These writers carried out an extended series of investigations on the depth temperature and physical and chemical condition of the earth's crust. Chemical analyses of rocks and soils were made and the changes wrought by physical and chemical forces were noted. On these were based theories as to the formation of rocks, soils, minerals, and ore deposits. The erosive properties of soil water were found to be limited to a depth not exceeding 20,000 feet, although hydrostatic water bodies are rarely found as low as half that distance, the rise in temperature precluding their existence. The work of these men revealed the part played by vulcanism in rock changes, and the effects produced through hot solutions and magmatic intrusions. Various systems of classification of minerals and ore deposits were developed. Richard Beck's, "The Nature of Ore Deposits" (1900), and Lindgren's "Mineral Deposits" (1919), are works which have contributed to the systematizing of economic geology from the mineral standpoint, and the establishment of epochs of metal generation. The ore deposits of the United States have been described in the monographs of the United States Geological Survey, and by Kemp, Spurr, Grabau and other writers. This branch of geology emphasizes the strong tendency to concentration shown by mineral elements. All climatic forces are found to aid this work. Underground waters, both flowing and stationary, are powerful assistants. Other phases of economic geology have been developed in studies of subterranean waters, microscopical petrology and mineralogy, the chemical analyses of rocks, etc. Among the leaders in this work have been Pirsson, Emmons, Iddings, Washington, Van Hise, Clarke, and others. The enormous metallurgical industries of to-day are all dependent upon scientific principles chiefly discovered and applied in the nineteenth century. Metallurgists in the previous century knew that by adding certain metals to molten steel it could be hardened. A method of this kind was published by Réaumur in 1722. Tool points, he showed, could be hardened if the steel when red hot was forced into solid tin, lead, copper, silver or gold, thus producing an alloy stronger and harder than the pure steel. A series of calorimetric researches on metallic alloys, carried on by Bergman, led to the discovery that steel differs from iron merely in the carbon contents. Clouet, in 1798, followed this by an experiment in which he melted up a little crucible iron with a diamond and obtained a mass of steel. This created a sensation and led to many other experiments on the metallurgy of cast and wrought iron and steel.