DISCOVERED BY Dll HIBBERT IN SHETLAND. 241 



only a polished line, and leaves none of its own substance. Its 

 hardness seems to be 1.5. 



In several specimens I have observed a distinct crystalline 

 structure, in the form of the regular hexahedral prism. The 

 pyramidal form being therefore excluded, it will belong to the 

 Rhomboidal system of Mohs. 



Its specific gravity is 2.336. It adheres very slightly to the 

 tongue ; and it will constitute a new Genus of the 5th Order, 

 or that of Mica in the 2d Class of Mohs' system, unless the 

 order of Talc-Mica is modified to receive it. 



Locality. — Dr Hibbert found this substance, in 1817, at 

 Swinaness in Unst, one of the Shetland Isles, traversing serpen- 

 tine in all directions, being mixed with the Magnesian Car- 

 bonate of Lime, and forming veins from half an inch to six 

 or eight inches broad. 



Chemical Character. — Hydrate of Magnesia dissolves entire- 

 ly in muriatic, nitric and dilute sulphuric acids, and I obtain- 

 ed from its solutions in the muriatic and sulphuric acids, the 

 deliquescent salt of Muriate of Magnesia, and regular crystals 

 of Sulphate of Magnesia. On some occasions a very slight ef- 

 fervescence takes place ; but this no doubt arises from adhering 

 particles of carbonate of lime, or from a small quantity of car- 

 bonic acid, which may have been absorbed by exposure to the 

 atmosphere. 



Xhe following analysis of this mineral has been made by D 

 Fyfe, since the preceding account of it was drawn up : 



Magnesia, - 69.75 



Water, - - 30.25 



100.00 



