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1. Letter from Sir D. Brewster relative to the specimens of Topaz with Pressure Cavities presented by him to the Museum of the Society


Dear Professor Balfour,—In vol. xvi. of the “Transactions of the Royal Society of Edinburgh,” I have described and given drawings of the pressure cavities which I discovered in topaz; and in vol. xxiii., just published, I have pointed out the geological relations of these cavities.

As the specimens of topaz containing them are so rare that I have found only five out of many hundreds which I have examined;— as the existence of such cavities with a polarising structure around them, proving that the topaz was in a soft or plastic state, will hardly be admitted by those who believe that the topaz was formed by aqueous deposition; —and as it is quite possible that other specimens containing such very minute cavities may never be found, even when diligently searched for, I think it right to present to the Society for preservation the five topazes in which the cavities were found.—I am, ever most truly yours.



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