318 D. Forbes — On Volcanos. 



the name of volcanic ash or dust has been applied, are instantane- 

 ously reduced to so fine a state of division, literally "blown to 

 atoms," as to become converted into an almost impalpable powder, 

 capable of being carried away by the winds prevailing during an 

 eruption to distances of even hundreds of miles from the orifice 

 from which they had been ejected, and ultimately settle down on 

 the land or in the sea to form deposits, whose nature would often be 

 a puzzle to geologists, did not the microscope at once reveal their 

 true mineral character and volcanic origin. 



Other particles, less finely divided, become granulated and fall 

 down from the air in the shape of small black grains known as 

 volcanic sand ; whilst still larger portions, owing to the bubbles of 

 vapour or gas entangled in their substance, descend as black porous 

 or spongy stones, from the size of a pea to that of one's head or 

 larger, and have received the names of lapilli, scoriee, or volcanic 

 cinders, from their presenting much the appearance of an ordinary 

 cinder from a coal fire. Although the scoriae thrown up by volcanos 

 are in major part of a dark colour, there are also others (called 

 trachytic) much lighter both in colour and weight, which are usually 

 more common at the commencement of an eruption; and the ordinary 

 pumice stone, which is imported in large quantities from the vol- 

 canos in the Lipari Islands, for the use of painters, etc., is an 

 example of this variety familiar to you all. 



A peculiar form of lava is produced by the currents of wind 

 blowing over the surface of the molten matter in the crater catching 

 up portions of it and drawing them out into long, slender filaments, 

 like hair or spun glass, of all shades of black, brown, or yellow. 

 In the Sandwich Islands, where this variety is very abundant, it is 

 called Pele's hair, from the name of one of their ancient goddesses. 



In the intervals of an eruption, or after the greatest force of the 

 rush has spent itself, the vapours often rise through the molten lava 

 in the crater in smart puffs, which carry up with them portions of 

 the fluid lava high into the air, whence they descend consolidated as 

 spheres or somewhat elongated bodies, consisting of an external shell 

 of solid lava, hollow, or only filled with vapour or gas in the centre. 

 From their resemblance to militaiy projectiles, these bodies, which 

 vary from the size of an orange to that of a pumpkin, have received 

 the name of volcanic bombs. 



The mineral matter thrown up into the air from a volcanic vent 

 necessarily descends again by virtue of its own weight; a portion 

 drops back into the crater, but the major part falling beyond it accu- 

 mulates around its brink to form a mound, which, since the larger 

 and heavier pieces are not projected to so great a distance as the 

 others, keeps, as it increases in size, raising itself more rapidly in 

 height nearest around the vent, than further off, and thus builds up 

 a hollow cone, the throat or chinmey of which is kept open, at least 

 during the continuance of an eruption, by the upward rush of the 

 gases and vapour forced through by the pressure below. 



The action of the heat, being, of course, much more intense in the 

 chimney or throat of the crater, now causes the at first comparatively 



