Principles of radiometric dating
is converted into another specific atom or isotope at a constant and known rate.
But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.
In the above table, note that the number is the mass number (the total number of protons plus neutrons).
Note that the mass number may vary for an element, because of a differing number of neutrons.
Before then, the Bible had provided the only estimate for the age of the world: about 6,000 years, with Genesis as the history book.
Hutton's theories were short on evidence at first, but by 1830 most scientists concurred that Noah's ark was more allegory than reality as they documented geological layering.