XVI. Account of the Native Hydrate of Magnesia^ discovered 

 hif Dr HiBBERT in Shetland. By David Brewster, 

 LL. D. F. R. S. LoND. & Sec. R. S. Edin. 



(Read January 8. 1821. J 



JL HE 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, traver- 

 sing serpentine in every direction, in veins 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 f . The con- 

 nection between the primitive form of minerals and their 

 iiumber 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 Rhomboidal or 

 the Pyramidal system of Mohs. 



In 



* See Bhuce's American Mineralogkal Journal, vol. i. p. 26.-30. 

 t See Phil. Trans. 1814. p. 213., and 1818, p. 211> 



