The discovery of the noble gases and their incorporation into the periodic system are examined in this paper. A chronology of experimental reports on argon and helium and the properties relevant to their nature and position in the periodic system is presented. Proposals on the nature of argon and helium that appeared in the aftermath of their discovery are examined in light of the various empirical and theoretical considerations that supported and contradicted them. ``The piece that would not fit'' refers (...) not only to argon, the element that at first seemed not to fit into the periodic system, but also to the piece or pieces of evidence that various researchers and observers were prepared to discard or discount in coming to terms with the newly discovered gases. (shrink)