both of the former and the latter.

THE AGE BY RADIOACTIVE TRANSFORMATIONS

Rutherford suggested in 1905 that as helium was continually being
evolved at a uniform rate by radioactive substances (in the form
of the alpha rays) a determination of the age of minerals
containing the radioactive elements might be made by measurements
of the amount of the stored helium and of the radioactive
elements giving rise to it, The parent radioactive substances
are--according to present knowledge--uranium and thorium. An
estimate of the amounts of these elements present enables the
rate of production of the helium to be calculated. Rutherford
shortly afterwards found by this method an age of 240 millions of
years for a radioactive mineral of presumably remote age. Strutt,
who carried

19

his measurements to a wonderful degree of refinement, found the
following ages for mineral substances originating in different
geological ages:

Oligocene - 8.4 millions of years.
Eocene - 31 millions of years.
Lower Carboniferous - 150 millions of years.
Archæan - 750 millions of years.

Periods of time much less than, and very inconsistent with, these
were also found. The lower results are, however, easily explained