Illium 349 was an extremely rare, radioactive element used exclusively and sparingly in Enlarging Rays to restore shrunken beings to full size.
The element was first isolated by Zak-Kul, in the Bottle City of Kandor and required a device -- an Enlarging Ray -- to moderate its effects. Illium 349 could generally be handled or carried in a pocket without affecting the matter in contact with it, and the Enlarging Ray required minuscule quantities of it.
Illium 349 was so effective that, when Zak-Kul escaped Kandor, shrunk Superman, and took his place on Earth; Superman only needed to find three specks of the element to operate the Enlarging Ray three times. The Ray could be made to work in reverse as well, meaning that Illium 349 could also facilitate the shrinking of matter, in the style of The Atom. Superman, for example, used one of his three specks of Illium 349 to shrink small enough to escape Kandor by travelling between the molecules of glass.
Unfortunately for the people of Kandor, the last of the Illium was used to return Zak-Kul to the bottle, and for a time there was no more Illium. Van-Zee could not find a single speck when he sought to escape Kandor and court Lois Lane.
Despite Illium's relative ease of handling, certain precautions had to be taken when attempting to moderate its radioactivity. Superman's Kandorian ally Nor-Kann experimented with Illium, and caused a sudden chain reaction which irradiated his brain; killing him very swiftly.
After Nor's death, Kandorian research into Illium 349 was curtailed. Fifteen years later, Superman found a way to combine stellar energy from a supernova with the few grains of Illium that remained; and power a ray which would enlarge all of Kandor. As Nor-Kann had predicted, the ray was not completely stable, as it safely enlarged the inhabitants of Kandor; but the buildings turned to dust, much like the previous attempt by Than-Ol.
- Zak-Kul's Enlarging Ray bares a striking resemblance to Brainiac's Hyper-Ray; first seen three issues previously in Action Comics #242. Both would restore Kandorians to their original size and both ran on a very limited and rare element/energy. Thus Illium 349 was likely invented by Brainiac, and not Zak-Kul, and simply remained unnamed until Action Comics #245.
- The Wild Man of Zaga from the Interplanetary Circus can eat anything but Illium; thus the bars of his cage are made from it in Superman #145. Illium 349 is so rare that it is most likely a rare radioactive isotope of Illium; so Illium may not have the same size changing effects.
- Illium 349 is often spelled with a hyphen as Illium-349.
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