PHENOMENA OF ERUPTION 37 



which glow at night, with the heat of the molten mass below, and 

 which is perforated by various openings. From some of these 

 steam is given out, from others molten lava wells up occasionally. 

 In openings of a third class the lava may be seen rising and sink- 

 ing, until a great bubble forms on its surface and bursts with a 

 loud roar, scattering the hardened lava scum about the crater, in 

 fragments of various sizes, some very fine, others coarse. The 

 bubble proves to be of steam, and when set free, the steam globule 

 rises to join the cloud which always overhangs the mountain. 

 The bursting of the steam bubble is followed by a rush of steam 

 through the mass of the lava, the pressure is relieved, and the lava 

 column sinks down out of sight, until the steam pressure again 

 accumulates and the performance is repeated. 



Evidently, one active agent in these phenomena is imprisoned 

 steam in its struggles to escape. Different as are the manifesta- 

 tions at other volcanoes, steam is an important cause of the erup- 

 tion in all cases, though the conditions under which it acts vary 

 widely. Little or no combustion is involved, and that not as a 

 cause, but as an effect of the activity. 



In Vesuvius essentially the same phenomena may be observed, 

 but on a far grander and more terrible scale. Earthquakes usually 

 announce the coming eruption, increasing in force until the out- 

 break occurs. Terrific explosions blow out fragments of all sizes, 

 from great blocks to the finest and most impalpable dust. The 

 finer fragments arise chiefly from the scattering of the partly hard- 

 ened lava by the force of the explosion, but in part also from the 

 crashing together of the blocks as they rise and fall through the 

 air. Inconceivable quantities of steam are given off with a loud 

 roar, which is awe-inspiring in its great and steady volume. The 

 condensation of such masses of vapour produces torrents of 

 rain, which, mingling with the " ashes " and dust, gives rise to 

 streams of hot mud that flow for long distances and are often 

 exceedingly destructive. Great floods of molten rock, or lava, 

 issue from the crater, or burst their way through the walls of the 

 cone, and pour down the mountain side, until they gradually 

 stiffen by cooling. 



