Thomas Young, in 1802-7, studied the mechanical properties of iron and steel and developed the theory of the modulus of elasticity. A patent was issued to the Rev. Robert Stirling, in 1817, for a regenerative iron smelting furnace. The next year Samuel Baldwin Rogers substituted iron bottoms for sand bottoms in puddling furnaces. Faraday and Stodart produced the first alloy of nickel and steel in 1820, and in 1822 Faraday showed that there is a fundamental chemical difference between hard and soft steel. The first patent for a hot blast for iron furnaces was granted to James Beaumont Neilson in 1828. All these discoveries led to important improvements in iron making. The steam hammer was patented by Nasmyth in 1842, and between 1843 and 1848 Thomas Andrews conducted valuable investigations into the heat of combination. The ground was now prepared for one of the greatest of metallurgical inventions--the conversion of pig iron into steel by an air blast in a Bessemer converter. This invention not only vastly extended the use of steel, but drew attention to the valuable oxidizing effects of a hot air blast and in that way induced many important improvements in the metallurgy of copper, lead, and zinc. Siemens, Whitworth, Bell, Graham, Percy, Richards, Martin, Thomas, Holley, Hewitt, Fritz, Howe, Jones, and others made further important improvements in the metallurgy of iron and steel in the United States and Europe. One of the early American iron smelters was built by Governor Keith, in 1726, in New Castle County, Delaware. A rolling mill and forge were subsequently built at Wilmington. The first American smelted iron was shipped to England from smelters in Maryland and Virginia in 1718. The Bessemer steel process was introduced into the United States by Abram Hewitt at the Troy smelter, New York, in 1865. From these beginnings the iron industries of the United States have grown so that they now produce more than two-fifths of the world's annual supplies. The alloys of iron and steel have now attained importance and a new science known as metallography has developed. Professor Arnold, of Sheffield, Sherard Cowper-Coles, Roberts-Austen, Sorby, Tschermak, Tschernoff, Wüst, and Ziegler have been active promoters of this branch of metallurgy, and a closely related one dealing with the effects of the heat treatment of metals. Developments in the iron industries led to others in the metallurgy of copper, lead, and zinc. The application of the blast furnace to copper, lead, and zinc smelting was chiefly made in America. One of the early furnaces was built in Leadville, Colorado, in 1877. From that time, pyritic smelting has been chiefly developed by American metallurgists. The metallurgy of lead, copper, and zinc has reached a similar high plane to that attained by iron and steel.