From the discovery of radioactivity to the development of the K-Ar dating method

In this paper I try to explain why the potassium-argon dating method was developed much later than other radiometric methods (like U-He and U-Pb), which were established at the beginning of the 20^th century. In fact the pioneering paper by Aldrich and Nier (1948) was published 50 years after the discovery of polonium and radium, when nearly all the details concerning potassium isotopes and radioactivity of potassium-40 had been investigated. The role of Marie Curie’s concept of the nature of radioactivity in the discovery of the radioactivity of potassium is emphasized.