We are told by Priestley that, when he was in Paris in 1774, he exhibited the method of obtaining dephlogisticated air from _red precipitate_ to Lavoisier and other French chemists. We shall see hereafter what important results to science followed from this visit to Lavoisier. Let us shortly review Priestley's answer to the question, "What happens when a substance burns in air?" Beginning to make chemical experiments when he had no knowledge of chemistry, and being an extremely rapid worker and thinker, he naturally adopted the prevalent theory, and as naturally interpreted the facts which he discovered in accordance with this theory. When a substance burns, phlogiston, it was said, rushes out of it. But why does rapid burning only take place in air? Because, said Priestley, air has a great affinity for phlogiston, and draws it out of the burning substance. What then becomes of this phlogiston? we next inquire. The answer is, obviously it remains in the air around the burning body, and this is proved by the fact that this air soon becomes incapable of supporting the process of burning, it becomes phlogisticated. Now, if phlogisticated air cannot support combustion, the greater the quantity of phlogiston in air, the less will it support burning; but we know that if a substance is burnt in a closed tube containing air, the air which remains when the burning is quite finished at once extinguishes a lighted candle. Priestley also proved that an air can be obtained by heating _red precipitate_, characterized by its power of supporting combustion with great vigour. What is this but common air completely deprived of phlogiston? It is dephlogisticated air. Now, if common air draws phlogiston out of substances, surely this dephlogisticated air will even more readily do the same. That it really does this Priestley thought he had proved by his experiment with clean iron nails (see p. 60). Water was regarded as a substance which, like air, readily combined with phlogiston; but Priestley thought that a candle burned less vigorously in dephlogisticated air which had been shaken with water than in the same air before this treatment; hence he concluded that phlogiston had been taken from the water. After Cavendish had discovered (or rather rediscovered) hydrogen, and had established the fact that this air is extremely inflammable, most chemists began to regard this gas as pure or nearly pure phlogiston, or, at least, as a substance very highly charged with phlogiston. "Now," said Priestley, "when a metal burns phlogiston rushes out of it; if I restore this phlogiston to the metallic calx, I shall convert it back into the metal." He then showed by experiment that when calx of iron is heated with hydrogen, the hydrogen disappears and the metal iron is produced.