204 JE. T. Hardman — Substitution of Zinc for Magnesium. 



quantity, viz. copper and zinc. The zinc re-actions were very 

 pronounced. 



6. " Mountain Leather." — ^Variety of asbestus. From Portlock's 

 Collection, Geological Survey Museum, Dublin. Locality unknown ; 

 but associated with asbestus and other basaltic minerals from Antrim. 

 Zinc very apparent, all the re-actions being perfectly unmistakable. 

 A trace of lead (?) was also observed. 



7. Augite. — Very large crystals in a trappean ash. Locality un- 

 known. From Geological Survey Collection. About four or five 

 grains of the powdered mineral, fused with carbonate of soda on 

 charcoal, yielded a sufficiency of metal to identify with certainty. 

 Besides zinc, copper was present. All the zinc re-actions were most 

 distinct. 



All the above minerals and rocks were selected at random, on 

 account of containing, or being themselves, magnesian compounds, 

 and they are numbered in the order in which they were examined, 

 every one resulting in " a find." In many cases the examination by 

 the blowpipe and its immediately connected wet tests was found to 

 be amply sufficient to prove the presence of the metal ; especially as 

 from the comparatively large amount of the substance under inves- 

 tigation taken, — four or five grains,' — there was little possibility of 

 error. But where the result was at all doubtful, it was confirmed by 

 one or more analyses in the wet way. 



The amount of zinc appeared to be most plentiful where the 

 largest quantity of magnesium existed ; but as I am as yet only con- 

 cerned in proving the presence of the former metal in the above 

 minerals, I am not now in a position to positively assert this ; de- 

 ferring the quantitative analyses until I have continued the investi- 

 gation so far as to enable me to select the most typical examples for 

 estimation. In the mean time these notes on the subject may not 

 be without some interest, bearing as they do on a matter of much 

 importance from a mineralogical as well as a chemical standpoint. 



It may be thought that the zinc might as well be considered to 

 replace the other members of the isomorphous group, which are 

 known to occur in traces and occasionally in quantity in some of 

 these minerals. But this would be a replacement of a replacing 

 element; and I believe it is invariably considered that the accessory 

 metal substitutes itself for part of the essential one. On this ground 

 alone, all the minerals here referred to being strictly magnesian ones, 

 the zinc must be regarded as replacing magnesium. At the same 

 time I do not doubt that in other instances where there is no magne- 

 sium, it may be equally found to replace any of the other members. 

 But when we remember the affinities of the two metals, it will hardly 

 be considered a far-fetched notion to suppose, that were a preference 

 possible, the zinc would associate itself with the magnesium, in the 

 same way as some other metals act towards each other, notably the 

 Platinic group ; which possessing a very peculiar relation to one 



1 This was rendered possible by tbe use of paraffin oil in the blowpipe lamp, by 

 which a very large and hot flame was procured, capable of decomposing a much larger 

 quantity of rock. For reductions, and where great heat is required in blowpipe 

 analysis, it will be found most useful where gas is unattainable. 



