[125] Polonium, which was named by Mme. Curie in honour of her native country, has been shown to be identical with Marckwald’s ‘Radio-tellurium,’ which was named by Rutherford ‘Radium F’; it is one of the degradation products of radium. It has been mentioned, in the accounts of the rare earth minerals given above, that almost all these minerals are radioactive, _i.e._ have the property of emitting specific radiations. Moreover, radioactivity, to any considerable extent at least, is, with a few important exceptions, confined to the minerals which have been already described. It has been shown by many investigators, chief amongst whom are Strutt and Boltwood, that the activity is usually due to the presence of uranium or thorium, or both.[126] [126] Hauser and Wirth (_Ber._ 1910, ~43~, 1807) observed activity in some zirconium minerals containing neither thorium nor uranium. After the discovery of helium in Cleveite (a variety of pitchblende, _vide_ p. 13) in 1895 by Ramsay, a large number of minerals were examined for this gas, and it was found that almost all the rare earth minerals contain helium. The fact that these minerals are also for the most part radioactive, naturally suggested some relation between the activity and the presence of helium, and led directly to the discovery that radium is continuously producing helium; and it became apparent that helium has been accumulating in these minerals since their formation, by the decay of radioactive elements. The question of the origin of helium in minerals will be touched on again. In 1904 Boltwood advanced the theory that radium is produced by the degradation of uranium, the parent-element having, however, a much greater half-life period. If uranium continuously produces radium, whilst the latter decays much more rapidly than the former, it must follow that in minerals containing uranium a state of equilibrium is reached between uranium and radium, and the ratio of these two in all minerals should therefore be constant, and independent of the geological age. Boltwood examined a number of the minerals of which descriptions have been given in the preceding chapters, and found the ratio to be surprisingly constant.[127] Strutt also examined a large number of minerals,[128] and whilst on the whole his results seemed to support the theory, his values for the ratio were by no means so constant as those of Boltwood. Strutt included in his examination the interesting radium-containing mineral observed by Danne at Issy l’Evêque.[129] This was a pyromorphite (lead chlorophosphate) containing neither uranium nor thorium. Danne suggested that the radium was not an original constituent, but had been introduced by the action of percolating waters. This view was confirmed by McCoy and Ross,[130] who found that the activity was entirely confined to the surface layer. [127] _Amer. J. Sci._ 1904, [iv.], ~18~, 97; _Phil. Mag._ 1905, [iv.], ~9~, 599.