He surmised that this “air” must come from the iron, and yet, he noted, it differed from the ordinary inflammable air from iron by the remarkable blue colour of its flame, and he concludes that “this inflammable principle may come from some remains of the animals from which it is thought that all calcareous matter proceeds.” Priestley, we now know, had incidentally converted some of the fixed air into the only other oxide of carbon, but he failed to appreciate the significance of his observation, and the credit of the discovery of carbon monoxide belongs to Cruikshank. In his next paper on “Air in which Candles have burned,” Priestley made a discovery of the very highest importance. He had attempted to verify without success the allegation by the Count de Saluce, made in the memoirs of the Philosophical Society of Turin, that air vitiated by the combustion of candles could be restored by exposure to cold. “Though this experiment failed,” he says, “I have been so happy as by accident to have hit upon a method of restoring air which has been injured by the burning of candles, and to have discovered at least one of the restoratives which Nature employs for this purpose. It is _vegetation_. This restoration of vitiated air, I conjecture, is effected by plants imbibing the phlogistic matter with which it is overloaded by the burning of inflammable bodies. But whether there be any foundation for this conjecture or not, the fact is, I think, indisputable.” He then proceeds to give an account of his observations on the growing of plants in confined air which led to his discovery. “One might have imagined,” he says, “that since common air is necessary to vegetable as well as to animal life, both plants and animals had affected it in the same manner; and I own I had that expectation when I first put a sprig of mint into a glass jar standing inverted in a vessel of water: but when it had continued there for some months I found the air would neither extinguish the candle, nor was it at all inconvenient to a mouse, which I put into it.... Finding that candles would burn very well in air in which plants had grown a long time, and having had some reason to think that there was something attending vegetation which restored air that had been injured by respiration, I thought it was possible that the same process might also restore the air that had been injured by the burning of candles. “Accordingly, on the 17th of August 1771, I put a sprig of mint into a quantity of air in which a wax candle had burned out, and found that on the 27th of the same month another candle burned perfectly well in it. This experiment I repeated, without the least variation in the event, not less than eight or ten times in the remainder of the summer.