EARTH’S CRUST AND CENTER.
Our earth is called “terra firma;” it is regarded as the very embodiment of stability, but every waving outline, every hill and mountain peak, not less than the rumbling of the earthquake, and the bursting forth of volcanic fires, indicate that it has been, and may again be, the scene of mighty disturbances. Indeed, upon reflection, one wonders that we can live on it at all. The temperature of the earth increases one degree for every fifty feet as we approach the center. At this rate, at the depth of fifty miles the heat would be sufficient, according to Humboldt,[11] to melt the hardest rocks. Fifty miles is one one-hundred and sixtieth of the earth’s diameter. It thus appears that if we should have a globe three feet in diameter full of molten liquid, surrounded by a covering of infusible material one eighth of an inch in thickness, that film of solid matter would represent the earth’s crust. Think of it!
A “LEAD TREE.”
Ex.—Place in a glass a dilute solution of acetate of lead; suspend in it a strip of zinc. Some of the lead will be precipitated in crystals upon the zinc. This is caused by the zinc taking a portion of the acetic acid, and thus forming a new compound called zinc acetate, thereby liberating some of the lead.
Besides, that awful, fiery sea within is subject to tides, currents and convulsions that constantly threaten to disrupt and destroy this crust. It is supposed that masses of water percolate through cracks and fissures until they reach the internal fires and are suddenly converted into steam at an enormously high temperature, which gives it such tremendous expansive force as to shake the globe itself. This action, combined with the violent explosion of gases, creates the sublime and dreadful phenomena of