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XXXV. On the probable Cause of cei-tain Optical Properties 

 observed by Sir David Brewster in Crystals of Chabasie. By 

 James F.'W. Johnston, A.M., F.R.S.E., F.G.S., 4-c., Pro- 

 fessor of Chemistry and Miiieralogy in the University of Dur- 

 ham.* 

 AT the Meeting of the British Association in Edinburgh, Sir 

 David Brewster brought under the consideration of the 

 Chemical section, some very interesting observations on the 

 variations which the doubly refracting power is seen to un- 

 dergo in different portions of the same ci'ystal of certain va- 

 rieties of chabasie. An abstract of this paper has since ap- 

 peared in the Fourth Report of the Association, p. 575 ; but the 

 leading fact is more precisely stated in the account of the Meet- 

 ing published in the Literary Gazette (for 1834,) No. 952, p. 690. 

 The double refraction in these crystals, which is positive in the 

 rhomboidal nucleus or centre of the crystal, was seen " to di- 

 minish in succeeding layers from a positive state till it disap- 

 peared altogether ; beyond this neutral line it became nega- 

 tive, and again gradually increased f." This observation was 

 brought forward partly with the view of illustrating, as it does 

 very beautifully, the importance of the optical character of mi- 

 nerals in throwing light upon the structure, composition, and 

 mode of formation of such of them as occur in a crystalline 

 state ; but partly also to show that chemical analysis is liable 

 to lead to error by treating as simple minerals what are in 

 reality only aggregates of different substances deposited in suc- 

 cessive layers around a common nucleus. 



That the layers which exhibit the difference of optical pro- 

 perties mentioned by Sir David, have a different composition, 

 is highly probable. A series of optical changes is produced 

 on some substances, as on glauberite and topaz, by the eleva- 

 tion of temperature ; or, as on unannealed glass, by sudden 

 cooling : it is possible, therefore, that the deposition of succes- 

 sive layers of the same chemical constitution under a pressure 

 or temperature constantly varying, might produce also phse- 

 nomena analogous to those observed in the present case. 

 There is not much apparent probability, however, that any 

 such variations actually took place in the circumstances under 

 which the crystals of chabasie were formed. That change of 

 atomic arrangement which gives rise to the interesting phae- 

 nomenon of dimorphism must necessarily, we may suppose, 

 give to the two forms very different optical properties : but 



* Communicated by the Author, 



■f- These phfenomeiia are also detailed, and an explanation attempted, in 

 a [)aper by Sir David Brewster on another subject, inserted in the Philoso- 

 phical Transactions for ls;50, part i. pp. 93, 94. 



