Prediction and the periodic table

The debate about the relative epistemic weights carried in favour of a theory by predictions of new phenomena as opposed to accommodations of already known phenomena has a long history. We readdress the issue through a detailed re-examination of a particular historical case that has often been discussed in connection with it-that of Mendeleev and the prediction by his periodic law of the three 'new' elements, gallium, scandium and germanium. We find little support for the standard story that these predictive successes were outstandingly important in the success of Mendeleev's scheme. Accommodations played an equal role-notably that of argon, the first of the 'noble gases' to be discovered; and the methodological situation in this chemical example turns out to be in interesting ways different from that in other cases-invariably from physics-that have been discussed in this connection. The historical episode when accurately analysed provides support for a different account of the relative weight of prediction and accommodation-one that is further articulated here. (C) 2001 Elsevier Science Ltd. All rights reserved.