Phlogistian

It [air] was liable, according to the phlogistians, to vitiation, by the addition to it of phlogiston, […]

Bryan Higgins was a "phlogistian", while William, correctly as it turned out, didn't believe in the concept.

In Davy's time it was still necessary in considering any novel fact to keep the phlogistian hypothesis in view, and to demonstrate its inability to explain the new discoveries.