Dr. Boyer tells the viewers that concrete is composed of cement, water, sand, and aggregate or coarse gravel in proper proportions. He notes that "Portland" is not a trade name for cement but rather a standard cement powder formula named by an English bricklayer in 1824 for the isle of Portland. The Romans used concrete, made from Mt. Vesuvius's volcanic rock and sand, to construct many of their famous buildings. Mixing concrete is compared to following a recipe since measurement is critical. Furthermore, the chemistry of concrete is such that it requires moisture for 7-10 days to properly harden. Dr. Boyer enumerates the versatility of concrete and shows how pre-casting concrete of precise forms speeds up construction. He then discusses concrete use in bridge building. Because concrete is strong in compression but relatively weak in tensile strength, reinforced concrete with embedded steel was developed in 1850, and pre-stressed concrete, held in a permanent squeeze with steel wires and bars, is now used in bridges and many other applications.