One of the important consequences of the discovery of radioactivity was to afford the scientist a means for solving the problem of the earth’s age. By “age of the earth” we mean here the time which has elapsed since the earth’s surface became fitted for the habitation of living beings. By means of radioactivity we can form an approximate estimate of the time which has passed since the formation of any given series of geological strata. Radium is our geological time clock. It is now known that all the common rocks and soils of which the earth’s crust is built up contain measurable amounts of radium. According to the computation made by Prof. John Joly, the total quantity of radioactive matter may be as much as one 500 billionth part of the whole volume of the globe, or something over half a cubic mile. All of the 36 known radio-elements are disintegration products of the primary radio-elements uranium and thorium--_i.e._, they are produced from one or the other of these in their long sequence of changes. And the rate at which the radioactive products change--their average life period,--from the first transmutation to the final product, radium lead, an isotrope of common lead, is accurately known. (Helium atoms are “the debris shed at the various stages of the transformation.”) It is now well established that a gram of uranium as found along with its products in rocks and minerals is changing at a rate represented by the production of 1.88 x 10^{-11} grams of helium and 1.22 x 10^{-10} grams of lead (isotrope) _per annum_. We do not know for a certainty, of course, that this rate of production has been maintained throughout geological time. In the opinion of Lord Rayleigh, we may safely assume that the rate of transformation has not changed, so that “it would seem that in the disintegration of a gram of uranium we have a process the rate of which can be relied upon to have been the same in the past as we now observe it to be” (_Nature_, October 27, 1921). Acting on Rutherford’s suggestion, the Hon. R. J. Strutt (later Lord Rayleigh) made a determination of the amount of radium in the superficial parts of the earth--which are alone accessible; and he also determined the ratio of the lead (isotope) to the uranium, which was found to be 1.3 (specifically, in the broggerite found in the Pre-Cambrian rocks at Moss, Norway). Now, if we assume--as the evidence seems to warrant--that the lead of this atomic weight (206.06) was all produced by uranium at the rate given above, we get an age of 925 million years for these rocks. Some minerals from other Archaean rocks in Norway give a rather larger figure. “In other cases,” says Lord Rayleigh, “there is some complication, owing to the fact that thorium is associated with uranium in the mineral and that it, too, produces helium and an isotrope of lead of atomic weight probably 208 exactly, about one unit higher than common lead.”