Illium-349 is a radioactive element. Its rays can be used to either enlarge or shrink physical matter, through an unknown process. The element is, therefore, a key component in a Reducing Ray.
Zak-Kul, a rogue Kandorian scientist, was studying the mineral and found ways to use it to enlarge himself out of his shrunken bottle city. However, he didn't get an opportunity to try this until Lois Lane let him out of his bottle accidentally. ("The Shrinking Superman")