How did Meselson and Stahl provide evidence for semi-conservative replication?

One sample of bacteria was grown in a nutrient broth containing light nitrogen (N¹⁴) and one was grown in a broth with heavy nitrogen (N¹⁵). The bacteria took up the nitrogen to make new nucleotides so it was incorporated into their DNA. A sample of each was taken and centrifuged, and the DNA grown in the heavy nitrogen settled lower in the tube than the light nitrogen did. The DNA grown in the heavy nitrogen broth was taken and replicated again but this time in the light nitrogen. When centrifuged, it settled at a point in between where the heavy and light nitrogen did, showing that it kept a strand of the old DNA and created a new strand with the light nitrogen. Therefore, the DNA replicated semi-conservatively.