can occur, as with uranium and plutonium. Inorganic compounds Most binary compounds of thorium with nonmetals may be prepared by heating the elements together. In air, thorium burns to form , which has the fluorite structure. Thorium dioxide is a refractory material, with the highest melting point (3390 °C) of any known oxide. It is somewhat hygroscopic and reacts readily with water and many gases; it dissolves easily in concentrated nitric acid in the presence of fluoride. When heated in air, thorium dioxide emits intense blue light; the light becomes white when is mixed with its lighter homologue cerium dioxide (, ceria): this is the basis for its previously common application in gas mantles. A flame is not necessary for this effect: in 1901, it was discovered that a hot Welsbach gas mantle (using with 1% ) remained at "full glow" when exposed to a cold unignited mixture of flammable gas and air. The light emitted by thorium dioxide is higher in wavelength than the blackbody emission expected from incandescence at the same temperature, an effect called candoluminescence. It occurs because : Ce acts as a catalyst for the recombination of free radicals that appear in high concentration in a flame, whose deexcitation releases large amounts of energy. The addition of 1% cerium dioxide, as in gas mantles, heightens the effect by increasing emissivity in the visible region of the spectrum; but because cerium, unlike thorium, can occur in multiple oxidation states, its charge and hence visible emissivity will depend on the region on the flame it is found in (as such regions vary in their chemical composition and hence how oxidising or reducing they are). Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium. All four thorium tetrahalides are known, as are some low-valent bromides and iodides: the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water. Many related polyhalide ions are also known. Thorium tetrafluoride has a monoclinic crystal structure like those of zirconium tetrafluoride and hafnium tetrafluoride, where the ions are coordinated with ions in somewhat distorted square antiprisms. The other tetrahalides instead have dodecahedral geometry. Lower iodides (black) and (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead contain and could be more clearly formulated as electride compounds. Many polynary halides with the