Thus, for instance, radium would finally be transformed into helium.[40] [Footnote 40: This opinion, no doubt formed when Sir William Ramsay's discovery of the formation of helium from the radium emanation was first made known, is now less tenable.

The latest theory is that the alpha particle is in fact an atom of helium, and that the final transformation product of radium and the other radioactive substances is lead.Cf.Rutherford, op.cit.

passim .-- ED.] It is possible, by considerations analogous to those set forth above in other cases, to arrive at an idea of the total number of particles per second expelled by one gramme of radium; Professor Rutherford in his most recent evaluation finds that this number approaches 2.5 x 10^{11}.[41] By calculating from the atomic weight the number of atoms probably contained in this gramme of radium, and supposing each particle liberated to correspond to the destruction of one atom, it is found that one half of the radium should disappear in 1280 years;[42] and from this we may conceive that it has not yet been possible to discover any sensible loss of weight.

Sir W.Ramsay and Professor Soddy attained a like result by endeavouring to estimate the mass of the emanation by the quantity of helium produced.
[Footnote 41: See _Radioactive Transformations_ (p.

251).