(The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight.
Some isotopes are stable, but some are unstable or radioactive.
In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling.
It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.
Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.
These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.
After estimating the rate of this radioactive change, he calculated that the absolute ages of his specimens ranged from 410 million to 2.2 billion years.
Though his figures were too high by about 20 percent, their order of magnitude was enough to dispose of the short scale of geologic time proposed by Lord Kelvin. This device separates the different isotopes of the same element and can measure the variations in these isotopic abundances to within one part in 10,000.