In this paper, we revisit the discovery of argon by Lord Rayleigh and William Ramsay. We argue that to understand historically how argon was detected, conceptualized, and accommodated into chemical knowledge we need to take into account the philosophical insight that scientific discovery is often an extended process. One of argon’s most intriguing properties was that it did not react with other elements. Reactivity, however, had been a constitutive property of elements. Thus, the discovery of argon could not have been accepted by chemists without a reconceptualization of ‘element’. Furthermore, there were difficulties with the accommodation of argon in the Periodic table, because argon appeared to undermine the conception of matter that underlay the Periodic table. The discovery of argon was complete only after those conceptual difficulties had been removed. This is why it has to be understood as an extended process, rather than as an event. Furthermore, we will suggest that some of the factors that complicated the discovery of argon were related to the legitimization of physical techniques of investigation in chemistry and the emergence of physical chemistry.