Thallium is a toxic, bluish-white metal with the symbol Tl and atomic number 81, Sir William Crookes discovered thallium in 1861. He named it after the Greek word thalos, which means the green color of a tree's leaves. Thallium is soft, heavy, inelastic, tasteless, and odorless. It has a melting point of 578.9 °F (303.9 °C) Thallium is used in the production of electronics, thermometers, lenses, and imitation jewels. It's also used in some chemical reactions and medical procedures. Thallium is a hazardous substance and a difficult-to-detect poison. It can cause severe damage to the central nervous system. Symptoms of thallium poisoning include gastrointestinal issues, neurological dysfunction, alopecia, and Mees' lines on the nails. Thallium was previously used as a rodenticide, but this use was banned in the United States in 1972. The United States has not produced thallium since 1984, and it's now imported. Thallium is found in trace amounts in the earth's crust, usually combined with other elements like oxygen, sulfur, and chlorine. Coal burning, smelting, and cement production are the primary sources of thallium release into the environment.