The metal-nonmetal borderline in Group VIA comes at selenium. It exists in a metallic, chainlike alIotrope as well as in rings. Selenium is a "one-dimensional semiconductor" rather than a "one-dimensional metal" in the sense that electrons flow along the chains only with difficulty. Visible light shining on metallic selenium provides enough energy to get the electrons moving, therefore selenium has a greatly enhanced electrical conductivity when illuminated. This is the basis of the "Xerox" process. A selenium-coated cylinder is given a uniform electrostatic charge in the dark, and then has the image of a printed page or diagram cast on it. Where the original diagram was blank, light strikes the selenium cylinder, and the surface electrostatic charge leaks away. Only on the dark regions, representing the printing or design on the original, is the static charge retained. A black, fusible powder then is dusted onto the cylinder, and sticks only to the charged regions. The drum is rolled against a sheet of paper to transfer the powder, which then is bonded to the paper by heat. The result is an image, in fused black powder, of the dark areas on the original.