In a classic experiment that demonstrated the semiconservative nature of DNA replication, cells were grown first in 14N-containing media and later in 15N-containing media, and DNA was separated by

Cesium chloride density gradient centrifugation

Explanation:

Centrifugation is a technique that separates the one type of molecules from the others based on some physical properties such as density, shape, molecular weight, etc.

Meselson and Franklin Stahl grew the E coli cells in the N-14 and N-15 medium. They isolated the DNA from E coli cells and separated the double helices by centrifugation in a cesium chloride density gradient.

The technique of density gradient centrifugation separates the heavy-heavy, light-light, and heavy-light DNA double helices into distinct bands as they differ from each other with respect to density.