In this book Chang seeks to question and challenge some of our most taken-for-granted assumptions about scientific knowledge, and its generation, dissemination and reception. Using episodes from the history of water, he re-evaluates, historically and philosophically, the scientific ‘fact’ that the substance we commonly recognize as water is really made up of two atoms of hydrogen and one atom of oxygen. The first three chapters focus on three landmark points in this history: the Chemical Revolution, during which water's compound nature was first advanced by Lavoisier; early electrochemical experimentation and the first electrolysis of water in 1800, following the invention of the voltaic pile; and the reforms in organic chemistry, which by the 1860s had resulted in the acceptance of modern molecular formulae such as H₂O. Hence from the mid-eighteenth century to the mid-nineteenth, water went from being one of the four elements to being H₂O. Or did it?

Chang's discussion strikes at the heart of the enduring knowledge problem in science: what is the status of scientific knowledge? Are scientific ideas and theories universal, necessary and certain or do they merely successfully explain, predict and control our experiences? What does it mean to know that water is H₂O? Based on the three case studies, Chang presents in the last two chapters a systematic philosophical and prescriptive statement in response to these questions. Espousing what he dubs ‘active scientific realism, which affirms that science should strive to maximize our contact with reality in order to learn as much as we can’ (p. xix), he recommends the pursuit of ‘active normative epistemic pluralism’, which in effect amounts to ‘having multiple systems of practice in each field of study’ (p. 253).

Chapter 1 re-evaluates the Chemical Revolution, one of the best-studied episodes in the history of science, and an exemplar of theoretical discontinuity and incommensurability in a distinct Kuhnian vein. Chang not only makes the case that phlogistic chemistry was not inferior to Lavoisier's oxygenist system, but claims that ‘there is no better case than the composition of water for illustrating the surprising cogency of the phlogiston theory’ (p. 6, original emphasis). Historians of science, and historians of chemistry in particular, should not be taken by surprise here, even though it is true that the simplistic opposition between ‘scientific’ oxygen and ‘irrational’ phlogiston still holds sway, especially in generalist accounts and in philosophy-of-science circles. Whereas advocates of oxygen viewed water as a compound of hydrogen and oxygen (two then newly discovered gases), for traditional chemists it was a combination of phlogisticated and dephlogisticated water. And, as Chang shows, chemists had perfectly good scientific reasons to believe in phlogiston, which they could recognize and manipulate experimentally, and which explained some phenomena far better than Lavoisier's oxygen and caloric did, particularly the emission of heat and light during combustion and the common properties of metals. The two systems had different strengths and weaknesses, advantages and limitations. Drawing on the important work of Robert Siegfried and Betty Jo Teeter Dobbs from the late 1960s, Chang rightly identifies the divergent ontological and epistemological commitments of the two camps. While oxygen made sense in the context of ‘practices of decomposition and recomposition’ in which weight changes were of primary significance, phlogiston worked well as part of ‘practices of transformation’ (p. 40, original emphasis).

In line with his active realism and pluralism, Chang proceeds to claim that phlogiston had been discarded prematurely, and that it would have been beneficial to keep it around alongside oxygen. It would have made for healthy pluralistic competition in chemistry, which in turn might ‘have given chemists a productive open end to start thinking about something like energy’ or otherwise encouraged ‘attempts to isolate the electric fluid from phlogiston-rich substances such as metals' (pp. 45–46). Chang does not wish to engage in counterfactual history; he wants us consider seriously the drawbacks of Lavoisier's ‘winning’ system, the advantages afforded by the ‘losing’ phlogiston, and the consequences to past and present chemistry of renouncing one in favour of the other. In this sense he paints a valuable picture of the ‘Kuhn-loss' entailed by the Chemical Revolution. Well aware of the dangers – as well as proclaimed advantages – of Whiggishness in the history of science, Chang outlines his position carefully in an attempt to defend it against potential knee-jerk charges. Although the challenge he poses to the Whiggishness phobia that has come to dominate our field remains problematic (and will fuel much welcome debate), it is worth stressing that Chang's ‘complementary’ effort is to determine ‘whether there were good reasons for abandoning the phlogistonist system, not only whether past scientists themselves thought so’ (p. 19).

The water controversy continued well into the nineteenth century, as Chang shows in Chapters 2 and 3. From antiquity to the late eighteenth century, water had been understood as an element. If the burning of hydrogen (or inflammable air) by Lavoisier in the early 1780s failed to prove decisively its compound nature, one might reasonably assume that the question was settled by the first electrolysis of water, performed in 1800 with a voltaic pile by William Nicholson and Anthony Carlisle. But as Chang argues, the ‘distance problem’ – the emergence of oxygen and hydrogen from electrodes that were separated by a macroscopic distance – turned water electrolysis into an anomaly rather than into positive evidence for its compound nature. Thus Johann Wilhelm Ritter could still argue that, while water was an element, electrolysis involved two syntheses. Similarly, a drawn-out and complex controversy in atomic chemistry plagued the question whether water was H₂O or HO, as Dalton had originally suggested.

This book builds on and extends Chang's ‘complementary-science’ project, which he introduced in his previous book Inventing Temperature: Measurement and Scientific Progress (2004). The attempt is to use the history and philosophy of science to recover and reassess neglected and discarded past theories and practices in order to complement scientific knowledge and enrich our views of how it should and could be attained. This is not an easy book to read, or to review. The argument is at once ambitious and challenging, detailed and sweeping, dense and somewhat elusive. Aiming to bring together approaches and insights from history, philosophy and scientific practice, Chang should be commended for his eagerness to confront big questions and to engage in a genuine interdisciplinary endeavour. Too many academic studies, while providing solid contributions to progressively narrowing fields, end up raising the perennial question – ‘so what?’ Whether you accept or reject Chang's high-risk, high-return type of work, this is unlikely to be your problem.