Period 6 element devices though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of alcohol-filled, galinstan-filled, digital, or thermistor-based instruments. It remains in use in scientific research applications and in amalgam material for dental restoration. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-wave ultraviolet light which then causes the phosphor to fluoresce, making visible light. Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray other metal resembles tin but discolors when exposed to air. The
Cadmium telluride photovoltaics in Mexico and Sweden. In 1984 astrophysicists identified tellurium as the universe's most abundant element having an atomic number over 40. Certain undersea ridges are rich in tellurium. The manufacture of a CdTe cell includes a thin coating with cadmium chloride () to increase the cell's overall efficiency. Cadmium chloride is toxic, relatively expensive and highly soluble in water, posing a potential environmental threat during manufacture. In 2014 research discovered that abundant and harmless magnesium chloride () performs as well as cadmium chloride. This research may lead to cheaper and safer CdTe cells. By themselves, cadmium and tellurium are toxic