DOMAIN XII. VOLCANIC. 



duces round it when it falls perpendicularly into 

 a standing water. 



Transparency. I cannot omit another remark. M. Faujas 

 says, that the edges of this glass where they are 

 very thin, if presented to a strong light, are 

 a little transparent. The transparency of the 

 thinnest parts of the glass on which I made my 

 observations, when compared to that of common 

 factitious glass, is certainly not equal to it : it is 

 not, however, so much inferior as this naturalist 

 seems to suppose. A scale three lines and a 

 half in thickness being presented to the flame of 

 a candle, afforded, in part, a passage to the light; 

 and another, two lines thick, being interposed 

 between the eye and external objects, permitted 

 a confused sight of them. Another, half a line 

 in thickness, being laid on a book, it might be 

 read with the greatest distinctness. I have en- 

 tered into these minute details the better to show 

 the perfect quality of this glass. 



Colour. " The opacity of this glass in the mass pro- 

 ceeds from a very subtile, and perhaps bitumi- 

 nous substance, incorporated with the vitreous 

 matter, and rendering it dark like a cloud. The 

 glass loses this substance if it be left for some 

 hours re-melted in the crucible, and it then be- 

 comes white." 

 " Bergman observed that the Islandic glass, 



