Complex sentence: DuPont adopted it in 1939 and was their slogan until the 1980s when the "Through Chemistry" bit was dropped; in 1999 it was replaced by "The miracles of science".
Split into simpler sentences: DuPont adopted it in 1939 and was their slogan until the 1980s when the "Through Chemistry" bit was dropped. And in 1999 it was replaced by "The miracles of science".