The theory apart, this paper is as important as these on ammonia and the marine acid air, and exhibits Priestley at his best. The observations he makes concerning the main properties of the new gas and its solubility in water, its inability to burn and to support flame, its heaviness, its power to unite with ammonia, to be absorbed by charcoal and to liquefy camphor, are all accurate. “Having hit upon a method of exhibiting some of the acids in the form of air, nothing could be easier than to extend this process to the rest.” Accordingly he attempted to procure what he called the _vegetable acid air_ by heating “exceedingly strong concentrated acid of vinegar,” and states that he succeeded in obtaining an air which extinguished the flame of a candle and was soluble in water. The paper is very short and is full of contradictions. In reality, as he subsequently found, he was dealing with vinegar largely adulterated with oil of vitriol. The “vegetable acid air” had no real existence. The next paper in the series is the most important of the whole, and the one of all others that has contributed most largely to Priestley’s reputation. It is entitled “Of Dephlogisticated Air, and of the Constitution of the Atmosphere,” and deals with the discovery of oxygen. It begins in the following characteristic fashion:— “The contents of this section will furnish a very striking illustration of the truth of a remark which I have more than once made in my philosophical writings, and which can hardly be too often repeated, as it tends greatly to encourage philosophical investigations, viz., that more is owing to what we call _chance_—that is, philosophically speaking, to the observation of _events arising from unknown causes_ than to any proper _design_ or preconceived _theory_ in this business. This does not appear in the works of those who write _synthetically_ upon these subjects, but would, I doubt not, appear very strikingly in those who are the most celebrated for their philosophical acumen did they write _analytically_ and ingenuously.