be 

 rather over 70 per cent, of arsenic present. It is quite 

 certain from rough analyses made of the two smaltites, that 

 this percentage of arsenic does not obtain in these specimens. 

 They probably contain the molecule (Ni, Co) As 2 , but are 

 for the larger part constituted of some molecule containing a 

 lesser percentage of arsenic. The gersdorfite (Nos. 5 and 34 ), 

 appearing in both the second and third groups, belongs to a 

 species which like the smaltites and chioanthites cannot be 

 considered to be fully understood. The recorded analyses 

 of gersdorfite vary considerably, and Dana suggests various 

 formulae. This mineral was from Siegen. A good analysis 

 on the apophorometer gave 37 per cent, of As and 47 per 

 cent, of Ni, including some Fe and Co. Sulphur was 

 copiously expelled. By difference it would make 16 per 

 cent. The relation between the numbers of atoms would 

 be As, 5 ; Ni, 8 ; and S, 5. Now there is some over- 

 estimation of the Ni, for its amount is obtained on the 

 assumption that the whole of the residue is NiO. The 

 sulphur is therefore probably under estimated and the nickel 

 over estimated. The result points evidently to the molecule 

 (NiCoFe) 3 (AsS) 4 as forming the greater part of the sub- 

 stance. Dana cites a fairly concordant analysis of the Siegen 

 mineral, i. e., As, 37*52 ; Ni, 40'97 ; S, 17*49 ; Fe, 4'19. This 

 would approximate nearly to the constitution suggested. 



While there are doubts about the placing of the diarsenide 

 molecule there are none, as I believe, about the correct 

 placing of the sesquisulphide molecule. For bodies carrying 

 their arsenic in this form all commence sublimation at the 

 low temperature characterizing the second group, and will 

 be found to slowly continue to break up within a range of 

 not more than 100 degrees above this. Suppose we place 

 such a substance as proustite in a thin layer on the hob. It 

 begins subliming at about 250°, and as the lower and hotter 

 particles become exhausted the rate of evolution falls off. 

 But if now we set the temperature to, say, 320°, and continue 

 changing the upper glass every 3 or 4 minutes, an almost 

 steady out-put of arsenious oxide will be observed, till finally 



