After a reference to a hypothesis of the origin and constitution of the atmosphere which occurs among the “Queries, Speculations and Hints” above referred to, and which is on a par with much in Priestley’s speculations, he proceeds to relate the circumstances which more immediately led to the most important of all his discoveries. It was the accident of possessing a burning lens of “considerable force,” for want of which he could not possibly make many of the experiments that he had projected.
“But having afterwards procured a lens of twelve inches diameter and twenty inches focal distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield, putting them into vessels [short, wide, round-bottomed phials], which I filled with quicksilver and kept inverted in a basin of the same. Mr Warltire, a good chemist, and lecturer in Natural Philosophy, happening to be at that time in Calne, I explained my views to him, and was furnished by him with many substances, which I could not otherwise have procured.
“With this apparatus, after a variety of other experiments, an account of which will be found in its proper place on the 1st August 1774, I endeavoured to extract air from mercurius calcinatus per se;[17] and I presently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle burns in nitrous air exposed to iron or liver of sulphur,[18] but as I had got nothing like this remarkable appearance from any kind of air besides this particular modification of nitrous air, and I knew no nitrous acid was used in the preparation of mercurius calcinatus, I was utterly at a loss how to account for it.
“In this case also, though I did not give sufficient attention to the circumstance at that time, the flame of the candle, besides being larger, burned with more splendour and heat than in that species of nitrous air; and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre, and it consumed very fast; an experiment which I had never thought of trying with nitrous air.
“At the same time that I made the above-mentioned experiment I extracted a quantity of air with the very same property from the common red precipitate[19] which, being produced by a solution of mercury in spirit of nitre (nitric acid), made me conclude that this peculiar property, being similar to that of the modification of nitrous air above mentioned, depended upon something being communicated to it by the nitrous acid; and since the mercurius calcinatus is produced by exposing mercury to a certain degree of heat, where common air has access to it, I likewise concluded that this substance had collected something of nitre, in that state of heat, from the atmosphere.
“This, however, appearing to me much more extraordinary than it ought to have done, I entertained some suspicion that the mercurius calcinatus on which I had made my experiments, being bought at a common apothecary’s, might, in fact, be nothing more than red precipitate; though, had I been anything of a practical chemist, I could not have entertained any such suspicion. However, mentioning this suspicion to Mr Warltire, he furnished me with some that he had kept for a specimen of the preparation, and which, he told me, he could warrant to be genuine. This being treated in the same manner as the former, only by a longer continuance of heat, I extracted much more air from it than from the other.
“This experiment might have satisfied any moderate sceptic; but, however, being at Paris in the October following, and knowing that there were several very eminent chemists in that place, I did not omit the opportunity, by means of my friend Mr Magellan, to get an ounce of mercurius calcinatus prepared by Mr Cadet, of the genuineness of which there could not possibly be any suspicion; and at the same time I frequently mentioned my surprise at the kind of air which I had got from this preparation to Mr Lavoisier, Mr le Roy, and several other philosophers, who honoured me with their notice in that city, and who, I daresay, cannot fail to recollect the circumstance.”
This last remark is significant in reference to a claim which was subsequently put forward that the real discoverer of oxygen was Lavoisier, and that he obtained it by heating mercuric oxide.[20]
Priestley also obtained the same air from red lead, which, he says,
“confirmed me more in my suspicion that the mercurius calcinatus must get the property of yielding this kind of air from the atmosphere, the process by which that preparation and this of red lead is made being similar. As I never make the least secret of anything that I observe, I mentioned this experiment also, as well as those with the mercurius calcinatus and the red precipitate, to all my philosophical acquaintance at Paris and elsewhere, having no idea, at that time, to what these remarkable facts would lead.” [Nitrous oxide.]
Priestley, on his return to England, made an experiment with Cadet’s preparation, which he found to behave precisely like that he had procured from Warltire. He observed that the new gas was only sparingly soluble in water and that its power of causing a candle to burn with a strong flame was in nowise diminished by agitation with water—facts which he said convinced him
“that there must be a very material difference between the constitution of the air from mercurius calcinatus and that of phlogisticated nitrous air, [nitrous oxide] notwithstanding their resemblance in some particulars.”
It was not, however, until the following March (1775) (he having meanwhile been intent upon his experiments on the vitriolic air [sulphur dioxide]), that he ascertained the real nature of the new air, and was led “though very gradually ... to the complete discovery of the constitution of the air we breathe.” By trials with the nitrous air and with mice he found that the new gas was eminently fit for respiration: nitrous air reduced its volume to a greater extent than in the case of common air, and a mouse lived longer in it than it would in the same volume of common air.
“Thinking of this extraordinary fact upon my pillow, the next morning I put another measure of nitrous air to the same mixture, and to my utter astonishment found that it was farther diminished to almost one-half of its original quantity.”
Priestley now utterly missed his way for a time. He sought to get the new air from the various oxides of lead, but the fetish of phlogiston again led him wrong, and eventually by a train of reasoning which is fully set forth in the paper, but which need not here be repeated, there remained, he says, no doubt in his mind