ON ITS EXTEUIOR. VOLCANOES. 225 



veiling companion, Gustav Bose, and subsequently Her- 

 mann Abich, have commenced, with much ingenuity 

 and success, the investigation of the structure of volcanic 

 rocks. 



The greater part of the vapour which rises from volca- 

 noes is pure aqueous vapour, which condenses and forms 

 springs, as the spring in the Island of Pantellaria, to which 

 the goat-herds resort for a supply of water. The current 

 which, on the morning of the 26th of October, 1822, 

 was seen to pour from the crater of Vesuvius through 

 a lateral opening, and was long supposed to have been boil- 

 ing water, appears from the careful examination of Monti- 

 celli, to have consisted of dry ashes, or of lava pulverised by 

 friction. The phenomenon of volcanic ashes, which darken 

 the air for hours and even for days, and in their fall cause 

 great damage to vineyards and olive trees by adhering 

 to the leaves, mark by their columnar ascent, upborne 

 by vapours, the termination of every great eruption. This 

 was the magnificent phaenomenon which, in the case of Ve- 

 suvius, the younger Pliny, in his celebrated letter to Corne- 

 lius Tacitus, compared to a lofty pine spreading out at its 

 summit into wide shadowing branches. The appearance of 

 flame, which has been described as accompanying the erup- 

 tions of scoriae, and the red glow of the clouds which hover 

 over the crater, are not certainly true flames, or to be attri- 

 buted to the combustion of hydrogen ; they are rather due 

 to reflections from the incandescent substances projected 

 high in air, and also to the ascending vapours illumi- 

 nated by the fiery sea within the crater itself. In regard to 

 the flames seen occasionally, as in the time of Strabo, to 

 issue from the deep sea during the activity of coast volca- 



