Avogadro and others postulated the existence of radicals early in the nineteenth century. Unfortunately, they did not fully understand how radicals could exist in nature and therefore they incorrectly proposed structures and mechanisms of formation. Due to this lack of understanding, at the end of the century it was fairly well established that radicals could not exist. Chemists did not have real evidence of the existence of radicals until the early twentieth century when Moses Gomberg discovered the triphenyl methyl radical. He proved this radical could exist with evidence based on reaction characteristics including color changes, molecular weight determination, and the specie's reactivity toward iodine, oxygen and nitric oxide. Still, his discovery was initially met with skepticism from his peers. Additional evidence was uncovered by F. Paneth in 1929 when he found experimental proof that tetramethyllead (Pb(CH3)4 generates radicals as well. Eventually enough evidence was collected that convinced chemists that free radicals do exist and that they do participate in reactions.