The second section of the volume deals with experiments and observations made in 1773 and the beginning of 1774, and opens with an account of the discovery of ammonia gas.
“After I had made the discovery of the marine acid air, which the vapour of spirit of salt may properly enough be called ... it occurred to me that by a process similar to that by which this acid air is expelled from the spirit of salt an alkaline air might be expelled from substances containing volatile alkali.
“Accordingly I procured some volatile spirit of sal ammoniac, and having put it into a thin phial, and heated it with the flame of a candle, I presently found that a great quantity of vapour was discharged from it; and being received in a vessel of quicksilver, standing in a basin of quicksilver, it continued in the form of a transparent and permanent air, not at all condensed by cold; so that I had the same opportunity of making experiments upon it as I had before on the acid air, being in the same favourable circumstances.... Wanting, however, to procure this air in greater quantities, and this method being rather expensive, it occurred to me that alkaline air might probably be procured, with the most ease and convenience, from the original materials, mixed in the same proportions that chemists had found by experience to answer the best for the production of the volatile spirit of sal ammoniac. Accordingly I mixed one-fourth of pounded sal ammoniac with three-fourths of slaked lime; and filling a phial with the mixture, I presently found it completely answered my purpose. The heat of a candle expelled from this mixture a prodigious quantity of alkaline air; and the same materials ... would serve me a considerable time without changing....”
He next studied the properties of the alkaline air. He found, of course, it was readily soluble in water.
“Having satisfied myself with respect to the relation that alkaline air bears to water, I was impatient to find what would be the consequence of mixing this new air with the other kinds with which I was acquainted before, and especially with acid air; having a notion that these two airs, being of opposite natures, might compose a neutral air, and perhaps the very same thing with common air. But the moment that these two kinds of air came into contact a beautiful white cloud was formed, and presently filled the whole vessel in which they were contained.... When the cloud was subsided there appeared to be formed a solid white salt, which was found to be the common sal ammoniac, or the marine acid united to the volatile alkali....
“Fixed air admitted to alkaline air formed oblong and slender crystals.... These crystals must be the same thing with the volatile alkalis which chemists get in a solid form by the distillation of sal ammoniac with fixed alkaline salts....
“Alkaline air, I was surprised to find, is slightly inflammable....
“That alkaline air is lighter than acid air is evident from the appearances that attend the mixture, which are indeed very beautiful. When acid air is introduced into a vessel containing alkaline air, the white cloud which they form appears at the bottom only and ascends gradually. But when the alkaline air is put to the acid the whole becomes immediately cloudy quite to the top of the vessel.”
Up to now Priestley had mainly confined himself to the narration of the new facts which he had discovered, barely mentioning any hypotheses that occurred to him.
“The reason why I was so much upon my guard in this respect was lest, in consequence of attaching myself to any hypothesis too soon, the success of my future inquiries might be obstructed. But subsequent experiments having thrown great light upon the preceding ones, and having confirmed the few conjectures I then advanced, I may now venture to speak of my hypotheses with a little less diffidence. Still, however, I shall be ready to relinquish any notions I may now entertain if new facts should hereafter appear not to favour them.”
In a paper on “Common Air Diminished and made Noxious by Various Processes” he attempts to apply the current doctrine of phlogiston to account for the various phenomena he has observed, and with what success may be inferred from his conclusion
“that in the precipitation of lime by breathing into lime-water, the fixed air, which incorporates with lime, comes not from the lungs but from the common air, decomposed by the phlogiston exhaled from them, and discharged, after having been taken in with the aliment, and having performed its function in the animal system.”
Priestley’s attempts at theorising brought little satisfaction to him or to his readers. Indeed he says:—
“I begin to be apprehensive lest, after being considered as a dry experimenter, I should pass into the opposite character of a visionary theorist.... In extenuation of my offence let it, however, be considered that theory and experiments necessarily go hand-in-hand, every process being intended to ascertain some particular hypothesis, which, in fact, is only a conjecture concerning the circumstances or the cause of some natural operation; consequently that the boldest and most original experimenters are those who, giving free scope to their imaginations, admit the combination of the most distant ideas; and that though many of these associations of ideas will be wild and chimerical, yet that others will have the chance of giving rise to the greatest and most capital discoveries, such as very cautious, timid, sober and slow-thinking people would never have come at.
“Sir Isaac Newton himself, notwithstanding the great advantage which he derived from a habit of patient thinking, indulged bold and eccentric thoughts, of which his queries at the end of his book of Optics are a sufficient evidence. And a quick conception of distant analogies, which is the great key to unlock the secrets of Nature, is by no means incompatible with the spirit of perseverance in investigations calculated to ascertain and pursue those analogies.”