VOLCANOES. 709 



of liquid lava, as in the lakes of lava in Kilauea, is covered through- 

 out with jets, like a vat of boiling water; and there is only a muttering 

 noise from the action. It looks like ordinary ebullition, only the jets 

 are jets of fiery liquid rock. They rise vertically, and fall back into 

 the pool, or on its sides, before they have cooled. A lake one thousand 

 feet in diameter (at a, Fig. 1110) was thus in brilliant play over its 

 whole surface, when visited by the author in 1840; and in more act- 

 ive times a large part of the area at bottom has been in this boiling 

 state. 



If the lavas be less liquid, the vapors are kept from escaping, by the 

 resistance, until they have collected in far larger bubbles; and, when 

 such bubbles burst, the projectile force may be enormous : it carries 

 the fragments far aloft, to descend in a shower of cinders of great 

 extent. 



Such bubbles, rising and bursting, were seen by Spallanzani in the crater of Strom- 

 boli, a high cinder-cone in the Mediterranean, north of Sicily. In times of moderate 

 action, at Vesuvius, the outbursts of cinders occur every three to ten minutes; but, in 

 a period of eruption, they are almost incessant. According to Sir Wm. Hamilton, the 

 cinders rose to a height of 10,000 feet, at the eruption of 1779, — a height indicating a 

 vast projectile force. Occasionally, masses of lava are thrown up, which descend like 

 huge cannon-balls, having been rounded by the rotation before they had cooled, and 

 rendered compact externally, while usually cellular within. Such masses are called 

 volcanic bombs. They may have lenticular as well as spheroidal shapes. The centre is 

 in some an aggregation of chrysolite, in others of older pieces of lava, or other mineral 

 matter, that was not in a state of fusion. They are sometimes twelve or fifteen feet 

 in diameter, and when so are ver\ T slow in cooling. 



2. Movements of the Lavas in the Crater. — (a.) Upward Move- 

 ment. — As the vaporizable substances (water, sulphur, etc.) and at- 

 mospheric air expand, while rising in the volcanic vent, they displace 

 correspondingly the lava, and so cause a general expansion of the 

 mass. This alone is sufficient to produce a rise of the lava in a con- 

 duit. 



The water is mainly from the rains which fall over the volcanic re- 

 gion. They descend toward the vent, and, as they approach the lava 

 conduit, are prevented from being driven back by the pressure of the 

 waters above : thus they pass into the lavas and become the great 

 source of their activity. In a similar manner, the salt waters of the 

 ocean will find their way to the lavas, provided there are no fresh 

 waters pressing seaward to prevent it. 



When the boiling of a viscid fluid in a tube causes its upper surface to ascend, be- 

 cause the liquid at top becomes inflated or frothy with vapor, it exemplifies, as Provost 

 long since remarked, the same principle, although the degree of inflation very far ex- 

 ceeds that in a dense lava. The fact of a rising in the volcano from this cause is 

 beyond question. 



This rising becomes apparent in overflowings from the pools of the 



