Page:Popular Science Monthly Volume 47.djvu/317

Rh that deposited the silver ores, still at work. In the mines of this region the miners, naked as savages, reeking with perspiration, drinking pailful after pailful of ice water (twenty tons of ice, or, in another case, ninety-five pounds per man, were used each day), could labor but ten minutes at the drift (in imminent danger of being scalded by striking a stream of hot water) before being overcome by the heat and reeling to a cooler place. Fainting, delirium, even death have been the effect of the reaction on coming to the surface. Verily the Cuban proverb, that a Yankee would be found to go after a sack of coffee though it were at the gates of hell, was not far from the literal truth.

However the rate of increase of temperature may vary, all indications thus agree that less than ten miles below us a red heat is attained and within twenty a white heat. Think of it! Ten miles below us it is red hot. Ten miles above we have the pitiless cold, far below zero, of interplanetary space. To what a narrow zone of delicately balanced temperature is life confined!

From the deeper zones of higher temperatures we have samples furnished us by the volcanoes, opened along great cracks in the earth, whence red or white hot foaming lava rises. They confirm our idea of the downward increasing heat of the earth. These outpourings of molten matter from volcanoes give us some idea also of the composition of the earth. To the path of investigation thus opened we shall return in a moment. They have also given rise to the very prevalent notion that the earth's surface is but a solid crust over a fluid interior of the consistence of lava. Observers on the Hawaiian Islands have even thought they could hear the dashing of the lava waves beneath. But it is not hard to see that the phenomena of volcanoes are far more complex than the mere welling up of a fluid interior. The lava is often more heavy than the crust, and it often stands at different heights in neighboring vents. Moreover, contemporaneous, not far distant vents sometimes furnish quite different material. This could hardly be possible if all volcanoes had a common source. The really essential and important part of a volcanic eruption is the escape of gases, which are or soon become largely steam. This forms the clouds which overhang a volcano, and descends in time of violent eruptions in torrential rains, such as buried Pompeii in mud. Hence, some have supposed that a volcanic eruption was due to the explosive action of sea water reaching the heated interior. But it is perhaps more probable that the gases which escape are originally contained in the lava and burst forth from the interior of the earth on their own account wherever a crack gives them a chance. According to this notion, the working of volcanoes is not unlike that of a bottle of ginger ale. All that is