go Human Physiology. 



direction. A different arrangement of atoms in opaque bodies 

 absorbs, or breaks up, or reflects the vibrations. 



The earth on which we live is a globe of matter, nearly eight 

 thousand miles in diameter, small and dense compared with 

 the outer planets of our solar system, Jupiter, Saturn, Uranus, 

 and Neptune. Of the great mass of the earth we have no 

 knowledge. It is believed to be composed of melted metals. 

 The external crust is supposed to be some twenty-five miles in 

 thickness a mere film on such a globe. In most deep mines, 

 and in artesian wells, heat increases regularly with the depth. 

 Volcanoes, hot springs, earthquakes, and many geological phe- 

 nomena, indicate the intense heat and liquid condition of the 

 earth's interior. 



The crust of the earth is composed of rocks, or gravel, sand, 

 clay at the surface, which has been rock, mingled with vegetable 

 and animal matter soil, or loam. Rocks are of two kinds ; 

 igneous and sedimentary. The igneous have been thrown up 

 from the molten masses of the earth's interior; as volcanic, 

 trappean, granite. The sedimentary rocks are formed of the 

 crumbled, broken down matter of the igneous rocks, or of 

 minerals dissolved in water, and slowly deposited at the 

 bottom of lakes and seas. These rocks and the earths into 

 which they crumble, under the action of water and frost, are 

 metallic oxides. Slate and clay are composed of aluminium 

 and oxygen. Rock crystal and sand, of silicium and oxygen. 

 Marble, limestone, chalk, of calcium and oxygen. Magnesium 

 and iron also enter largely into the composition of soils. Iron 

 rust ashes of iron is, in many places, reddening earth 

 and rock. Upon a close inspection we find that all this land 

 has been for ages under the deep sea. The solid rock is full 

 of shells of marine animals. The limestone strata, hundreds 

 of feet in thickness, has been deposited atom by atom in the 

 deep waters, and is composed of the shells of microscopic 

 animalculse. The flints are collections of shields found upon, 



