  The experiment is described schematically in Figure B. Meselson and Stahl placed a sample of DNA in a solution of cesium chloride. As the sample is rotated at high speed the denser material travels further away from the axis of rotation than does the less dense material. This results in a solution of cesium chloride that has increasing density as one goes further away from the axis of rotation. The DNA reaches equilibrium at the position where its density equals that of the solution. Meselson and Stahl grew E. coli bacteria in a medium that contained ammonium chloride \((\ce{NH4Cl})\) as the sole source of nitrogen. They did this for media that contained either \(\ce{^{14}N}\), ordinary nitrogen, or \(\ce{^{15}N}\), a heavier isotope. By destroying the cell membranes they could obtain samples of DNA which contained either  \(\ce{^{14}N}\) or \(\ce{^{15}N}\). They first showed that they could indeed separate the two different mass molecules of DNA by centrifugation (Figure C). The separation of the two types of DNA is clear in both the photograph obtained by absorbing ultraviolet light and in the graph showing the intensity of the signal, obtained with a densitometer. In addition, the separation between the two peaks suggested that they would be able to distinguish an intermediate band composed of hybrid DNA from the heavy and light bands. These early results argued both that the experimental apparatus was working properly and that all of the results obtained were correct. It is difficult to imagine either an apparatus malfunction or a source of experimental background that could reproduce those results. This is similar, although certainly not identical, to Galileo’s observation of the moons of Jupiter or to Millikan’s measurement of the charge of the electron. In both of those episodes it was the results themselves that argued for their correctness.