Tungsten is a heavy metal he is used to make filaments for lightbulbs, brake linings and missile nose cones, and in alloys to harden other metals.) The similarities between Fallon and Sierra Vista, and between them and two other towns--Elk Grove, Calif., and Hoisington, Kan.--that also have excess cases of childhood leukemia and rhabdomyosarcoma suggest that these towns might hold clues to the ca