Action of Water upon Glass. 263 



appear to be confined to mere surface. In collections of ancient 

 glass, specimens may be selected, exhibiting how extensively an 

 analogous action has been going on during the period they have 

 remained buried in the earth. These vitreous relics of antiquity are 

 often covered, to a considerable thickness, with opal pearly scales 

 of beautiful appearance, consisting almost wholly of silica, whose 

 alkali had been removed probably by the action of the water *. 



A fragment of transparent ancient glass was examined with 

 regard to its alkaline property, which it was found to enjoy in a 

 high degree, being sensibly alkaline (when in powder) to the 

 tongue, and its hot solution acting upon the cuticle. It appeared 

 to consist almost entirely of potash and silica ; not the smallest 

 trace of lead being discoverable in it ; several other coloured spe- 

 cimens of ancient glass, upon examination, were, in every case, 

 more highly alkaline than any modern glass containing lead, that 

 has hitherto been examined. 



The specific gravity of common flint-glass was taken by way 

 of comparison with the ancient fragments above mentioned, 

 the result of which is here given. Flint-glass, S.G., 3.208. 

 Ancient glass, 2.375. It may here be remarked, that the latter 

 acted powerfully upon the test-paper, by merely moistening it, 

 without reduction to powder. It cannot be surprising, there- 

 fore, that ancient glass, which may almost be called pure silicate 

 of potash, should be occasionally found in states of such rapid 

 decay, as the specimens in collections often exhibit. 



Another proof of the action of water, aided by other concomitant 

 circumstances, in producing decomposition upon glass, is an ac- 

 count given in Vol. I. p. 135 of this Journal, of some bottles of 

 wine found in a quantity of black mud at the bottom of an old 

 well, full of burned wood, supposed, upon good authority, to be 

 of anterior date to the fire of London. The siliceous earth, in 

 this instance, separated in films on the surface of the bottle, in 

 consequence of the abstraction of alkaline matter, probably 



* The opal is an hydrate of silica : may not its formation have taken place 

 by a similar agency acting upon natural combinations ? The removal of alkali 

 from siliceous compounds may Lave left opal thus constituted. 



