The chemical analysis of rocks and minerals
The French physicist Antoine Henri Becquerel wondered if X rays might be among the energy given off by fluorescent materials. In Becquerel discovered that potassium uranyl sulfate K2UO2 SO4 2 did create energy that penetrated black paper to expose photographic film. To his surprise the compound could do this even when it was not fluorescing. This energy did not only have penetrating power it also ionized the air as shown by an electroscope.
In the Polish-French physicist team of Marie Sklodowska Curie and her husband Pierre discovered that thorium also gave off radiation.
The phenomenon of emitting penetrating, ionizing radiation was given the name radioactivity by the Curies. The Curies found that the radiation was proportional to the Uranium content of the compound.
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This indicated that radioactivity was a property of atoms, not molecules. They also discovered that compounds containing U and Th emitted more radiation than either element by itself. The Curies deduced that other radioactive elements must be present, which led to their discovery of the elements polonium and radium Dalrymple, , p In the New Zealander-English physicist Ernest Rutherford and the English chemist Frederick Soddy published results from their experiments with radioactivity.
Rutherford and Soddy made four important observations.
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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. Second, atoms of radioactive elements are unstable and spontaneously decay to other elements by the emission of alpha and beta particles.
Radiometric dating
Thus the transmutation medieval alchemist desired was in fact being done by nature continuously. Third, radioactive decay is proportional to the number of atoms present. Finally, helium might be a product of radioactive decay Dalrymple, , pp In Rutherford suggested that radioactive decay could be used as a method to calculate absolute time. They were done without knowledge of isotopes or the decay rates and intermediate products of Uranium.
Experimental study of rocks
Furthermore, the decay of Thorium to lead was unknown. Even so, early work done by Bertram Borden Boltwood , an American chemist and physicist demonstrated several important consistencies. Finally, as Arthur Holmes , a British geologist pointed out, ages obtained using these new methods were often in agreement with what many geologists had speculated Dalrymple, , pp. The fact that this new method of dating utilized empirical evidence gathered in the field and analyzed in the lab with methods based upon experimentally determined physical principles made it promising and powerful.
It soon became clear that the Earth was much older than many of the earlier methods indicated. His observations and calculations put Earth's age at 2. This was a dramatic increase in the estimate of Earth's age for the time.
Argon–argon dating - Wikipedia
Boltwood's basic idea and technique have been used ever since , but advances in technology and knowledge of atomic structure have shown the earth to be even older. Uranium decay is so slow it can indicate geologic time. Boltwood's reasoning holds true for other radioactive elements such as carbon , which has been used to date artifacts within human history.