How was the methyl radical first demonstrated by Friedrich Paneth and W. Hofeditz in 1929?

In 1929, Friedrich Paneth and W. Hofeditz demonstrated the existence of the methyl radical through an experiment involving the thermal decomposition of tetramethyllead, ${\text{Pb(CH}}_3{)}_4$.

The researchers passed vapors of tetramethyllead mixed with hydrogen through a silica tube at low pressure. When a section of the tube was heated to approximately 800°C, the tetramethyllead decomposed, depositing a mirror of metallic lead on the tube's internal surface.

The gaseous products of this decomposition were then shown to be capable of removing a second lead mirror deposited at a cooler, more distant point in the tube. Because no known stable decomposition product could dissolve the lead, the scientists inferred that the high-temperature process generated highly reactive methyl radicals. These radicals traveled to the cool mirror and reacted with the lead to regenerate tetramethyllead. This experiment confirmed that methyl radicals are short-lived, highly reactive species that disappear rapidly, largely by dimerizing into ethane, ${\text{H}}_3{\text{C–CH}}_3$.1