How did Meselson and Stahl rule out the conservative model of DNA replication using equilibrium ...

First, they grew E. coli cells in a medium containing only high-density nitrogen (15N). Then they allowed the cells to undergo successive rounds of replication in the presence of normal density nitrogen (14N). They used equilibrium density gradient centrifugation after each round of replication to separate the DNA produced by density. If the conservative model was correct, they would have found no intermediate-density DNA after a round of replication. However, after one round of replication, they found that the entire amount of DNA had a density exactly intermediate between 14N and 15N. Subsequent rounds of the experiment showed that semiconservative replication was the correct model.