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 revision. Geologists were afraid to peep into a volcanic crater in action until Dr. Brun went and did so, and the results of his investigations have upset the theorizing of previous geologists who observed volcanoes from afar. Formerly it was supposed that water played an important role in volcanic activity; water, in contact with rock, molten and at a very high temperature, would be in an explosive condition, and water, therefore, was held to be the agent which shot the rocks and ash upwards. Brun, however, has pumped into his receiving vessels samples of the gases given off from volcanoes in their explosive condition, actually standing on the crest of the crater and dangling his receiving pipes into the uprushing vapours, and he finds that the gases given off are totally devoid of water. The gases actually present are carbon dioxide, carbon monoxide, marsh gas, nitrogen, and hydrogen, sometimes chlorine, and also a large quantity of ammonium chloride (sal-ammoniac) as a fine solid powder, which ascends and spreads out over the volcano as a mushroom-shaped cloud. Sometimes the cloud does not ascend, but, like a dense cloud of smoke, rolls down the hillside, and withers up all life which comes into contact with the intensely hot fumes. In 1902 the whole population of the town of San Pierre, in Martinique, was annihilated by the burning cloud (nuée ardente) which was rolled down from the volcano Mont Pelé. At other times the crater becomes full of water and forms a crater lake; on the Peak of Tristan da Cunha, which is a volcanic mountain, there is such a crater lake. If, now, the volcano becomes active, the ash is shot through the water, and the whole rolls down the side of the mountain as a hot liquid mud, engulfing animals and trees in its rush. Volcanic mud-rushes of