310 The Diamond. — Double Refraction. 



THK DIAMONr. 



Dr. Brewster, while exaiiiiiung the optical structure of amber, 

 was led to compare it with the diamond. Some singular ana- 

 logies were fomul in the two substances, and one diamond exa- 

 mined presented a new and unexpected phaenomenon, which pro- 

 mises to throw light on its origin and formation. The phaeno- 

 menon occurs also in amber. It is " tlie existence of small por- 

 tions of air witliin l)oth substances, the expansive force of which 

 lias communicated a polarizing structure to the parts in imme- 

 diate contact witli the air. This structure is displayed by four 

 sectors of polarized light encircling the globule of air, and can 

 be produced artificially, either in glass or in gelatinous masses, 

 by a compressing force propagated circularly from a point. It 

 is obvious that such an effect, cannot arise from any mode of 

 crystallization ; and if any proof of this were necessary, it might 

 be sufficient to state, that I have never observed the slightest 

 trace of it in more than 200 mineral substances which I have 

 examined, nor in any of the artificial salts foruied from aqueous 

 solutions. It can therefore arise only from the expansive force 

 exerted by the included air on the diumond and the amber, when 

 they were in suck a soft state as to be susceptible of compression 

 Jrom so small a force. That this compressible state of the dia- 

 mond could not arise from the action of heat, is manifest from the 

 nature and the recent formation of the soil in which it is found ; 

 that it could not exist in a mass formed by aqueous deposition, 

 is still more obvious ; and hence we are led to the conclusion, ren- 

 dered probable by other analogies, that the diamond originates, 

 like amber, from the consolidation of, perhaps, vegetable matter, 

 which gradually acquires the crystalline form by the influence of 

 time, and the slow action of corpuscular forces." This polar- 

 izing structure was found in flat diamonds regularly crystallized, 

 and also in one of a perfectly octoedral form. — Edin. Phil. Jour- 

 nal. 



DOUBLE REFRACTION. 



M. Soret has, \h the Journal de Physique (xc. p. 353), 

 given two simple methods of ascertaining the double refraction 

 of mineral substances. The apparatus for the first method is sim- 

 ply two phites of tourmaline, cut parallel to the axes of the crys- 

 tal, and placed crossways, so as to absorb all the light. The sub- 

 stance to be examined is to be placed between these plates : if 

 it be doubly refractive, the light re-appears through the tourma- 

 lines; if not, all remains dark. The second method consists in 

 placing the mineral to be examined over a hole in a card, and 

 examining the light transmitted through it by an achromatic 

 prism of Iceland spar. If the two images produced are coloured 

 differently, it indicates double refraction. 



BRITISH 



