“From repeated contact with sooty walls, we also became covered with this strange, light-absorbing powder, until we were enveloped in an invisible mantle, and also passed from each other’s sight. Eye alone answered to eye in their reflections of light. Too deeply impressed for conversation, we stood still with outstretched hands. My comrade asked at length, ‘May it not be even so in the valley of the Shadow of Death?’ And we looked for strength into each other’s eyes and linked our arms that we might have the companionship of touch. We were now thoroughly frightened, and turned to retrace our steps; but which way? We stood in a sea of nothingness--locked in the foundations of the mountain. The walls were lost to the sight, and were nothing to the touch. We stooped to the deep dust of the floor and held the flame to read our foot-prints; but the soil absorbed the light, as the sand of the desert does the raindrop. We reached forward, and the hand failed to meet the wall; we reached downward; there, too, was empty space. The light showed no defining edge between the solid rock and the void. We swung the lamp from the brink on which we lay; it revealed nothing. We dropped a heavy stone into the chasm and listened for the rebound. No sound was returned as it sank into the profound. We cast another stone across to test the width, but this, too, was lost to the senses. Silently they passed away, as the mist wreath on the hill side. And then we knew we had been preserved from death. A careless step and we had found a grave in the depths of the world’s foundations. We realized that we were lying in trembling safety on the threshold of the extinct volcano, and lifting our useless eyes from the impenetrable blackness, the awful whisper ‘Lost!’ passed between us. We were afraid to move; but the wasting oil of our lamp warned us that time must not be lost. Presently our ears caught the heat of surf on the rock as the tide came in, and following this direction, we finally reached the entrance, almost fainting from joy when we stood beyond this chamber of gloom. Once more we stood under the wondrous tracery and reflections of the outer gates of the inter-world of mysterious.” A most thrilling experience, and one giving a fine picture of what may be found in the mysterious depths of a lava bubble. In some cases the bubbles are very thin; and an unwary passer might be suddenly plunged into unfathomable depths should he tread on one. Usually, however, they are formed over horizontal currents or passages. We have endeavored to give the well-established facts concerning the principles of volcanic action. It only remains, ere we leave this phase of the subject, that we notice the one point on which as yet our knowledge is not clear. That point is, the source of the heat which produces the remarkable effects.