446 APPENDIX ON 



enes, afford examples. But there are a few cases 

 also, in which minerals differing in their crystalline 

 form, are similar in their chemical composition; as 

 appears to be the case with the common, and white, 

 iron pyrites. These anomalies will however, probably, 

 be reconciled by the future investigations of science. 



Dr. Brewster has, with that attachment which we 

 usually evince towards a favorite pursuit, given a 

 preference to the optical characters of minerals, as 

 the surest means of determining their species. See a 

 memoir by Dr. Brewster, in the Edinb. Phil. Journ. 

 vol. 7. p. 12. 



This memoir relates to a difference in the optical 

 characters of the Apophyllites from different locali- 

 ties, upon which Dr. Brewster proposes to erect a 

 particular variety into a new species under the name 

 of Tesselite. Berzelius, as it appears from a paper, 

 preceding that by Dr. Brewster. in the same volume 

 of the Journal, has, at Dr. Brewster's desire, ana- 

 lysed the Tesselite, and found it agreeing perfectly 

 in its chemical composition with the Apophyllites from 

 other places. Chemically, therefore, the Tesselite 

 does not appear a distinct species. 



A few days before Dr. Brewster's paper was pub- 

 lished, it happened that I had been measuring the 

 angles of the Apophyllites from most of the localities 

 in which they occur, all of which I found to agree 

 with each other more nearly than different minerals 

 of the same species frequently do. The Tesselite 

 is not therefore, crystallographically, a separate 

 species.* But when chemistry and crystallography 



* I have found several crystals of this substance corresponding in a 

 remarkable manner in their general form of flattened four-sided prisms, 

 terminated by four-sided pyramids with truncated summits, but with 

 their corresponding planes dissimilar. The planes which appear as the 

 summits of some of these prisms, being only the lateral planes of very 



