DR T. THOMSON'S Analysis of some Minerals. 459 



Glasgow. The following comparison between the Castlehill mi- 

 neral and prehnite will leave no doubt about its nature. 



It is composed of fibres slightly diverging, so is fibrous preh- 

 nite. 



Translucent, so is prehnite. 



Lustre vitreous, so is that of prehnite. 



The hardness is the same as that of prehnite. 



Specific gravity 2.900. I found that of a fine specimen of 

 prehnite from Kilpatrick 2.901. 



Before the blowpipe it behaves exactly as prehnite. 



Captain LEHUNT, at my request, analyzed the Castlehill mi- 

 neral. The following table contains the result of his analysis. 

 I have placed, in a second column, the analysis of a pure speci- 

 men of prehnite from the hills behind Port-Glasgow, which I 

 made some years ago, to show the identity of the two minerals. 



Castlehill Kilmacolm 



Prehnite. Prehnite. 



Silica, . .,:..... . 43.084 42.22 



Alumina, . . '. . . . 23.840 23.68 



Lime, . -<.''''. . . ." ; V : 26.164 23.52 



Protoxide of iron, . . . 0.640 3.06 



Protoxide of manganese, . 0.416 



Potash and soda, . . . 1.028 



Water, 4.600 5.58 



99.772 98.06 



Potash and soda had been already found in prehnite by 

 LAUGIER. They probably existed in the specimen analyzed by 

 me. The loss being only 2 per cent. I did not suspect the pre- 

 sence of an alkali, and, therefore, did not search for it. In the 

 specimens of prehnite analyzed by GEHLEN, the lime amounted 



VOL. XI. PART II. 3 M 



