Experiments and Observations on Different Kinds of Air
Air is not one thing: one part of it feeds every flame and every breath.
Heat a pinch of red powder with sunlight, and out comes an invisible gas that makes a candle blaze and a mouse thrive — the gas we now call oxygen.
The big idea
For thousands of years people thought air was a single, simple thing — one of the four "elements." Joseph Priestley showed it is not. By heating a red powder (an oxide of mercury) with a lens that focused the sun, he captured a new kind of air in which a candle burned with a startlingly bright flame and mice lived far longer than usual.
That one component of ordinary air — about a fifth of it — is what fire and breathing both need. A flame and a lung, it turns out, are after the same thing.
How it came about
Priestley was a Dissenting minister and a tireless experimenter who had already bottled and described a whole menagerie of gases, and invented soda water along the way. On 1 August 1774, working at Calne in Wiltshire, he aimed his big burning lens at red oxide of mercury and was astonished by the flame the released air produced. Months later he plucked up the courage to breathe it, and joked that this "pure air" might one day become a fashionable luxury.
But Priestley explained his discovery with an old idea called phlogiston — a supposed fire-stuff — and got the story backwards. In Paris that October he described the experiment to Antoine Lavoisier, who saw what it really meant, named the gas oxygen, and built modern chemistry on it. A Swedish chemist, Carl Wilhelm Scheele, had quietly found the same gas even earlier. All three deserve a share; Lavoisier understood it best.
Why it mattered
Once you know that air is a mixture, and that one part of it drives both burning and breathing, the whole of chemistry opens up. You can explain rust and fire and why we breathe; you can weigh what a reaction takes in and gives off. Priestley's gas was the loose thread that, pulled, unravelled the ancient four-element view of the world.
A way to picture it
Think of air as a crowd in a room, only a fraction of whom are willing dance partners — that fraction is oxygen. A candle and a mouse are both looking for partners. In ordinary air they find enough. In Priestley's "dephlogisticated air" almost everyone will dance, so the flame roars and the mouse is full of energy. Seal them in a small room and let them use up the partners, and the dancing stops — the flame gutters out, the air goes stale.
Where it sits
Priestley stands at the doorway of modern chemistry. Behind him is the ancient idea of four elements; just ahead is Lavoisier weighing his way to the oxygen theory, and then Dalton's atoms and Mendeleev's periodic table, where oxygen takes its place as element number eight. His own discovery that plants renew air was an early glimpse of what we now call photosynthesis.
…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 pre-conceived theory in this business.
But what surprized 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.
The feeling of it to my lungs was not sensibly different from that of common air; but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury.
Hitherto only two mice and myself have had the privilege of breathing it.