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7. Gilbert Newton Lewis, a chemist, helped to build the modern electron theory of valence, a theory explains the forces holding atoms together in molecules.

8. Because of a high birthrate and considered tremendous immigration, the United States population increased tremendously from 31 million in 1860 to 76 million in 1900.

9. For most of their history especially since the 1860's, New York City has been undergoing major ethnic population changes.

10. Rocks have forming, wearing away and re-forming ever since the Earth took shape.