Like Wedgwood, Priestley was a core member of the Lunar Society, a small group of Midlands-based industrialists, intellectuals and 'natural philosophers' (the 18th century term for scientists), who used to meet at each full moon to discuss the experiments and inventions they were working on.
(There was nothing sinister about the timing of these meetings: in the days before street lighting, the full moon simply made it easier for the Society's members to find their way home in the early hours of the morning.)
None of the Lunar Society's members were professional scientists; in the late 1700s there was no such thing. 'Natural philosophy' was a fashionable hobby for anyone who had sufficient money, free time and intellectual curiosity to dedicate to it. Neither were their scientific experiments particularly rigorous, at least not compared with the way scientific research is conducted today. Experimentation was largely driven by an enthusiasm for creating unusual effects, rather than for patiently testing and refining theories.
As a result, experiments were often more showmanship than science. The painting that Chuffy! mentioned in the comments of my Wedgwood post depicts a natural philosopher 'performing' an experiment in which a bird in a jar is observed to fall unconscious when the air around it is sucked out with a pump. Similarly showy experiments demonstrating the strange and recently-discovered properties of electricity were also very popular.
Joseph Wright of Derby, An Experiment on a Bird in the Air Pump, 1768
The above painting slightly pre-dates Joseph Priestley's experiments on air, but it's easy to imagine Priestley in the role of Wright's natural philosopher.
Priestley performed hundreds of experiments involving air and the absence of it, using a succession of unfortunate mice to observe how long an animal could survive in conditions such as a sealed jar; a sealed jar with the air sucked out of it; a sealed jar with a mint plant growing in it; and, most famously, a sealed jar filled with a gas Priestley had created from burning mercury calx, in which a mouse miraculously survived for half an hour or more.
Despite the fact that these experiments earned Priestley the Copley Medal, the equivalent in its day of Nobel Prize, Priestley was barely aware of what he'd discovered. He thought he had created air that was free of 'phlogiston', a gas that medieval scientists believed was released by burning substances and which, when absorbed to saturation point by the surrounding air, caused fires to go out. Even though the phlogiston theory was already obsolescent in Priestley's time, he continued to believe in it, preventing him from seeing that what he had actually done was to create pure oxygen.
One of the many themes of Johnson's book is to question the 'Great Man' approach to history by showing that Priestley, in isolation, was in fact quite a rubbish scientist. He performed hugely important experiments and made hugely important discoveries, but consistently failed to apprehend their meaning. If Priestley had been operating in an intellectual vacuum, his contribution to scientific progress might have been negligible.
Fortunately, what he did very well was to share the information about his experiments with anyone who was interested. When Priestley discovered that, contrary to expectations, a mint plant not only flourished happily in a sealed jar but also allowed a mouse sealed in the same jar to stay alive for a surprisingly long time, it was his chum Benjamin Franklin who realised that Priestley had stumbled upon something of fundamental importance to the way our planet works:
"That the vegetable creation should restore the air which is spoiled by the animal part of it looks like a rational system, and seems to be of a piece with the rest."Franklin glimpsed in the experiment something that Priestley couldn't see: that plants and animals need each other to survive, not just in tiny microcosmic environments like Priestley's sealed jar, but also on a planetary scale. If he'd thought about it any more, Franklin might have anticipated ecosystem theory by about two hundred years, but he was too bound up with the revolutionary politics of the nascent United States to devote much time to thinking about Priestley's mint experiments.
The fact that both Franklin and Priestley were as active politically as they were scientifically gives Johnson's book its major theme: how the interconnectedness of different disciplines (science, business, religion, politics) in the 18th century fuelled an explosion in scientific discovery, political progress and cultural advancement. Whereas today, scientists are scientists, politicians are politicians, captains of industry are captains of industry and religious leaders are religious leaders, during the Enlightenment it was eminently possible to be all of these things simultaneously - with each discipline informing and illuminating the others.
The free flow of ideas between disciplines is one of the reasons for the sudden flowering of science and culture in the late 18th century. In his book, Johnson also argues for the significance of geographical location, both at the micro scale (in the late 18th century, intellectuals from different walks of life congregated in the coffee-house opposite St Paul's cathedral, excitedly discussing their ideas while hopped up on this new imported brew) and the macro one (Johnson makes a lot of the fact that the Lunar Society was based adjacent to the coalfields of the north of England; the great store of energy laid down in the carboniferous era - thanks to the very process of photosynthesis that Priestley and Franklin had glimpsed - fuelling intellectual progress as well as the furnaces and engines of the world's first factories.)
Information flows, patterns repeating at different scales and the importance of geographical location are all classic Steven Johnson themes and are presented very well and very entertainingly in The Invention of Air. Which is just as well, because as a biography it's not great: Johnson seems far more interested in what Priestley symbolised than what Priestley was actually like. But perhaps this is deliberate - after all, Johnson doesn't subscribe to the Great Man view of history, preferring to see progress as a collaborative and often accidental process that owes more to information exchange and quirks of geography than to individual genius.
(Which incidentally makes a nonsense of the first two endorsements on the back cover of the book, praising Johnson for presenting Priestley as 'brilliant...one of the most fascinating personalities of his era' and as 'a new American hero'.)
It's a view that resonates well today: one of the things I like very much about Twitter, for example, is that you get intellectuals from different disciplines - including Steven Johnson himself - discussing their thoughts and ideas in a public forum where anyone can follow and contribute to the conversation, like an Information Age version of the London coffee-house.
Whether Twitter will give rise to a new Enlightenment remains to be seen, but one of the messages of this book is that the oxygen of information exchange can cause important ideas to grow and flourish, resulting in bursts of progress that change the world for the better. And I like that thought very much.