150 ' Analyses of Booh. [Ato. 



lawB of decrement, but Mr. Brooke, at the recommendation of 

 Mr. Levy, has substituted spherical trigonometry for it in this 

 section. 



To this outHne succeeds a section on the direct determination 

 of the laws of decrement from the parallelism of the secondary 

 edges of crystals, according to the methods pointed out by 

 Haliy, Monteiro, and Levy. A section follows, on the Methods 

 of Drawing the Figures of Crystals, some of the examples in 

 which are particularly elegant ; and a short essay on Minevalo- 

 gical Arrangement, with an Alphahetical Arrangement of Mine- 

 rals, their Synonymes and Primary Forms, terminate the volume. 



We intended to discuss in this place certain arguments employed 

 by Mr. Brooke, respecting the difficulties of mineralogical arrange- 

 ment, which we conceive to be somewhat fallacious, as well as to 

 examine in what respect his ahecedarium of mineralogy is really 

 preferable to such arrangements as have a more natural character. 

 We also intended to ofler a few remarks on certain subjects of 

 mineralogical chemistry adverted to in the list of minerals ; but 

 as we have now neither space nor time for the necessary exten- 

 sion of this article, we must leave all these subjects to the discern- 

 ment of Mr. Brooke's readers ; at the same tmie strongly recom- 

 mending the work in general to their careful study, as the only 

 comprehensive treatise on Crystallography which has yet ap- 

 peared, in this country at least. We will conclude with the im- 

 portant statement given in the section on Arrangement, respect- 

 ing Dr. Brewster's preference of the optical characters of minerals, 

 as the surest means of determining their species. 



" Dr. Brewster has, with that attachment which we usually 

 evince towards a favourite pursuit, given a preference to the 

 optical characters of minerals, as the surest means of determin- 

 ing their species. See a memoir by Dr. Brewster in the Edinb. 

 Phil. Journ. vol. vii. p. 12. 



" This memoir relates to a difference in the optical characters 

 of the Apophyllites from different localities, upon which Dr. 

 Brewster proposes to erect a particular variety into a new species 

 under the name of Tesselite. Berzehus, as it appears from a 

 paper, preceding that by Dr. Brewster, in the same volume of 

 the Journal, has, at Dr. Brewster's desire, analysed 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 published, it 

 happened that I had been measuring the angles of the Apophyl- 

 lites 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 



• *' 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 

 pyrauiiUs with truncfttoU sunumts, hut with their QQrnifj)onding ^lams dimmtlau The 



