Explanation

In Meselson and Stahl’s experiment, bacterial cells were grown for several generations in a medium containing a heavy isotope of nitrogen $(N^{15})$ and then, were transferred to a new medium containing the normal lighter isotope $(N^{14})$. At various times thereafter, samples of bacteria were collected and their DNA was dissolved in a solution of cesium chloride, which was spun rapidly in a centrifuge. Because the cesium ion is so massive, it tends to settle towards the bottom of the spinning tube, establishing a gradient of caesium density.