  We follow the trajectory of John H. Van Vleck from his 1926 Bulletin for the
National Research Council (NRC) on the old quantum theory to his 1932 book, The
Theory of Electric and Magnetic Susceptibilities. We highlight the continuity
of formalism and technique in the transition from dealing with spectra in the
old quantum theory to dealing with susceptibilities in the new quantum
mechanics. Our main focus is on the checkered history of a numerical factor in
the Langevin-Debye formula for the electric susceptibility of gases. Classical
theory predicts that this factor is equal to 1/3. The old quantum theory
predicted values up to 14 times higher. Van Vleck showed that quantum mechanics
does away with this "wonderful nonsense" (as Van Vleck called it) and restores
the classical value 1/3. The Langevin-Debye formula thus provides an
instructive example of a Kuhn loss in one paradigm shift that was regained in
the next. In accordance with Kuhn's expectation that textbooks sweep Kuhn
losses under the rug, Van Vleck did not mention this particular Kuhn loss
anywhere in his 1926 NRC Bulletin (though he prominently did flag a Kuhn loss
in dispersion theory that had recently been regained). Contrary to Kuhn's
expectations, however, he put the regained Kuhn loss in susceptibility theory
to good pedagogical use in his 1932 book. Kuhn claimed that textbooks must
suppress, truncate, and/or distort the prehistory of their subject matter if
they are to inculcate the exemplars of the new paradigm in their readers. This
claim is not borne out in this case. Because of the continuity of formalism and
technique that we draw attention to that Van Vleck could achieve his
pedagogical objectives in his 1932 book even though he devoted about a third of
it to the treatment of susceptibilities in classical theory and the old quantum
theory in a way that matches the historical record reasonably well.
