During Meselson and Stahl’s experiments, heavy DNA was distinguished from normal DNA by centrifugation in CsCl gradient. Caesium chloride is highly water soluble heavy salt. When spun in centrifuge at high speed (say 50,000 revolutions per minute) the salt forms a density gradient with heaviest most concentrated region at the bottom and successively less concentrated lighter one towards the surface. When DNA is mixed with caesium chloride it will settle down at a particular height in centrifugation, heavier one higher up.