ABSTRACTS 



OP 



THE PAPERS PRINTED IN 

 THE PHILOSOPHICAL TRANSACTIONS. 



Additional Observations on the optical Propertiesand Structure of heated 

 Glass and unannealed glass Drops. By David Brewster, LL.D. 

 F.R.S. Edin. and F.S.A. Edin. In a Letter addressed to the Right 

 Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read November 10, 

 1814. [Phil Trans. 1815,^. 1.] 



J.N this continuation of the author's remarks on Rupert's drops, he 

 observes, that when they are made of colourless glass, and are held 

 before a sheet of white paper, even the unbroken drops exhibit some 

 appearances which he considers as marks of a crystalline texture ; 

 and when they are broken in such a manner that the parts are re- 

 tained in their relative situation, the structure shows a cleavage in 

 the direction of lines diverging from the apex, or rather in the form 

 of conical layers, having a common axis, with their apex towards the 

 blunt end of the drop. 



Since the surface of these drops is cooled suddenly by plunging 

 into water, and fixed before the central parts are contracted, the 

 author conceived that their specific gravity might be less than that 

 of annealed drops ; and indeed a difference was found upon trial, 

 amounting to Vrth part in the aggregate bulk of its external dimen- 

 sions ; but it appeared, on examination, that the contraction of the 

 glass had taken place from within outwards, so as to leave cavities in 

 the interior sufficient to account for the difference of weigltt, without 

 supposing any intrinsic difference of density in the glass itself. 



Dr. Brewster further observed, that by heating a drop to redness, 

 and allowing it to cool gradually, these cavities disappeared entirely, 

 so as to prove that they did not arise from included air, but merely 

 from the internal contraction of the glass. 



Since considerable difficulty occurs in preserving unannealed drops 

 of flint glass, the author has paid attention to the circumstances most 

 favourable to their preservation, and recommends removing them 



VOL. II. B 



