Pctures of radioactive dating
First, the decay of what we now call the parent isotope is exponential.
Furthermore, the formation of the daughter isotope is exponential and inverse to the parent decay.
Even so, early work done by Bertram Borden Boltwood (1870-1927), an American chemist and physicist demonstrated several important consistencies.
First, the same Pb/U ratios are obtained in samples from the same geologic age.
The Curies found that the radiation was proportional to the Uranium content of the compound.
In 1896 Becquerel discovered that potassium uranyl sulfate (K2UO2 (SO4)2 did create energy that penetrated black paper to expose photographic film.
Third, radioactive decay is proportional to the number of atoms present.