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This paper reconstructs what may have led the American professor of chemistry and natural philosophy John William Draper to introduce a new kind of radiation, which he dubbed ‘Tithonic rays’. After presenting his and earlier empirical findings on the chemical action of light in Section 3, I analyze his pertinent papers in Section 4 with the aim of identifying the various types of arguments he raised in favor of this new actinic entity (or more precisely, this new natural kind of ray besides optical, thermal and perhaps also phosphorogenic rays). From a modern perspective, all of these obviously belong within the electromagnetic spectrum, but not so for many thinkers of the 19th century. I close with remarks about why Draper's interpretation was abandoned in the second half of the 19th century (he himself recanting only in 1872), and why I think such a natural history of argumentation (as one might call my approach in Section 4) may be useful for a comparison-oriented history of science.