How did methyl radicals cause a lead mirror to disappear in the Paneth and Hofeditz experiment?

In the 1929 experiment conducted by Friedrich A. Paneth and W. Hofeditz, the disappearance of a lead mirror was caused by the chemical reaction between methyl radicals and metallic lead.1

During the experiment, tetramethyllead was decomposed by heat in a silica tube, which deposited a primary mirror of metallic lead on the tube's surface. The gaseous products of this decomposition were then passed over a second, distant lead mirror located at a cooler point in the tube. The methyl radicals generated in the high-temperature zone reacted with the lead in the second mirror to regenerate tetramethyllead, effectively causing the mirror to dissolve and disappear. This provided the first experimental demonstration of the existence of short-lived, highly reactive free radicals.12