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AiiT. XX . — Account of the Native Hydrate of Magnesia, dis- 
covered by Dr Hibbert in Shetland. By David Brewster, 
LL.D., F.R.S. Lond. & Sec. R. S. Edin.f 
The Native Hydrate of Magnesia was first discovered and 
ranked as a separate mineral by the late Dr Bruce of New 
York. It was found only at Hoboken in New Jersey, tra- 
versing serpentine in all directions, in veins of from a few lines 
to two inches in thickness. Its specific gravity was 2.13, and 
it yielded, upon analysis, 70 parts of pure magnesia, and 30 of 
water “[■. 
In the year 1813, I received some fragments of this rare mi- 
neral from our late eminent countryman Dr John Murray, and 
though it exhibited no traces of a crystalline structure, I found 
it to be a regularly crystallised mineral, with One axis of double 
refraction perpendicular to the laminae The connection be- 
tween the primitive form of minerals and their number of axes 
of double refraction, which I observed at a subsequent period, 
enabled me to determine that the native hydrate of magnesia 
belonged either to the Rhombolddl or the Pyramidal system 
of Mohs. 
In this state of our information respecting native magnesia. Dr 
Hibbert, (who has distinguished himself bv his excellent mine- 
ralogical survey of Shetland, and augmented our national re- 
sources by the discovery of chromate of iron in large quantities), 
put into my hands a mineral from Shetland, which had been 
considered by mineralogists SL^White Talc, but which he was per- 
suaded differed materially in the nature of its ingredients from 
that substance. In consequence of being familiar with the Ho- 
boken magnesia, I considered the Shetland specimen as the 
same mineral, and I put this opinion beyond a doubt, by esta- 
blishing the identity of their optical properties, and also by a 
chemical examination of the two substances. 
Mineralogtcal Character . — The structure of native hydrate 
of magnesia is distinctly lamellar. The laminae sometimes di- 
* From the Transactions of the Royal Society of Edinburgh^ vol. ix, p, 239, 
*f- See Bruce’s American Mineralogical Journal^ vol, i. p, 26.— 30* * 
X See FUL Trans. 1814, p. 213, and 1818, p. 211. 
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