Complex sentence: The philosophical concept that matter might be composed of units that cannot be further divided has been around for millennia, but the theory was mostly ignored until the work of John Dalton in the early 19th century.
Split into simpler sentences: The philosophical concept that matter might be composed of units that cannot be further divided has been around for millennia,. However, the theory was mostly ignored until the work of John Dalton in the early 19th century.