“That candles will burn only a certain time in a given quantity of air is a fact not better known than it is that animals can live only a certain time in it; but the cause of the death of the animal is not better known than that of the extinction of flame in the same circumstances; and when once any quantity of air has been rendered noxious by animals breathing in it as long as they could, I do not know that any methods have been discovered of rendering it fit for breathing again. It is evident, however, that there must be some provision in Nature for this purpose, as well as for that of rendering the air fit for sustaining flame; for without it the whole mass of the atmosphere would, in time, become unfit for the purpose of animal life; and yet there is no reason to think that it is, at present, at all less fit for respiration than it has ever been. I flatter myself, however, that I have hit upon two of the methods employed by Nature for this great purpose. How many others there may be I cannot tell.” One of these methods he eventually finds to be, as in the first case, the action of vegetation, and he proves by a number of decisive experiments “that plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing and tend to keep the atmosphere sweet and wholesome when it is become noxious in consequence of animals either living and breathing, or dying and putrefying in it.” The other method he conceived to be the action of water, since he found that by vigorous agitation with water, air which breathing had rendered noxious could again be breathed for a further period. “I do not think it improbable but that the agitation of the sea and large lakes may be of some use for the purification of the atmosphere, and the putrid matter contained in water may be imbibed by aquatic plants, or be deposited in some other manner.” When a confined volume of common air is placed in contact with a mixture of iron filings and sulphur made into a paste with water, a certain portion of the air is imbibed by the paste. This fact was first observed by Hales. Priestley repeated the observation and found that about a fifth or rather more of the volume of the air was thus absorbed. He noted that the residual “air” was rather lighter than common air, it had no action on lime-water and was exceedingly noxious to animals, by which is meant that it could not be breathed by them. Priestley had thus prepared nitrogen, but he failed to recognise the individuality of this gas.