Sir Mark was diverted from his previous work by the enthusiastic desire to have a hand himself in this important new field. He set to work to build an apparatus capable of accelerating much larger numbers of protons than the accelerator of Cockroft and Walton. In doing so he developed an intense source of protons which was adopted as a standard component of such accelerators. With this apparatus, in 1933, Oliphant and Rutherford studied the disintegration of lighter elements by high-speed protons. More important experiments were made possible by the discovery of deuterium, which is heavy hydrogen twice as heavy as ordinary hydrogen, and by the gift to Lord Rutherford by Professor G.N. Lewis, of a small quantity of 'heavy water'.
Heavy water contains deuterium rather than ordinary hydrogen as a constituent.
Most importantly Sir Mark showed that deuterons could interact with each other to produce a third form of hydrogen, this one three times heavier than ordinary hydrogen, to which the name tritium was given. This was the first example of fusion, producing heavier nuclei from two lighter ones. In the same way Sir Mark discovered helium-3, a light form of helium. At this time, around 1933, Lord Rutherford was arguing that the notion of useful energy from nuclear interactions was, and I quote, 'moonshine', because the very high energy needed to initiate a reaction would result in some loss of energy. Of course, what Sir Mark had shown in his experiments, was how two lighter hydrogen atoms can fuse to form heavier ones with the release of energy.