Zenium

Zenium is an ultra-rare element that can generate an exchange ray that switches objects through the X-dimension. Kull-Ex, a Kandorian scientist, uses it successfully to exchange places with Superman. (S No. 134/2, Jan 1960: "The Revenge against Jor-El")

Supergirl locates a nugget of Zenium in a meteorite to exchange Kull-Ex and Superman back to their previous places. (S No. 134/3, Jan 1960: "The Duel of the Supermen)