How did ethyl radicals demonstrate their presence with zinc and antimony mirrors?

The existence of ethyl radicals is demonstrated by their high reactivity, which allows them to dissolve metallic mirrors of zinc and antimony.1

When ethyl radicals—typically generated from the decomposition of tetraethyllead—come into contact with these metal mirrors, they react to form stable ethyl derivatives, specifically diethylzinc (${\text{Zn(C}}_2{\text{H}}_5{)}_2$) and triethylantimony (${\text{Sb(C}}_2{\text{H}}_5{)}_3$). The disappearance of the metal mirror serves as evidence of the radicals' presence, as these reactive species are otherwise too short-lived for direct detection. This experimental approach mirrors the earlier method used by Friedrich Paneth and W. Hofeditz to identify methyl radicals using lead mirrors.12