8 06 Prof. J. Joly on the 



the substance appears to consist of the sulphide o£ silver. 

 Sometimes a little arsenic may obstinately linger in such cases, 

 but essentially the whole of the arsenic comes away before 

 the temperature of the higher group is attained. All these 

 bodies do not behave precisely as proustite. There seems to 

 be an interesting difference with respect to the evolution 

 of sulphur, a difference which certainly is not accountable 

 on the observations to be presently given relating to the 

 sulphides. In the case of sartorite, for instance, a plentiful 

 liberation of sulphur attends the decomposition of the sub- 

 stance. Tt is difficult to detect the liberation of this element 

 from the proustite. In this direction some insight may be 

 obtained as to interactions progressing on the hob. 



Much remains to be ascertained. From the limited scope 

 of the experiments influences may escape notice which may 

 prove of importance. Thus it happens that the results in 

 group III. are mainly confined to compounds of iron, nickel, 

 and cobalt. We are not, therefore, in a position to examine 

 whether the atomic weight of the stable element or its 

 periodic position exerts an influence. The copper arsenide, 

 domykite, appears exceptional in its temperature of de- 

 composition. Tennantite is, however, normal. 



Of the second group we seem justified in stating what has 

 been already said of the similar antimony-bearing bodies ; 

 that the nature of the values n, m, and R in the general 

 formula n R S, m As 2 S 3 , does not seem to affect the tem- 

 perature of volatilization. 



Throughout the table it does not appear that the presence 

 of sulphur, save when combined as the sesquisulphide 

 molecule, exerts much influence on the sublimation tem- 

 perature. There is no notable difference between the vola- 

 tilization temperature of As and AsS ; and, consistently, 

 NiAs and NiAsS show no difference in stability. The sesqui- 

 sulphide of arsenic shows a stability above that of either of 

 the constituent elements. 



It is important in dealing with substances containing both 

 arsenic and sulphur to distinguish between their sublimates 

 upon first appearance. I have found that two characters 

 serve to discriminate between these sublimates and to separate 

 them however faintly developed. The characteristic odour of 

 80 2 reveals the smallest trace of sulphur when properly sought 

 for. The observation is best made by warming for a moment 

 the watch-glass on the convex side over a low bunsen flame and 

 as the heat soaks through observing the odour while holding* 

 the glass in a vertical position. It will also then be found 

 that a separation of the two sublimates may be effected, if 



