460 DR T. THOMSON'S Analysis of some Minerals. 



to 26 per cent., as well as in the Castlehill specimen analyzed by 

 Captain LEHUNT. 



If we admit a small excess of silica, and consider the oxides 

 of iron, manganese, and the potash and soda, as accidental ingre- 

 dients, the composition of prehnite will be 



10 atoms silicate of alumina, 

 7 atoms silicate of lime. 



Reckoning from the Castlehill variety, all the other Scottish 

 prehnites, so far as I have analyzed them, contain rather less 

 lime. The surplus of silica amounting to about one-seventh of 

 the whole, is probably, in the mineral, united to the oxides of 

 iron and manganese, and to the potash and soda. How far these 

 may be essential ingredients remains still to be discovered *. 



* The name Wollastonite was given by HAUY to table-spar, or bisilicate of lime ; 

 but mineralogists in general have refused to adopt this appellation. I have been in- 

 duced, therefore, in order to commemorate the many obligations which mineralogy 

 owes to Dr WOLLASTON, to apply the term Wollastonite to a mineral which I believe 

 to be new, and which has a very close relation to the species which HAUY designa- 

 ted by that name. 



It occurs in veins in a greenstone which is situated near Kilsyth, on the banks 

 of the Forth and Clyde Canal, and possesses the following characters : 



Its colour is white, with a slight shade of green. Its texture is fibrous, and the 

 fibres are in tufts diverging from a centre, thus exhibiting marks of an imperfect 

 crystallization. The mineral is translucent on the edges, and has a lustre inclining 

 to silky. The fracture is splintery, and the fragments are sharp-edged. 



The hardness is intermediate between that of selenite and calcareous spar. Its 

 specific gravity is 2.8760. 



Before the blowpipe it melts with some difficulty into a white enamel. This fu- 

 sion is not accompanied by any froathing. With borax it fuses into a bead-yellow, 

 while hot, but becoming colourless on cooling. With biphosphate of soda in consi- 

 derable excess, it fuses into a colourless bead, leaving a silica skeleton. With car- 

 bonate of soda it effervesces, and fuses into an opaque bead, with a reddish-blue 

 colour. 



