290 Jif. JRqsp*9 Analysis qftM [Oct, 



qther mineral may be applied to this. It appear^ also that the 

 same prpportion obtaips between the two seleniurets of copper. 



The proportions of the elements of these two minerals are not 

 perhaps definite. We may call the least fusible one, which ^Isp 

 contams the smallest quantity of copper, cupriferous seleniur^i of 

 lead; and the other, wliich has a larger quantity of gopper,j,i^^(i 

 is more fusible, seleniuret of lead and copper. ^ I . i ilijry' 



6. Seleniuret of Lead %cith Selemurel of Utercur^'i—jiie 

 analyses of this compound were more troublesome than those of 

 the other seleniurets, because the seleniuret of mercury is not 

 combined with the seleniuret of lead in any definite proportion. 

 Different portions of the same specimen are so unequally com- 

 posed that two pieces of the same mass gave very different 

 results. A seleniuret of lead which contains no seleniuret of 

 mercury, cannot be distinguished by its external appearance 

 from those seleniurets which contain either much or little of it. 

 They have the same colour;, are only found massive, and disse- 

 minated in bitter spar. Some specimens have a small grained, 

 saccharoidal fracture ; plhers are coarse grained, and afford 

 parts which have a pretty distinct triple cleavage, according to 

 the planes of the cube. I observed in several specimens of this 

 mineral that the seleniuret of lead most remote from the bitter 

 spar, contained the greatest quantity of seleniuret of mercury, 

 and that that in immediate contact with it was quite free from 

 it. When the mineral has a distinct cleavage, only the latter 

 presents lamellar parts : the first (that in contact with the bitter 

 spar) is always fine grained saccharoidal. It is easy to ascer- 

 tain whether the seleniuret of lead contains much seleniuret of 

 mercury, or not; for the pure seleniuret of lead does not fuse, 

 and gives no sublimate when heated in a small matrass ; but if 

 it contain seleniuret of mercury, the latter rises, and forms a 

 very crystalline sublimate, the quantity of which is proportionate 

 to that of the seleniuret of mercury in the mineral. If it be 

 large, the assay boils up strongly at first, whilst the seleniuret of 

 mercury sublimes, and only infusible seleniuret of lead is left. 

 A small portion qi seleriiate pf mercury is usually formed by the 

 action of the air in the matrass, which is rather more volatile 

 than the seleniuret. The latter may be wholly converted into 

 seleniate by heating the assay in an open tube. The fused sele- 

 niate of mercury forms yellowish drops which have some resem- 

 blance to the oxide of tellurium, whose presence I suspected in 

 these minerals before I had satisfied myself that they contain 

 mercury. The presence of mercury is detected by heating the 

 mineral in a sqiall matrass with a little dry carbonate of soda, 

 when the mercury sublimes. It is also sometimes obtained^ a# 

 v/ell as the seleniuret of mercury, by heating the mineral with-? 

 out the soda, but in the Jatter case, its production i^ OYying \o 

 the bitter spar which decomposes the assay. 



