To completely disprove the possible contention that nitre was necessary for the production of “fire-air,” some “calx of mercury” (or red oxide), which had been prepared by boiling mercury for a long time in contact with air, was heated. The products were metallic mercury and “fire-air”. “This is a remarkable circumstance, that the fire-air which had previously removed from the mercury its phlogiston in a slow calcination, gives the same phlogiston up to it again, when the calx is simply made red-hot.”[19] Is it not remarkable that the true explanation should not have forced itself upon Scheele’s mind, which was so acute, and so capable of forming true deductions? The next set of experiments dealt with the phenomena of respiration. A rat, confined in air until it died, polluted the air with one-thirtieth of aerial acid. Respiration from Scheele’s own lungs had the same effect. A few flies, bees, and caterpillars also polluted the air in the same way. Peas, roots, herbs, and flowers all converted about one-fourth part of ordinary air into “aerial acid”. “These are accordingly strange circumstances, that the air is not noticeably absorbed by animals endowed with lungs, contains in it very little aerial acid, and yet extinguishes fire. On the other hand, insects and plants alter the air in exactly the same way, but still they convert the fourth part of it into aerial acid.”[20] And so he makes experiments which prove that it is the fire-air which is converted into “aerial acid” by peas; and that “fire-air” is absorbed by fresh blood, and acquires no aerial acid from it. And, further, he was able to breathe fire-air for a long time, especially if a “handful of potashes” was put into the bladder. A couple of large bees, confined in “fire-air,” along with milk of lime, consumed practically the whole of the air in eight days. But plants, confined in “fire-air,” along with milk of lime, would not grow; however, they yielded a little aerial acid. Scheele is again puzzled here by the circumstance that the blood and the lungs have not the same action on air as insects and plants, inasmuch as the former convert it into vitiated air, and the latter into aerial acid. We now know that air will not support life of warm-blooded animals when the oxygen falls below a certain not very small amount, while insects appear to be capable of exhausting the oxygen to a great extent; and it is probable that the plants, under the unnatural circumstances in which they were placed, gave off a considerable amount of carbon dioxide. Scheele’s explanation in terms of phlogiston is not successful. He wrote:--“I am inclined to believe that fire-air consists of a subtle acid substance united with phlogiston, and it is probable that all acids derive their origin from fire-air. Now if this air penetrates into plants, these must attract the phlogiston, and consequently the acid, which manifests itself as aerial acid, must be produced.”[21]