(For the significance of this fact see Radioactivity.) The richest known source is thorianite, which consists mainly of thorium oxide, and contains 9.5 cc. of helium per gram.
Much the best practical source of helium is thorianite, a mineral imported from Ceylon for the manufacture of thoria.
If thorianite cannot be obtained, monazite, which is more abundant, may be utilized.
Thorianite, however, contains no silica, and until it is shown that metallic oxides behave in the same way this explanation must be accepted with reserve.