Although Soddy named them, he was the first to refer to elemental variants which would eventually be called isotopes.
When working with potassium selenocyanate, and observing it through the newly invented spectroscope, he found a green line in its spectrum, which was eventually found to be the element thallium. He created the radioscope, a toy shaped like a paddlewheel which spun when light was shone on it, which was one of the motivators of Maxwell's kinetic theory of gases. Although Soddy named them, he was the first to refer to elemental variants which would eventually be called isotopes. FTP, name this scientist, who passed current through an ionized gas in his namesake tube, discovering what he called "molecular rays" and paving the way for Thomson's discovery of the electron.
When working with potassium selenocyanate, and observing it through the newly invented spectroscope, he found a green line in its spectrum, which was eventually found to be the element thallium.
Sir William Crookes