ON ITS EXTEKIOR. VOLCANOES. 223 



height of many thousand feet. The electric tension is in- 

 creased by the suddenness of the condensation, and also, as 

 Gay-Lussac has shewn, by the formation of such an enor- 

 mous surface of cloud. Forked lightnings dart from the 

 column of ashes, and (as at the close of the eruption of 

 Vesuvius, near the end of the month of October 1822) the 

 rolling thunder of the volcanic storm is heard, and clearly 

 distinguished from the sounds which issue from the interior 

 of the volcano. We learn from Olafsen's relation, that in 

 Iceland (at the volcano of Katlagia, 17th October, 1755), 

 eleven horses and two men were killed by lightning from the 

 cloud of volcanic steam. 



Having thus pourtrayed, as part of the general view of 

 nature, the structure and dynamic activity of volcanoes, we 

 have next to glance briefly at the diversity of their material 

 products. The subterranean forces dissolve old combinations, 

 and form new ones ; but they operate also by displacing 

 the otherwise unchanged substances whilst in a state of 

 liquefaction by heat. The greater or less pressure under 

 which the solidification either of liquid or of merely softened 

 substances takes place, appears to be the principal cause of 

 the difference between " plutonic" and " volcanic" rocks. 

 The molten rock which has issued in a distinct current 

 from a volcanic opening is called lava; and where such 

 currents meet, and are impeded in their course by opposing 

 obstacles, they spread out in breadth, and cover large areas, 

 in which they solidify in superposed strata. These few 

 sentences contain all that can be affirmed generally respect- 

 ing the products of volcanic activity. 



Fragments of the rocks which have been broken 



through by volcanic disturbance are sometimes inclosed in 

 VOL. i. R 



