To decide this question, Scheele burned in air substances such as phosphorus, which do not produce by their combustion any kind of “air.” The result was that the air lost 9 volumes out of an original 30, or about one-third of its bulk. A flame of hydrogen burning in air caused it to lose one-fifth of its volume. On burning a candle, some spirits of wine, or some charcoal, in a confined quantity of air, very little, if any, diminution of volume was noticed; but on shaking the air with milk of lime, contraction ensued, but not to the same extent as when phosphorus was burnt in it. This greatly puzzled Scheele; we now know that such combustibles are not able to remove all the oxygen, but that they are extinguished when only a portion of each has entered into combination. Here, again, however, his memory comes to his help, for he says, “It is known that one part of aerial acid mixed with ten parts of ordinary air extinguishes fire; and there are here in addition, expanded by the heat of the flame and surrounding the latter, the watery vapours produced by the destruction of those oily substances. It is these two elastic fluids, separating themselves from such a flame, which present no small hindrance to the fire which would otherwise burn much longer, especially since there is here no current of air by means of which they can be driven away from the flame. When the aerial acid is separated from this air by milk of lime, then a candle can burn in it again, though only for a very short time.”[15] Thus the question was correctly solved. Scheele’s acumen led him at once to make experiments admirably adapted to discover the true reason; he was not turned aside by any imaginary difficulties, but went straight to the point. He next burned sulphur in confined air, and found little alteration of volume, but on shaking with clear lime-water, absorption took place, and one-sixth of the air was removed. “The lime-water was not in the least precipitated in this case, an indication that sulphur gives out no aerial acid during its combustion, but another substance resembling air; this is the volatile acid of sulphur, which occupies again the empty space produced by the union of the inflammable substance with air.”[16] The next set of experiments were devised “to prove that ordinary air, consisting of two kinds of elastic fluids, can be compounded again, after these have been separated from one another by means of phlogiston.”