In 1903, Frederick Soddy showed that the decay of radium produced helium gas.
Soddy carried out the experiment with Sir William Ramsay at University College London. A radium sample was enclosed in a thin-walled glass vessel placed inside an evacuated glass bulb. After the experiment had run for a long period, spectral analysis detected helium in the surrounding space.
The result was striking because it connected a radioactive transformation with the appearance of a recognizable chemical element. It showed that radium decay was not simply an emission of invisible rays; it also produced material particles. Later work by Ernest Rutherford and Thomas Royds established that alpha particles are helium nuclei, providing the nuclear explanation for the helium observed in the experiment.
The common confusion is with uranium, thorium or polonium, all of which belong to radioactive decay chains. The historically important 1903 experiment specifically concerned radium and helium production.