THE Boulder Dam forms a huge concrete plug between the walls of the Black Canyon on the Colorado river. According to Science Service of Washington, D.C., it weighs six million five hundred thousand tons. As this concrete sets, the slow chemical reaction that takes place gives off heat. Researches by the U.S. Bureau of Reclamation have shown that in the Boulder Dam sufficient heat would be generated to melt a cube of ice as high as a 24-story building. If no means were adopted to keep cool this great block much damage might be done, as during the protracted cooling and shrinking period there would be a serious risk of dangerous cracks occurring. To obviate this risk, as each section of the concrete is poured it is riddled with coils of pipe. About 560 miles of tubing will be used, and this will be kept in place permanently as the cement hardens.