During the eighteenth century, as it became clear that metals gained weight after they were oxidized, phlogiston was increasingly regarded as a "principle" rather than a material substance. By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to hydrogen. Joseph Priestley, for example, in referring to the reaction of steam on iron, while fully acknowledging that the iron gains weight after it binds with oxygen to form a calx, iron oxide, iron also loses "the basis of inflammable air (hydrogen), and this is the substance or principle, to which we give the name phlogiston". Following Lavoisier's description of oxygen as the "oxidizing principle" (hence its name, from Ancient Greek: , "sharp"; , "birth" referring to oxygen's supposed role in the formation of acids), Priestley described phlogiston as the "alkaline principle."