146 Proceedings of the Royal Irish Academy. 



JXXIII. — Ptjethee Eeseaeches on the stjpposed Substitftion of Zikc 

 roK Magj^esitm in Mineeaxs.^ By Ed-waed T. Haedhan, F. C.S., 

 &c., Geological Survey of Ireland. 



[Eead, February 25, 1878.] 



SoiTE time ago, during the analysis of chalk from the county Tyrone, 

 I noticed that the specimens examined contained traces of zinc, and I 

 also found that metal in small quantity in the overlying hasalts. It 

 subsequently occurred to me that, owing to its marked relations in 

 physical and chemical characters to magnesium, zinc might he ex- 

 pected to occur in rocks or minerals containing compounds of the latter 

 metal. Accordingly, I made some analyses of such magnesium mine- 

 rals as I had at hand, and the results, which fully equalled my expec- 

 tations, were laid before this Academy, and printed in the Proceedings 

 for 1874. Those analyses comprised some eight or ten rocks or mi- 

 nerals characterised by magnesian compounds. Since then, in the 

 intervals of other chemical research, I have continued this investi- 

 gation, and in nearly every instance I have obtained small quantities 

 of zinc combined in these magnesian rocks or minerals, and also in a 

 few cases in minerals of the metals belonging to the same isomorphous 

 groups as zinc and magnesium — for instance, in iron pyrites, and 

 in limestone, in which there was little or no magnesia. I give below 

 a list of twenty different specimens from various places, in all of 

 which zinc is unmistakeably present, and often in very appreciable 

 quantity. 



Method of Analysis. — In all but one or two cases the analysis was 

 twofold. First, an examination with the blowpipe was made, and 

 then if zinc was indicated, a complete analysis in the wet way. In 

 many instances the blowpipe results were so strongly marked, and 

 so unmistakeably showed the presence of zinc, that a wet analysis was 

 really superfluous. But to put the matter beyond all question, it was 

 j)erformed on a sufficiently large quantity of the rock or mineral. 



Por details as to the analytical methods adopted, I shall only 

 refer to my foimer Papers on this subject, where they are given fully. 

 There is one point worth mentioning, however, in this conjiexion. 

 It appears to be usually the impression that the only reliable blow- 

 pipe tests for zinc are the white incrustation, and the green colour 

 imparted by nitrate of cobalt ; and that it is too volatile to be reduced 

 to the metallic state on charcoal. Such appears to be the idea on 

 which are based the directions for its detection, in many books on Chemi- 

 cal Analysis, or special works on the Blowpipe, but it is an erroneous 



' See ' ' Analysis of Chalk, County Tyrone, -witli Note on the Occurrence of Zinf. 

 therein," Journal Roy fd Geological Society of Ireland, vol. iii., p. 159. Also Geological 

 2fogazi7ic, vol. x., p. 434 ; and " On a supposed Substitution of Zinc for Magnesium 

 in Minerals," Proceedings Eo'yal Irish Academy, vol. i. Ser. 2 (Science), p. 534. 



