Radon

Discovered by Fredrich Ernst Dorn of Germany in 1898, radon was originally called radium emanation. It was isolated in 1908 and named niton. It's name originates from the word radium. It is a heavy radioactive gas, and it is colorless at regular temperatures. It is the heaviest of all gases. At very cold temperatures it appears yellow. Inhalation of radon is very bad, and in fact it has been tested and radon is the second leading cause of cancer.

Radon occurs naturally in some spring waters. Ernest Rutherford is sometimes mistakenly credited with the discovery of radon... however, he discovered the alpha particle radiation given OFF by radon.