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Experimenting theory: The proofs of Kirchhoff's radiation law before and after Planck * This paper has grown out of a presentation to the Quantum Theory Centenary at Berlin in December 2000 and has benefited from comments by Olivier Darrigol, Klaus Hentschel, Dieter Hoffmann and Erhard Scholz. I would like to thank Michael Eckert and Ulrich Majer for reading of earlier drafts of the paper and for stimulating discussions.