The compound he had placed in the cage was dichlorodiphenyltrichloroethane (DDT), or, more precisely, 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane, which a Viennese pharmacologist named Othmar Zeidler had first synthesized in 1874.
Müller quickly realized that DDT was the chemical he had been searching for.