202 E. T. Hardman — Substitution of Zinc for Magnesium. 



and exposed before tJie blowpipe on cbarcoal. The support became 

 covered with the usual zinc oxide incrustation, which, treated with 

 nitrate of cobalt, gave a vivid green. The fused bead and support were 

 levigated with water in an agate mortar, when numerous spangles 

 of white metal were seen, which, when treated with a drop of water 

 very slightly acidulated with hydrochloric acid, dissolved with 

 evolution of hydrogen. As the solution (a) could only contain zinc, 

 with a trace of ferrous iron, and of lime, this experiment was con- 

 clusive. 



The above results were obtained with separate portions of the 

 rock, treated in the following different ways : — 



(1). The powdered rock was fused in a platinum crucible with 



carbonate of soda. 

 (2). The powdered rock was boiled in strong hydrochloric acid. 

 (3). The powdered rock was boiled in nitro-hydrochloric acid. 



It is somewhat remarkable that only a trace of titanium was 

 observed in this specimen. 



In my former note on this subject, I endeavoured to account for 

 the presence of zinc by the supposition that it existed as a distinct 

 mineral, and pointed out the possibility of its being introduced by 

 the infiltration of water ; but subsequent examination and considera- 

 tion has led me to what appears to be the true clue. Zinc is very 

 closely allied to magnesium in characteristics and behaviour, both 

 in the metallic state and in combination with other elements. In 

 many points the resemblance is very strong ; ^ their salts have a 

 similar composition, and these, as well as the natural compounds, 

 or minerals, are isomorphous.^ Consequently, following the law of 

 isomorphism, they should be mutually replaceable ; and wherever 

 we find the one in any quantity, we should expect to find the other 

 encroaching on it. It is curious that while in most mineralogical 

 treatises the interchangeability of the protoxides of iron, manganese, 

 calcium, and magnesium is laid down, that of zinc with these, so 

 far as I am aware, has not been yet shown,^ and in the very few 

 instances in which zinc is given as an accessory metal, its presence 

 seems to be considered rather as an accidental circumstance, than 

 as the result of any chemical law. In few cases has it been given 

 in the composition of a mineral, unless when present in such 

 quantity as to make a very serious item in the analyses, as in those 

 of Automolite, or zinc spinel (20 to 35 per cent, of Zn 0), and 

 Franklinite (17 per cent. Zn 0) ; while in most minerals traces of 

 Mn, Fe, Mg, are constantly recorded as replacing part of the 

 essential metals. 



Believing, therefore, that zinc compounds might thus be calculated 

 to occur in most magnesian rocks or minerals, I have tried several, 

 and so far the result has almost exceeded my expectations, for in 

 every single case the metal was proved. As yet I have merely 



1 Fownes' Manual of Chemistry, 10th ed. p. 393; also Galloway's Qualitative 

 Analysis, p. 49. 



- hupra cit.; also Dana's Manual of Mineralogy, p. 74. 

 ^ Although in one or two cases implied. 



