“I concluded that this subtle air did not arise from the copper, but from the spirit of salt; and presently making the experiment with the acid only, without any copper, or metal of any kind, this air was immediately produced in as great plenty as before; so that this remarkable kind of air is, in fact, nothing more than the vapour, or fumes of spirit of salt, which appear to be of such a nature that they are not liable to be condensed by cold, like the vapour of water and other fluids, and therefore may be very properly called an _acid air_, or more restrictively the _marine acid air_.” The new gas discovered by Priestley we now call hydrogen chloride. Ordinary hydrochloric acid is simply an aqueous solution of it. “Water impregnated with it makes the strongest spirit of salt that I have seen, dissolving iron with the most rapidity.... Iron filings, being admitted to this air, were dissolved by it pretty fast, half of the air disappearing and the other half becoming inflammable air, not absorbed by water. Putting chalk to it, fixed air was produced.” He subsequently found that the marine acid air was more conveniently made by the action of oil of vitriol upon common salt. From the “miscellaneous observations” with which this section of the volume concludes, there can be little doubt that Priestley, without knowing it, had prepared oxygen gas from nitre as far back as 1771. The accounts he gives of the behaviour of the gas obtained by heating nitre in a gun-barrel plainly indicate this fact. “A candle,” he says, “not only burned but the flame was increased, and something was heard like a hissing similar to the decrepitation of nitre in an open fire.” He also noted the effect of nitrous air upon it and concludes that “this series of facts relating to air extracted from nitre appear to me to be very extraordinary and important, and in able hands may lead to considerable discoveries.” 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.