Henri Becquerel and the Curies helped to discredit what notion held for a very long time?

Henri Becquerel and the Curies helped to discredit the notion of "phlogiston," a hypothetical substance once believed to be released during combustion and associated with the idea that matter could only transform rather than change fundamentally. Their discoveries in radioactivity revealed that elements could emit energy and transform into different elements, demonstrating that matter could undergo significant and spontaneous changes. This challenged the long-standing views of chemistry and physics, leading to a deeper understanding of atomic structure and radioactivity.