168 Prof. Johnston oji the Cause of 



crystal ; the form of the crystal, its transparency, apparent 

 homogeneity, &c. will be unaffected; its optical properties 

 alone will present a change, and one exactly such as we are 

 considering*. 



But if silica be isomorphous with pure chabasie, and be the 

 cause of the phaenomena in question, the quantity found in the 

 mineral should be in some degree variable. Granting the 

 positive nucleus of pure chabasie to have a fixed composition 

 associated with a constant form, analysis ought to show that 

 the quantity of silica present in some crystals is greater than 

 in others. Now chemists have recognised two varieties of 

 chabasie; one from Aussig in Bohemia, and from Fassa, 

 analysed by Hoffman, and from Faroe analysed by Arfvred- 

 son ; represented by the formula, 



NlS3 + 3AS^ + 6Aq, 



Kj 

 and another from Nova Scotia, analysed by Hoffman, from 

 Gustafsberg, by Berzeiius, and from Kilmacolm, by Mr. Con- 

 nell, represented by the formula, 



N^S^ + 3AS^- + 6Aq. 



kJ 



The only difference between these two formulae is that the 

 latter contains one half more silica in the first member than 

 the former does. This circumstance is entirely in accordance 

 with the view presented in this paper. If the first of the 

 two formulae be the correct one, the presence of the additional 

 silica in the second should produce a perceptible change in 

 the optical properties. It is impossible to say, however, that 

 even in that formula the quantity of silica indicated may not 

 be greater than the pure mineral may contain. The fact that 

 silica is isomorphous with chabasie renders any formula for 

 this substance exceedingly doubtful. Crystals from the same 

 locality may not always contain the same quantity of silica; it 

 is probably the paucity of analyses only that prevents us from 

 knowing of crystals from one and the same locality to which 

 each formula would apply. Sir David Brewster has not stated 

 from what locality his crystals were obtained f ; but the analysis 

 of the actual specimen, and not the locality, would be required 

 to enable us to say which formula would apply to it. Were they 



* In the paper already referred to, Phil. Trans. 1830, p. 94, Sir David 

 Brewster supposes the change to be due to the presence of a foreign body. 



t One of the specimens I have since learned is from the Giant's Cause- 

 way. 



