Readers should be familiar with David Miller's extensive work on James Watt: his re-examination of the ‘true myth’ of that kettle, his consideration of the candidate ‘discoverers’ (including Watt) of the compound nature of water, his analysis of Watt's patent practice, and his investigation of Watt's exploits (with Thomas Beddoes) in pneumatic chemistry. James Watt, Chemist realigns and expands these studies to substantiate two chief claims. Rather than being a peripheral interest, chemistry was central to Watt's philosophical and practical concerns, and, in its creator's terms, Watt's steam engine should be understood chemically – with steam a chemical compound of water and (material) heat.

After a deft analytical biography, usefully supplementing Richard Hills's multivolume life, the book splits in two. ‘Representations’ recounts an orgy of portrayals of Watt in statues, images and texts as craftsman, philosophical engineer, ‘engineering scientist’, ‘proto-thermodynamicist’ and, more rarely, chemist. While Christine Macleod's masterly Heroes of Invention (2007) looked outwards to economy and politics, Miller peers inwards to the ‘knowledge ecology’ (p. 8). He finds that, as ‘mechanical’ representations of Watt's learned character triumphed in public, a private ‘chemical’ Watt – known to Watt's inner circle until about 1800 – was obscured. Why? Because Watt's chemistry (phlogistic in nature, accepting oxygen, only partially Lavoisierian) proved serviceable to few: supporters and detractors alike in the ‘water controversy’ found it unpalatable. Watt called himself an ‘engineer’ (defiantly, according to Miller in a careful gloss) before the Royal Society; he was a ‘philosopher’ when it came to business or reputation building. Especially from the 1790s, Watt was repeatedly recast for different audiences and interests. His death was of (refreshingly) little consequence: silencing but one narrator, it liberated many others, none of them invested in Watt as ‘chemist’.

‘Realities’, the title of the second part, seems to mean Miller's representations of realities as he reconstructs them (p. 3): we strip away the accreted myths to see the kernel we ourselves make. This part revives the chemical Watt from scattered fragments: a chemical apprenticeship in Glasgow with Black (their correspondence beginning in earnest only after Black left for Edinburgh); subsequent networks, notably with Priestley; pottery, bleaching and pneumatic medicine pervasive, bridging practice and principles; theoretical coherence and sophistication evident, at least relative to the frustratingly publication- and system-shy Black. Building upon notions of material heat, and of steam as water and heat compounded, Miller elucidates Watt's cryptic phlogistian statement of water's compound nature that had so embarrassed, confused or disgusted later commentators. Yet contemporaries revelled in this ‘ecology of steam’ (p. 125): De Luc's meteorology and Hutton's geology perhaps inspired, and were inspired by, steam and the steam engine. Against this global backdrop (one which, pace Priestley, has little place for divine design or agency), Watt's steam engine improvements were manifestations of his chemical philosophy. The punchline, revealed early, is that the steam engine was a ‘chemical device’ (p. 10). Yet as commentators from Cardwell on have found, and Miller now finds differently, the relationship between Watt's experimental investigations of steam and latent heat (originally considered chemically, later sanitized as theory-lite expressions of thermal laws) and the steam engine improvements ‘is not straightforward’ (p. 140). It is almost disappointing that the link between latent heat (construed chemically) and the separate-condensing engine was not so much direct as ‘complex and contingent’ (p. 143). Crudely (and Miller's analysis is anything but) the issue of the cylinder's heating and cooling had little to do with latent heat; questions of back pressure in the cylinder and expansive working did, however, both relate to Watt's understanding of latent heat (and were essentially chemical).

Discerning ‘reality’ from ‘representation’ is never easy. It is a particularly Herculean task for Miller, who must wrestle the narrative of the ‘route to the separate condenser’, the meaning of the steam ‘indicator’, and the foundations of ‘engineering science’ from the arms of physicists, and some historians, unable to forget what Watt did not believe: that heat is motion. Although Miller admits that Watt's own phrase ‘perfect engine’ (one in which no steam is wasted) ‘must in some sense be reliable’ (p. 147), he plays up the fabricated nature of Watt's multiple accounts of the invention of the separate condenser, and argues that later influential interpreters, like Hills, have invested the term with more significance than it can support. Miller is concerned that ‘perfect engine’ (a chemical thing, for Watt) too easily morphs into mid-nineteenth-century physical formulations like ‘ideal (heat) engine’. Watt should not, now, be configured as a prescient advocate of the energy physics of J.P. Joule, William Thomson, P.G. Tait and Macquorn Rankine – precisely because those north British philosophers, along with their sometime mentor James David Forbes, remade Watt both as advocate of an unhandy ‘engineering science’ ‘devoid of manual dexterity’ (Forbes, quoted on p. 70) and as sleepwalking thermodynamicist. Miller takes great pains to recover what Watt's circle were at pains to conceal and what two revolutions (one chemical, one thermodynamic) have further obscured: the conception, in two senses, of the steam ‘indicator’ and its ‘indicator diagram’. As I noted in 1992, Rankine had recast Watt's indicator diagrams as ‘diagrams of energy’ by January 1854, in an opportunistic theoretical reconstruction which (we now see from Miller) strategically failed to excavate the epistemological archaeology of Watt's indicator. It is not entirely clear whether Miller feels that Crosbie Smith and his acolytes are, by omission if not design, Rankine's followers. More ‘gloves off’, however, is his rebuttal of Arnold Pacey's claim that Watt's circle toyed with the idea of a ‘heat engine’, imagining a flow of heat in an engine long before Carnot and Clapeyron.

Miller exhibits phenomenal expertise uncovering ‘what Watt was’ for whom and why: self-described (unhelpfully) as ‘engineer’; a ‘philosopher’ in the courts; usually, posthumously, ‘mechanical’ (as craftsman or engineer). For whom, then, was Watt, usefully, ‘chemical’? With immense skill Miller has reconstructed and represented James Watt, Chemist. The paradox is that, in a book so concerned with interested representations, this Watt was – by the author's demonstration – of limited, and decreasing, use to Watt himself and to most powerful groupings. In a study concerned, I think, to establish knowledge as in important senses social, the ‘real’ chemical Watt was only fleetingly a matter of limited consensus. Thanks to those revolutions in chemistry and physics few, bar the odd historian (like chemist George Wilson), wanted to own ‘James Watt, chemist’. It is slightly disconcerting, then, that the ‘chemical’ Watt is located in ‘Realities’ – when most historical actors conspired, through purposeful narratives, collectively and variously to obscure him. If they did not make him out wrong, they had few incentives for an unclouded chemical synthesis. Are all figures obscured by representation, or was Watt just unlucky?

Miller's latest study is a tremendous piece of scholarship. It shows a slightly unnerving mastery of the vast archives and a refreshingly coherent approach to the literatures of history of science and technology, STS and cultural history, and should be read not just by by students of Watt but also by scholars concerned with chemistry, engineering, commemoration and reputation building from the mid-eighteenth century.