<p>Mary and Michael Laing’s 2006 essay urging that Dulong and Petit’s law not be ignored has gone substantively unanswered for two decades. We argue that the answer requires more than the inclusion of one law in a curriculum that already includes many. The standard chemistry curriculum is built around what we call the cumulative axis—the historical sequence Dalton, Thomson, Rutherford, Bohr, Schrödinger—and the atom that Petit and Dulong introduced in 1819 does not lie on that axis. We develop a four-dimensional argument that the Petit–Dulong atom is generated by a methodologically distinct experimental practice, answers a different question (the identity question rather than the individuation question), refers as a different natural kind term in the sense of Stanford and Kitcher’s Causal Theory of Reference (CTR2), and is penetrated rather than absorbed by Einstein-Debye quantum refinement. We then redeploy Meyer and Land’s notion of the threshold concept in a deliberately inverted form, proposing the Petit–Dulong threshold as a diagnostic threshold concept whose function is not to be traversed by the learner but to render a curricular omission visible. The standard pedagogical reading of student conflict at this threshold as misconception is, on our analysis, a misdiagnosis: the conflict is the symptom of a structural curricular omission, not a learner deficit.</p>