The Groundwork of the Universe, &c. 73 



The development of radioactivity has thrown light upon 

 another science : geology ; and it is now possible to calculate the 

 minimum period during which certain geological strata have 

 remained undisturbed. How this is done can be made clear by a 

 simple example. 



Suppose that into a sealed flask a (juantity of pure uranium-1 

 be introduced. In the course of its disintegration, this element 

 will give rise on the one hand to helium and on the other to lead, 

 as the end product of its degeneration. Since the process goes 

 on at a mathematically calculable rate, it will be possible at any 

 moment to ascertain how long it was since the experiment started, 

 provided that we know (a) the quantity of uranium-1 left 

 unchanged ; (6) the amount of helium or of lead ])resent. The 

 factor (6) tells how much uranium-1 has disintegrated ; and by 

 adding that to the value in (a) the total amount of uranium-1 

 originally present is found. Thus we know the amount of 

 uranium-1 originally present and we know also the quantity 

 which has disintegrated during the period of confinement in the 

 flask : these data suffice for the calculation of the period which 

 has elapsed since the flask was closed. 



Now in Nature, the flask is replaced by a mass of rock in 

 which some uranium is imbedded. We analyse this rock and 

 estimate in it the quantity of helium occluded in the pores of the 

 minei'al and we also determine the amount of lead present. Our 

 analytical results give us all the data required for the calculation 

 of the period which has elapsed since the uranium became 

 enclosed in the stratum. It should be noted that since some of 

 the helium may have diffused through the matei-ial of the rock, 

 and thus become lost, the period calculated from the uranium- 

 helium ratio in the rock is a minimum value. On the other hand, 

 when the age of the rock is determined from the uranium-lead 

 ratio in the mineral, it must be borne in mind that we are making 

 the assumption that all the lead present has.been derived from 

 uranium ; whereas actually there may have been some ordinary 

 lead present along with the uranium at the start. In the case of 



