Meselson and Stahl determined that DNA replication in E coli is semiconservative. What additive did they initially supply to the medium in order to distinguish "new" from "old" DNA?

Nitrogen 15, N15 isotope,

Explanation:

Generally Nitrogen has two isotopes N14 and N15. The N14 has 7 protons and  7 neutrons with the atomic mass  of 28( n+p). However the N15 has 7 protons and 8 neutrons.Since Neutrons are heavier than protons and constituted greater percentage of the atomic mass of an element the N15 isotope is heavier than the N14 isotope.

Therefore, Ammonium chloride solution  containing  the labeled  heavier N15 was initially added to the bacterial growing medium; so that the bacterial can use it to make their DNA, and divided manytimes so that N15 isotopes is fully incorporated  Therefore  the DNA strands which incorporated this will be heavier,(old) and easier to segregate to the lower band  during centrifugation , for identification compare to the newer lighter ones.

Thus the parent( heavy/old) containing DNA N15 and( New/light) in the daughter DNA N14 strands can  easily be separated for identification  during semi- conservative replication.