The element cosmoium had the symbol "Cs", which is the same atomic symbol used for Cesium. It is presumably named for the fictional element from John W. Campbell, Jr's. "Invasion of the Infinite." Cosmoium was supposedly composed of solidified light, in the form of exotic cosmic ray photons instead of ordinary light. Its specific gravity was 5,007.89 and tensile strength about two hundred thousand times that of steel.