Meselson and Stahl invented what new technique in their quest to prove that DNA replication is semi-conservative?

Meselson and Stahl devised the technique of density gradient centrifugation.

They grew bacteria in a medium containing a heavy isotope of nitrogen, 15N. After a few generations, some bacteria had DNA with only 15N in its bases. The researchers then placed some of these bacteria in a medium in which the only source of nitrogen was 14N. By centrifugation, the researchers could show that the DNA in the next generation of bacteria was intermediate in mass between DNA containing only 15N and DNA containing only 14N. This proved that the semi-conservative model was the correct model of DNA replication.