Radiometric dating instruments
With the discovery of radiometric dating, it became possible for the first time to attempt precise figures.Radiometric dating works on the principle that certain atoms and isotopes are unstable.When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex fossil, her discovery raised an obvious question -- how the tissue could have survived so long?The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes.Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.
By the mid- to late 1800s, geologists, physicists, and chemists were searching for ways to quantify the age of the Earth.
During the 19th century, and even well into the twentieth, geological chronology was very crude.
Dates were estimated according to the supposed rate of deposition of rocks, and figures of several hundred million years were bandied out; usually arrived at through inspired guesswork rather than anything else.
Only hard parts, like bones and teeth, can become fossils.
But for some people, the discovery raised a different question.
Usually, atoms have an equal number of protons and neutrons.