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XIX. — On the Structure and Optical Phenomena of Ancient Decomposed Glass. 

 . By Sir David Brewsteb, K.H., D.C.L., F.R.S., &c. (Plates X., XI.) 



(Read 5th January 1863.) 



The disintegration of solid bodies by means of active or feeble solvents, or by 

 those invisible processes which go on during long periods of time, has, so far as I 

 know, been studied neither by the chemist nor the natural philosopher. In 1837 

 I submitted to this Society a paper " On the Optical Figures produced by the 

 Disintegrated Surfaces of Crystals,"* containing experiments which I believe have 

 not been repeated, and results which no person has attempted to explain.-j- 

 Since that paper was published, my attention was called to the structure and 

 properties of decomposed glass, in consequence of having had occasion to study 

 the action of its coloured films, in absorbing definite parts of the spectrum. The 

 specimens, however, which I received for this purpose from the late Marquis 

 of Northampton, Mrs Buckland, and Mr Children, and referred to in my 

 " Experiments on the Connection between the Phenomena of the Absorption of 

 Light and the Colours of Thin Plates," | did not enable me to investigate the con- 

 dition of the elementary films, and the process by which the various states of the 

 glass were produced ; but having received, while in Rome in 1857, very fine speci- 

 mens from the museum of the Marquis of Campana, and more recently other speci- 

 mens from Nineveh, from Mr Layard, I have been able to obtain the results con- 

 tained in the following paper, and represented in the drawings which accompany it. 



The decomposition of glass in its early stages is finely seen in the brilliant 

 colours which cover its surface. These colours are those of thin plates, and tints 

 of the third and fourth orders in Newton's scale are produced in the course of 

 thirty or forty years on the inner surfaces of panes of glass in stable windows. 

 The elementary films which display these colours are not in optical contact with 

 the glass beneath them, though they adhere to it very firmly. They may be 

 removed by the point of a knife ; and in this state the powder is white by reflected 

 light, and semitransparent when surrounded with water. 



* Edin. Trans., vol. xiv. p. 164. 



f Since this paper was written, I have received from Professor Von Kobell, of the University 

 of Munich, a very interesting paper, entitled " Ueber Asterismus und die Brevusterschen Licht- 

 figuren," in which he gives an account of my experiments, and adds many new and important ones 

 of his own, illustrated with three plates. It is published in the Sitzungsberichte der Koniglichen 

 Baierischen Akademie der Wissenschaften: Sitzung der Math.-Phys. classe, 8 Feb. 1862. A very 

 full and excellent abstract of this Paper appeared in the Parthenon of 6th Sept. 1862. 



I Phil. Trans., 1837, p. 245. 



VOL. XXIII. PART II. 3 H 



