Thorium is dimorphic, changing at 14000C from a cubic to a body- centered cubic structure. Thorium oxide has a melting point of 33000C and is the highest of all oxides. Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be carefully handled. The principal use of thorium has been in the preparation of the Welsbach mantle, used for portable gas lights.
Glasses containing thorium oxide have a high refractive index and low disper- sion.
Consequently, they find application in high quality lenses for cameras and scientific instruments. Thorium oxide has also found use as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid. Twelve isotopes of thorium are known with atomic masses ranging from 223 to 234. All are unstable. Th232 goes through six alpha and four beta decay steps before becoming the stable isotope Pb208 Th232 is sufficiently radioactive to expose a photographic plate in a few hours.