It has been already stated in Part I (_vide_ p. 73) that a strongly radioactive product. Radiothorium, has been obtained from the mineral thorianite. This body is produced by the atomic degradation of thorium, and an intermediate body, mesothorium, has been found to be formed during the change. Mesothorium is a substance which, though it appears to be chemically identical with radium, has an activity equal to three hundred times that of radium, and when in equilibrium with its degradation products the ‘rays’ it emits are very similar to those of the latter element. Since mesothorium is a degradation-product of thorium, it occurs in minute quantities in all thorium minerals, and by reason of the possibility of using it as a substitute for radium, its extraction becomes a matter of importance. Soddy[498] has shown that if a barium compound be added to monazite before the treatment with sulphuric acid, the mesothorium remains with the barium sulphate; this is readily separated from the heavy unchanged grains of sand, and is purified, and finally obtained as chloride by treating the solution with hydrogen chloride. On recrystallisation of the barium chloride, the active products are concentrated in the less soluble part, and it is possible to prepare on the commercial scale a mixture which, though it contains only 0·25 per cent. of mesothorium, has an activity equal to that of pure radium bromide. This mixture contains 25 per cent. of radium compounds, radium being present as an original constituent of monazite; owing to the chemical identity of radium and mesothorium,[498] the latter cannot be separated, but Soddy, by removal of much of the barium compound in the laboratory, has obtained a product four times as active as the pure radium salt. [498] _Proc. Chem. Soc._ 1910, ~26~, 336, and _E._ 25504, November, 1910. See also Hahn, _Chem. Zeitg._ 1911, ~35~, 845. It is probable that the treatment of monazite will in the future be modified by the addition of barium sulphate before the sulphuric acid decomposition, to allow of the commercial extraction of its mesothorium.