But what surprised me more than I can well express was, that a candle burned in this air with a remarkably vigorous flame....

I was utterly at a loss how to account for it." [Illustration: FIG.

XVI.] The apparatus used by Priestley, in his experiments on different kinds of air, is represented in Fig.XVI., which is reduced from an illustration in Priestley's book on _Airs_.
Priestley had made a discovery which was destined to change Alchemy into Chemistry.

But he did not know what his discovery meant.

It was reserved for the greatest of all chemists, Antoine Lavoisier, to use the fact stumbled on by Priestley.
After some months Priestley began to think it possible that the new "air" he had obtained from calcined mercury might be fit for respiration.