 As reactants and products came to be routinely weighed, it became clear that metals gain weight when they become a calx. But according to the phlogiston theory, the calx involves the loss of phlogiston. Although the idea that a process involving the loss of a substance could involve the gain of weight seems strange to us, phlogiston theorists were not immediately worried. Some phlogiston theorists proposed explanations based on the ‘levitation’ properties of phlogiston, what Priestly later referred to as phlogiston’s ‘negative weight.’ Another explanation of the phenomenon was that the nearly weightless phlogiston drove out heavy, condensed air from the pores of the calx. The net result was a lighter product. Since the concept of mass did not yet play a central role in chemistry, these explanations were thought to be quite reasonable.