I enjoyed this book even while I quickly came to distrust it. Although it wasn't one of my areas of specialization, I did some work on the history of science while in grad school and I even had a job transcribing the lectures of a prominent philosopher of the history of science. To supplement this I read a number of key books focusing on the history of the discipline.
The problem I have with this book is that it is misleading. To steal a phrase of Somerset Maugham (writing about himself), Joseph Priestley is a good scientist of the second rank. In virtually every account of the history of science or intellectual history he is regarded as a talented dilettante, a gifted amateur. He certainly played a role in the history of science, performing experiments that more important thinkers were able to utilize to further science, but Priestley himself frequently failed -- and Johnson does hint at this without emphasizing its significance -- to understand the full implications of the results of his experiments. He was extremely weak as a theoretician, which is why he is not accounted among the great scientists.
Why is this misleading? Well, historians of science do not regard Priestley as a key or even especially important figure. At no point does Johnson hint that this is the widespread assessment of Priestley's place. It is a tad misleading to state that his contemporaries had one opinion without proceeding to remark that their successors do not share that opinion. Johnson talks of Antoine Lavoisier and Joseph Priestley as the two leading chemists, but it is intensely deceptive to talk as if they were competitors for pride of place. Lavoisier is one of the great geniuses in the history of science. In fact, modern chemistry is usually credited with beginning with him.
Another example. Any credible account of the history of the theory of ecosystems is not going to begin or even include Joseph Priestley, but Johnson implies that the science began with him. This is a preposterous stretch.
In other words, the book is simply not reliable. It doesn't attempt to disclose the general opinion of Priestley's place in history by philosophers and historians of science. By leaving this all unsaid, he implies that Priestley was a much more important than in fact he was.
All of this is a tremendous disservice to Priestley, who while not a genius and not a scientist or thinker of the first rank, was unquestionably an immensely interesting and fascinating figure. The problem with the book is that it wants to go beyond this to portray Priestley as something that he was not. He definitely played a role in the growth of science. But he was not an Antoine Lavoisier.
Still, if one grasps this fundamental weakness in the book, it can be a fun and interesting lead. Much like another Englishman whose interests ran in all imaginable directions, the Rev. George Berkeley (who had a town adjacent to San Francisco named after him), he is an immensely likable individual. One is impressed by his passionate quest for knowledge, his generosity of spirit, his progressive attitudes, and his great goodheartedness. I'm not quite sure why Joseph Priestley as he actually was seemed inadequate to Johnson; I'm not sure why such a fundamentally sympathetic figure needed to be elevated to a pivotal figure in the history of science.
So I'm in a dilemma about this book. It is a fun and interesting read. And it does a good job of explaining why we should care about Joseph Priestley. Yet he outrageously exaggerates his place in thought. I had other problems with the book (some of his metaphors are stretched to the extreme), but this was the major one. It reminds me of various rock historians who try to make us believe that the Doors and Jim Morrison were the equal of the Beatles, the Who, and the Rolling Stones, whereas in fact they didn't even come up to the level of the Kinks.
I do completely agree with Johnson about one thing. The incredible narrowness of most supposedly educated people today is appalling. Johnson begins the book by quoting a former undergraduate classmate of mine, Mike Huckabee (who even in the couple of theology classes we had together at Ouachita Baptist University did not especially distinguished himself), who when running for president disdained the knowledge of science (actually, he was trying to avoid stating that he denied the validity of science). Modern science actually began among Christians who believed that the universe, as the creation of a rational God, had a logical, rational structure that his creatures, created in his image, could understand. Isaac Newton and Rene Descartes, for instance, were deeply religious and practicing Christians (Newton wrote far more on Christian prophecy, for instance, than he did on physics, while Descartes' entire project was to create a view of the world compatible with the Christian Platonism of Augustine rather than the Aristotelianism of Thomas) Aquinas. Both would have found Huckabee's irrationalism un-Christian. No doubt one of Huckabee's motives was to avoid alienating minimally educated individuals who would have found his no-nothingism grounds for disqualification in a presidential candidate. But it is also quite true that far too many people today do not strive to comprehend the world around them. I find Joseph Priestley's passion for knowledge to be both admirable and inspirational. But it doesn't elevate him to the level of the top rungs of science. He was not a Lavoisier. He was several rungs below a James Clerk Maxwell. And frankly I believe one of the disservices of the book was to make Priestley take on a role that does not befit him. As I said earlier, he was a good scientist of the second rank. He was, however, an absolutely outstanding human being.