AT the discussion on isotopes at the Royal Society on March 3 the question was raised as to within what limits of accuracy the conclusion is justified that the atomic volume of the various isotopes of lead is constant, and the following collected results may therefore be of interest. There are two sets of data. In one the density and atomic weight of lead from thorite have been compared with the values of ordinary lead, and in the other a similar comparison has been made for the lead derived from two uranium minerals. These two sets, of course, cannot be compared together, as the densities of specimens are comparable only when they have been prepared under identical conditions. With due attention to this point the relative densities are, in the case of lead, capable of determination to a very high degree of accuracy.