XXIII. Account of a Remarkable Structure in Apophyllite, with 

 Observations on the Optical Peculiarities of that Mineral. 

 By David Brewster, LL. D. F. R. S. Lond. & Sec. R. S. 

 Edin. 



(Read 2\st April 1817, 5th March and 11th December 1821. 



JL HE variety of Apophyllite to which I have given the name 

 of Tesselite *, from the beautiful combination of tesselae which 

 it exhibits in polarised light, is one of the most singular sub- 

 stances in the mineral kingdom. The agency of that species 

 of light, indeed, seemed necessary to the developement of its 

 peculiar structure ; but I have since succeeded in confirming 

 and extending my former results, by the application of the mi- 

 croscope ; and have been led to observe several new pheno- 

 mena, which not only throw much light on the structure and 

 properties of this curious mineral, but which affect the funda- 

 mental principles of crystallography. 



The tesselated Apophyllite from Faroe crystallises in right 

 rectangular prisms, with flat summits, which generally exhibit 



at 



* My first experiments on Apophyllite are published in the Edinburgh Philo- 

 sophical Journal, vol. i p. 1. 



