230 



are less rich in oxid of iron, lose their color and 

 tumefy. What is this substance, the disengaging 

 of which reduces the obsidian to the state of 

 white pumice, sometimes fibrous, and at other 

 times spongy, with oblong cells ? It is easy to 

 perceive, that it easily loses a coloring principle; 

 and that the deprivation of color is not merely 

 apparent, that is to say, it is not owing to the 

 extreme tenuity to which the laminae and fibres 

 of the volcanic glass are reduced. Can we ad- 

 mit, that this coloring principle * is a hydruret 

 of car bon, analogous to that which perhaps exists 

 in the flint so easy to whiten by fire ? Some ex- 

 periments, which I made at Berlin in 1806 jointly 

 with Messrs. Rose and Karsten, on the obsidians 

 of TenerifFe, Quito, Mexico, and Hungary, in 

 porcelain retorts, did not yield any results that 

 were satisfactory. 



Nature probably employs very different means 

 to produce the spongy and vitreous pumices of 

 Teneriffe, the pumices with parallel fibres of the 

 /Eolian islands and of Llactacunga and the 

 capillary vitrifications of the Isles of Bourbon, 



* It is remarkable, that this principle is not always equally 

 volatile. Mr. Gay-Lussac saw lately obsidians of Faroe not 

 whiten at a degree of heat, whieh totally deprived of color 

 obsidians of Mexico, though from exterior appearance it would 

 have been difficult to distinguish these substances from one 

 another. 



f Between Quito and Riobamba. . 



