and the Occurrence of the Elements in Nature. 



199 



Fe occurs mostly as oxide, but also frequently as sulphide; 

 Co occurs mostly as sulphide and arsenide, but sometimes as 



oxide; 

 Ni occurs almost always as sulphide and arsenide, and only 



very rarely as oxide. 

 Iron, the first member of the triad, thus more nearly re- 

 sembles elements of the even series ; whereas nickel, the last 

 member, more nearly resembles elements of the odd series. 

 This shows therefore in a marked manner that in Group VIII. 

 we have a gradual passage from even series on the one hand 

 to odd series on the other. 



§ 12. The halogens CI, Br, I, and F, being the most electro- 

 negative of the elements, occur in nature in combination with 

 the most electropositive metals as chlorides, bromides, iodides, 

 and fluorides, &c, and are never (if we omit a few metallic 

 oxychlorides, and sulphochlorides) found in combination with 

 either oxygen or sulphur. 



§ 13. The only other point necessary to mention is that, as 

 regards the odd members of any one group, the tendency to 

 occur in nature as sulphides increases, whereas the tendency 

 to occur as oxides or double oxides diminishes, as the atomic 

 weight increases, thus : — 



fNa*, frequently occurs as double oxide (sili- 

 cates, Chili saltpetre &c.) and never as 

 sulphide. 

 Cu, occurs commonly both as sulphide (Cu 2 S, 

 CuS, Cu Fe S 2 , &c), and as oxide or double 

 oxide (Cu 2 0, CuO, malachite, &c). 

 Ag, generally as sulphide and never as oxide. 

 Au, only in free state. 



'Mg, always occurs as double oxide (MgC0 3 &c). 

 Zn, most commonly occurs as sulphide, but 



sometimes also as oxide. 

 Cd, only found as sulphide and never as oxide. 

 Hg, only found as sulphide or in metallic state, 

 and never as oxide. 



( Si, always occurs as oxide or double oxide 

 (silicates) . 

 /^ ttt J Sn, almost always occurs as oxide, but some- 



Qma P l times as sulphide. 



Pb, almost always occurs as sulphide, and only 

 rarely as oxide (minium). 

 And a similar thing for Groups III., V., and VI. 



* Omitting its occurrence as chloride, since it does not affect the 

 present question. 



Group I. < 



Group II. < 



