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.”