1 70 Sir D. Brewster's Obscrvaiions on Chabasie. 



and turpentine produce effects represented by opposite signs, 

 the rotation produced by oil of lemons being +, that of oil of 

 turpentine ~ and for a given thickness of oil only half that 

 of oil of lemons. The effect produced by one thickness of oil 

 of lemons is almost exactly neutralized by two thicknesses or 

 twice the weight of oil of turpentine : and this whether the two 

 liquids be in separate tubes, or he previously mixed and presented 

 to the ray of light iii one and the same tube. This is precisely 

 analogous to what, in the above paper, we suppose to take 

 place between the molecules of quartz and those of chabasie. 

 Portobello, July 22, 1836. 



XXXVI. Observations relative to the preceding Paper. By 



Sir David Brewster. 

 I N examining the optical properties of the different chabasies, 

 I found that the variety from the Giant's Causeway differed 

 so essentially from the ordinary kinds as to entitle it to the 

 distinction of a new mineral. Its double refraction was consi- 

 derably greater^ and its ordinary refraction considerably less 

 than that of the common kind. I have since had occasion to 

 examine a most interesting variety of chabasie brought from 

 Faroe, and presented to me by W. C. Trevelyan, Esq. It 

 consists of minute and perfect rhombs sticking loosely to- 

 gether, as it were in stalactites, and these minute crystals are 

 perfectly transparent and much better crystallized than any 

 other specimens which I have seen. Each rhomb, however, 

 was a composite crystal, and the faces of composition co- 

 incided with the diagonals of its rhomboidal faces. 



The remarkable property which Mr. Johnston has referred 

 to in the preceding ingenious paper, does not exist in these 

 minute crystals from Faroe, and I did not observe it in the 

 specimens from the Giant's Causeway. If it exists, therefore, 

 only in the common chabasie, it will not be difficult to put 

 Mr. Johnston's hypothesis to the test of direct experiment, 

 because this chabasie must, if the hypothesis be true, contain 

 a greater quantity of silex than the other varieties; in order, 

 however, to establish the hypothesis it must also be proved 

 that the outer layers of the rhomb contain more silex than the 

 inner rhomb or nucleus; and if the additional quantity of 

 silex is very small, it may exist as an extraneous ingredient, 

 diminishing its double refraction, not by an opposite double 

 refraction of its own, but merely by separating the particles of 

 chabasie, and diminishing the force of aggregation on which the 

 double refraction of the mineral has been supposed to depend*. 



Edinburgh, July 23, 1836. 



* See Phil. Trans. 1830, p. 93. [An abstract of the paper here referred 

 towill be found in Phil. Mag. and Annals, N.S. vol. vii. p. 356.— Edit-] 



