10 Prof. J. Joly on the 



The mineral is certainly not petzite. The latter body has 

 the composition AuTe, 3AgTe, and a density of 8'7-9'0. 

 The analysis suggests PbTe 3 as being approximately the com- 

 position o£ the telluride. It may be a new species. 



The second telluride calling for notice is No. 18. This 

 substance, on its chemical composition, should not give the 

 low- temperature sublimate. That it does so may, perhaps,. 

 be ascribed to the large amount of the very volatile element 

 combined with the tellurium. 



Bismuth. 



Four oxides are observed. (1) The bismuthous oxide, the 

 most stable oxide, is plentifully obtained as a sublimate at 

 high temperatures. If a little of it is scraped from the glass 

 and heated on the hob, it will be found to fuse at about 900° 

 and volatilize at about 1010°. As evolved from the element 

 or from compounds, a higher temperature is generally 

 required in order to obtain the sublimate : from 1200° to 

 1300°. It is gradually formed on the hob as the substance 

 decomposes or oxidizes. Thus, if pure elemental bismuth is 

 treated, the metal undergoes colour changes from yellow to 

 orange, red, ash-red, to purple. At a little over 900° there 

 is fusion of the oxide, and at a temperature rather over 

 1200° the sublimate is obtained. 



The sublimate is canary -yellow to pale whitish-yellow or 

 nearly white in colour. It is nebulous and hazy in form ; 

 never definite and streaky when formed as described. Under 

 the microscope it is granular. It is non-volatile and is soluble 

 in HC1, but not soluble in strong solution of KHO. 



This oxide may appear in small traces at a lower hob- 

 temperature if there is deflagration or sudden decomposition 

 of the mineral. It then takes a well-defined form, repro- 

 ducing the chance pattern of the deposit on the hob. The 

 sudden high temperature developed on deflagration may be 

 responsible for its sudden and limited development. It is 

 whitish in colour and shows the same physical and chemical 

 properties described above. It has been obtained from 

 colloidal bismuth at 640° ; from emplectite at 600° : from 

 cosalite and tetradymite at about 700° ; from aikinite at from 

 510° to 690°. Heating may, in many cases, be so carefully 

 regulated that deflagration is avoided, and in these cases the 

 bismuthous oxide is only obtained at the usual high tempe- 

 rature. In quantitative work the low-temperature evolution 

 of the oxide is so limited in amount as to render it 

 unimportant. 



