Developed under Johann Joachim Becher and Georg Ernst Stahl, phlogiston theory was an attempt to account for products of combustion.[87] According to the theory, a substance called phlogiston was released from inflammable materials through burning. The resulting product was termed calx, which was considered a 'dephlogisticated' substance in its 'true' form.[88] The first strong evidence against phlogiston theory came from pneumatic chemists in Britain during the later half of the 18th century. Joseph Black, Joseph Priestley and Henry Cavendish all identified different gases that composed air; however, it was not until Antoine Lavoisier discovered in the fall of 1772 that, when burned, sulphur and phosphorus “gain[ed] in weight”[87] that the phlogiston theory began to unravel.