Temperature of Sublimation. 86'6 



We can explain these facts in either of two ways, (a) As 

 arsenic is expelled from the substance fresh combinations 

 occur within it. Thus if a molecule RAs 2 loses As, the 

 molecule RAs may be formed and this may only break up in 

 the higher group. In this case it should be noticed that 

 either an equal or less amount of arsenic will be liberated in 

 a succeeding stage. If there were a number of stages in 

 which half the arsenic came away each time, of course there 

 would be a geometrical decrease. Other laws of decrease 

 might obtain. I have found, however, that there is often 

 an increase in the amount liberated at a higher stage. If an 

 arsenide in the course of its decomposition passes from 

 group II. to group III., there is often a very marked increase; 

 but if after this the temperature has to be carried very high, 

 into what may be called, provisionally, the fourth group, to 

 complete the liberation of arsenic, the sublimate in the 

 fourth group is smaller than that obtained in the third. 

 The explanation (a) cannot, therefore, generally be true, and 

 we must adopt the alternative explanation : (6) that the 

 several molecules decomposing at the different group 

 temperatures existed in the substance from the first. There 

 seems no reason to doubt that this is often or generally the case. 



It will be seen from this that a substance assigned to a 

 particular group on the strength of a first small sublimate 

 may essentially belong to some higher group. It should 

 appear in more than the one group. 



The fact of sublimate beino- obtained in more than one 

 group is not, I think, an objection to the suggested groups. 

 Rather it is, as I believe, in their support. Many recognized 

 mineral formulae might involve this passage from group to 

 group ; e.g., such as (CoNi) As + RAs 2 for certain smaltites. 

 But it would be an objection to the proposed groups if a 

 substance to which we must assign a molecular structure 

 proper to a lower group only, decomposed in a higher. I 

 was at first confronted with what appeared like some diffi- 

 culties of this kind. I shall now refer to these. 



The mineral tennantite was examined on two Cornish 

 specimens. The formula assigned to this mineral in Dana's 

 ' System of Mineralogy ' is 40u 2 S, As 2 S 3 . Now three con- 

 cordant analyses by different observers give approximately 

 the percentages S 27 ; As 19 ; Cu 49 ; Fe 3. These percentages 

 give as the relative numbers of molecules 17 S; 5 As; 16 Cu; 

 1 Fe. These may be supposed to form 11 CuS,FeS, 5 OuAsS. 

 The formula is, I venture to think, a probable one and would 

 bring the substance into the third group as its temperature of 

 sublimation actuallv does. 



