The very finely-divided volcanic dust is often borne to enormous distances from the volcano out of which it has been ejected. The force of the steam-current carrying the fragments into the atmosphere is often so great that they rise to the height of several miles above the mountain. Here they may actually pass into the upper currents of the atmosphere and be borne away to the distance of many hundreds or thousands of miles. Hence it is not an unusual circumstance for vessels at sea to encounter at great distances from land falling showers of this finely divided, volcanic dust. We sometimes meet with this far-travelled, volcanic dust under very unexpected circumstances. Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of Bonn, who showed me a quantity of fine volcanic dust which had during the past winter fallen in considerable quantities in certain parts of Norway. This dust, upon microscopic examination, proved to be so similar to what was known to be frequently ejected from the Icelandic volcanoes that a strong presumption was raised that volcanic outbursts had been going on in that island. On returning to England I found that the first steamer of the season had just reached Leith from Iceland, bringing the intelligence that very violent eruptions had taken place during the preceding months. [Sidenote: DISPERSION OF PUMICE AND VOLCANIC DUST.] This finely-divided volcanic dust is thus carried by the winds and spread over every part of the ocean. Everyone is familiar with the fact that pumice floats upon water; this it does, not because it is a material specifically lighter than water, but because cavities filled with air make up a great part of its bulk. If we pulverise pumice, we find the powder sinks readily in water, but the rock in its natural condition floats for the same reason that an iron ship does--because of the air-chambers which it encloses. When this pumice is ejected from a volcano and falls into a river or the ocean, it floats for a long time, till decomposition causes the breaking down of the thin glassy partitions between the air chambers, and causes the admission of water into the latter, by which means the whole mass gets water-logged. Near the Liparis and other volcanic islands the sea is sometimes covered with fragments of pumice to such an extent that it is difficult for a boat to make progress through it, and the same substance is frequently found floating in the open ocean and is cast up on every shore.