_Radium F_, after separation, will give out only α rays. Its activity, after separation, will decrease according to an exponential law, falling to half value in 143 days. Since radium in radio-active equilibrium contains four products which emit α rays, the number of α particles expelled per second from radium F will, weight for weight, be about 800 times as numerous as from new radium in radio-active equilibrium. Since the α particles from radium F produce only about half as much ionization as the α particles from the other radium products, the activity of radium F, measured by the electric method, will be about 400 times that of radium. =240. Origin of radio-tellurium and polonium.= It is now necessary to consider whether these products of radium have been previously separated from pitchblende, and known by other names. We shall first consider the α ray product, radium F. The radio-tellurium of Marckwald and the polonium of Mme Curie both resemble radium F in giving out only α rays, and in being deposited on a bismuth disk from a solution. If the active constituent present in radio-tellurium is the same as radium F, its activity should decay at the same rate as the latter. The writer[320] has carefully compared the rates of decay of the activity of radium F and of the radio-tellurium of Marckwald and found them to be the same within the limits of experimental error. Both lose half of their activity in about 143 days[321]. A similar value of the rate of decay of radio-tellurium has been obtained by Meyer and Schweidler[322]. The experiments on radio-tellurium were made upon the active bismuth plates supplied by Dr Sthamer of Hamburg, which were prepared under Marckwald’s directions. An additional proof[323] of the identity of these two products was obtained by comparing the absorption of the α rays by aluminium foil. The α rays from different products are projected with different velocities, and, in consequence, are unequally absorbed by matter. The absorption of the rays from the two products by aluminium foil agreed very closely, indicating the probable identity of the substances from which they were emitted. There can thus be no doubt that the active constituent present in the radio-tellurium of Marckwald is identical with the product radium F. This is a very interesting result, and shows how the close examination of the successive transformations of the radio-active bodies may throw light on the origin of the various substances found in pitchblende.