Zenium

Zenium is an ultra-rare element which can generate an exchange ray that can switch objects through the X-dimension. Kull-Ex a Kandorian scientist, uses it successfully to exchange places with Superman. (S Jan 1960, No. 134/2: "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 Jan 1960, No. 134/3: "The Duel of the Supermen)