334 
Mr Griffiths’s Experiments 
irregular composition, parting readily with its alkali by the ac- 
tion of water, it became a matter of some interest to determine 
how far certain natural combinations of potash with siliceous 
matter were equally active to the same tests, especially as in 
green-bottle glass, which contains little alkali, it is thus rendered 
evident. No analogous effect could, however, be produced by 
powders of felspar, basalt, greenstone, granite, obsidian, pumice, 
and some others, even when boiled with water, a method which 
never failed to produce it rapidly with glass, although cold water 
is perfectly sufficient. 
Some interesting conclusions may be drawn from the above 
experiments, which may tend to explain several well-known 
phenomena. 
In the first place, with regard to the glasses employed, in the 
laboratory, or for domestic uses, it must be evident that water 
has the power of acting upon and dissolving the alkali at the 
surface, and leaving an insoluble portion spread as a coating- 
over the interior of the vessel, defending it from further imme- 
diate action. 
Where, however, time can be allowed, the effect does not ap- 
pear 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 
specimens of ancient glass, upon examination, were, in every 
* The opal is a 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 have left opal thus constituted. 
