456 Sir David Brewster on the Structure 



stances had a similar locality, and had also carbon for their base, it became of 

 some importance to discover that their general polarizing- structure was the 

 same. The analogy, however, to which I wish to direct the attention of the 

 Society is founded on the existence of small portions of air within both sub- 

 stances, the expansive force of which has communicated a polarizing structure 

 to the parts in immediate contact with the air. This structure is displayed in 

 four sectors of polarized light encircling the globule of air, and can be pro- 

 duced 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 

 sljo-htest trace of it in more than 200 mineral substances which I have exa- 

 mined, nor in any of the artificial salts from aqueous solutions. It can, there- 

 fore, arise only from the expansive force exerted by the included air in the 

 diamond and the amber, when they were in such a soft state as to be suscep- 

 tible of compression from so small a force. That this compressible state of 

 the diamond could not arise from the action of heat is manifest from the na- 

 ture and 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 rendered probable by other analogies, that the 

 diamond originates, like amber, from the consolidation of, perhaps, vegetable 

 matter, which gradually acquires a crystalline form by the influence of time, 

 and the slow action of corpuscular forces. 



'' As the preceding results were obtained from flat diamonds, which did not 

 seem to have been regularly crystallized, I was anxious to detect the same 

 structure in those which had a regular crystalline form. With this view I 

 examined several of the diamonds in Mr. Allan's collection, and was fortunate 

 enough not only to detect in a perfect octohedral crystal the same structure 

 which I had obse-rved in the flat specimens, but also an air-bubble of consi- 

 derable size, which had produced by its expansion the polarizing structure 

 already described." 



Since these observations were written, Dr. Voysey has shown that the 

 matrix of the diamonds produced in Southern India is the sandstone breccia 

 of the clayslate formation ; and Captain Franklin has found that in Bundel 

 Kund the rocky matrix of the diamond is situated in sandstone which he 

 imagines to be the same as the new red sandstone of England, that there is 

 at least 400 feet of that rock below the lowest diamond beds, and that there 

 are strong indications of coal underlying the whole mass. The following are 

 Captain Franklin's observations on the origin of this mineral: 



