name of “scoria” is applied.  But when the lava contains no ready-formed crystals, but consists entirely of a glassy substance in a more or less perfect state of fusion, the liberation of steam gives rise to the formation of the beautiful material known as “pumice.”  Pumice consists of a mass of minute glass bubbles; these bubbles do not usually, however, retain their globular form, but are elongated in one direction through the movement of the mass while it is still in a plastic state.  The quantity of this substance ejected is often enormous.  We have seen to what a vast extent it was thrown out from the crater of Krakatoa.  During the year 1878, masses of floating pumice were reported as existing in the vicinity of the Solomon Isles, and covering the surface of the sea to such extent that it took ships three days to force their way through them.  Sometimes this substance accumulates in such quantities along coasts that it is difficult to determine the position of the shore within a mile or two, as we may land and walk about on the great floating raft of pumice.  Recent deep-sea soundings, carried on in the Challenger and other vessels, have shown that the bottom of the deepest portion of the ocean, far away from the land, is covered with volcanic materials which have been carried through the air or have floated on the surface of the ocean.

Fragments of scoria or pumice may be thrown hundreds or thousands of feet into the atmosphere, those that fall into the crater and are flung up again being gradually reduced in size by friction.  Thus it is related by Mr. Poulett Scrope, who watched the Vesuvian eruption of 1822, which lasted for nearly a month, that during the earlier stages of the outburst fragments of enormous size were thrown out of the crater, but by constant re-ejection these were gradually reduced in size, till at last only the most impalpable dust issued from the vent.  This dust filled the atmosphere, producing in the city of Naples “a darkness that might be felt.”  So excessively finely divided was it, that it penetrated into all drawers, boxes, and the most closely fastened receptacles, filling them completely.  The fragmentary materials ejected from volcanoes are often given the name of cinders or ashes.  These, however, are terms of convenience only, and do not properly describe the volcanic material.

Sometimes the passages of steam through a mass of molten glass produces large quantities of a material resembling spun glass.  Small particles of this glass are carried into the air and leave behind them thin, glassy filaments like a tail.  At the volcano of Kilauea in Hawaii, this substance, as previously stated, is abundantly produced, and is known as ’Pele’s Hair’—­Pele being the name of the goddess of the mountain.  Birds’ nests are sometimes found composed of this beautiful material.  In recent years an artificial substance similar to this Pele’s hair has been extensively manufactured by passing jets of steam through the molten slag of iron-furnaces; it resembles cotton-wool, but is made up of fine threads of glass, and is employed for the packing of boilers and other purposes.