This paper reconstructs what may have led the American professorof chemistry andnatural philosophy John William Draper to introduce a new kind ofradiation, whichhe dubbed `Tithonic rays''. After presenting his and earlierempirical findings onthe chemical action of light in Section 3, I analyze his pertinentpapers in Section 4with the aim of identifying the various types of argumentshe raised infavor of this new actinic entity (or more precisely, this newnatural kind of raybesides optical, thermal and perhaps also phosphorogenic rays).From a modernperspective, all (...) of these obviously belong within theelectromagnetic spectrum,but not so for many thinkers of the 19th century. I close withremarks about whyDraper''s interpretation was abandoned in the second half of the19th century (hehimself recanting only in 1872), and why I think such a naturalhistory ofargumentation (as one might call my approach in Section 4) may beuseful for acomparison-oriented history of science. (shrink)