Why was tetramethyllead heated in the experiment that demonstrated methyl radicals?

In the 1929 experiment conducted by Friedrich A. Paneth and W. Hofeditz, tetramethyllead was heated to approximately 800° C to induce its thermal decomposition.1

This high-temperature decomposition was necessary to break the chemical bonds of the tetramethyllead, resulting in the formation of methyl radicals and the deposition of a metallic lead mirror on the internal surface of the silica tube. By heating the compound, the researchers were able to generate these highly reactive, short-lived species in the gas phase, which they subsequently identified by observing their ability to dissolve a second lead mirror located at a cooler point in the tube.1