[128] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312. _Ibid._ 1907, A, ~80~, 56. [129] _Compt. rend._ 1905, ~140~, 241. [130] _J. Amer. Chem. Soc._ 1907, ~29~, 1698. Mlle. Gleditsch has also examined the question of the uranium-radium ratio in minerals. Her earlier work[131] gave ratios which, whilst constant for each mineral species, varied in much the same manner as Strutt’s for different species, and afforded very little support to Boltwood’s theory. Her more recent results,[132] however, are much more closely in accord with the theory, which has been still further strengthened by the work of Pirret and Soddy[133] and of Marckwald and Russell.[134] It may now be regarded as firmly established that radium is in the line of direct descent from uranium. [131] _Compt. rend._ 1909, ~149~, 267; _Le Radium_, 1909, ~6~, 165. [132] _Le Radium_, 1911, ~8~, 256. [133] _Phil. Mag._ 1911, [vi.], ~21~, 652. [134] _Ber._ 1911, ~44~, 777. Boltwood had assumed that the helium in radioactive minerals is produced from the uranium, during its disintegration. Strutt, however, disputed this; his experiments showed that very little helium is found even in the richest radium-uranium minerals unless thorium is also present. Thus pitchblende contains a very high percentage of uranium, but relatively little helium (there is usually a considerable thorium percentage here too, so that nothing conclusive can be deduced from this). Adams[135] found that carnotite, a mineral very rich in uranium, but containing no thorium, contains no helium at all; he explained its absence by the very loose texture and permeability of the mineral, which would allow the gas to escape. Strutt concluded that whilst helium is undoubtedly produced by disintegration in the uranium series, in minerals it is produced more by thorium or, as more recent work indicates, by radio-thorium, than by uranium. [135] _Amer. J. Sci._ 1905, [iv.], ~20~, 256.