THEORY OF PHLOGISTON The theory of phlogiston first proposed by Becher and greatly advanced by Stahl went back to the old idea of Jabir that com bustible bodies lost something when they burnt. Becher's terra pinguis became Stahl's phlogiston, which was not fire itself but the material of fire. Metals were composed of a calx, different for each metal, combined with phlogiston, which was the same in all metals and common to all combustibles. When a metal, such as lead or tin, was calcined in the air, phlogiston was evolved and a calx was left behind. When a candle or charcoal was burnt in the air it gave off phlogiston (manifest as heat and light) and very little residue was left : wax and charcoal mainly consisted of the material of fire. When a calx was heated with charcoal out of contact with air the phlogiston of the charcoal recombined with the calx and the metal was re-generated. In this way a gen eralization was made which accounted for the two opposite processes of oxidation and reduction, and moreover was able to predict certain chemical consequences which were verified later by experiment. The heat given off by animal bodies and its restoration by food were also explained by phlogiston. The fact that combustion and life ceased in a confined volume of air was due to the swift whirling motion of the phlogiston that filled the air, which when saturated could take no more. In considering the wide acceptance of Stahl's theory it must be borne in mind that it gave expression to the very striking facts that in com bustion something was emitted by the burning body, viz., heat, and in the reduction of a calx by charcoal something—potential energy, or the power of giving out heat again—was restored to it.