Hakdmax — 0)1 the Substitution of Zinc for Magnesium. 149 



difficult by the presence of the former metal, no less than four distinct 

 wet analyses from different portions of the mineral were made. In 

 all of these both nickel and zinc "were present, the former somewhat 

 abundantly. 



(18). Talc Roclc from Crohy Head, county Donegal. Prom Geolo- 

 gical Survey Collection. A white or cream-coloured rock. The 

 blowpipe examination proved in this the presence of zinc, together 

 with small traces of copper and lead. A proof experiment with 

 another portion of the mineral, boiled in strong hydrochloric acid, 

 showed the zinc to be present in appreciable quantity. 



(19). Iron Pyrites. The last mineral contains numerous small 

 crystals of iron pyrites. These, examined in the usual way, yielded 

 zinc. As I have already remarked, ferrous iron belongs to the mag- 

 nesium group. 



(20). Actinolite Rock from Cannaver Island, Lough Corrib. Similar 

 to No. 13. Blowpipe analysis proved this to contain zinc in the same 

 quantity as in j!^o. 13. 



(21). &er][jentine from Liss-oughter, county Gal way. "With the 

 blowpipe a remarkably distinct indication of zinc. The mineral, re- 

 duced with carbonate of soda, yielded quite enough metal for identifi- 

 cation. Besides zinc, nickel is also present in some quantity, and there 

 are traces of silver and tin. 



"What I wish to urge upon your attention, as the result of these 

 investigations, is the almost invariable occurrence of zinc in the mine- 

 rals examined. I have already shown that the presence of zinc as an 

 accessory component of minerals has been almost entirely neglected — 

 in fact it is only mentioned where it occiirs in considerable quantity, 

 as in Pranklinite or Automolite ; and so uncommon is it looked on as 

 an accessory, that the only augite in which its presence had been re- 

 corded before I had commenced this research was dignified with a 

 special name — Jeffersonite.^ 



"When a metal not usually occurring in rocks in any large quantity 

 is recorded, it is usually because it exceptionally occurs so abundantly 

 that its presence cannot well be overlooked ; and it is only in such 

 cases that zinc has been hitherto observed. It appears, however, 

 that, like many other substances, it only requires to be sought after ; 

 and that its presence is not simply accidental, but the result of the 

 invariable chemical laws of affinity and isomorphism ; and I submit 

 that zinc is as much to be regarded as an almost constant associate of 

 the magnesium group as indium and osmium with platinum ; nio- 

 bium with tantalum ; rhuthenium and rhodium with palladium, and 

 so on. 



In all the instances I have noted in this and my former Paper, 

 the quantity of zinc is small ; but this again is really in favour of my 

 view. Had the metal occurred in large quantity in portions of the 



Dana, System of Mlncra'ogy (187-3, p. 215;. 



