Friends of the salt and earth varieties

Affinity, that Elusive Dream
January 16, 2004

In 1809, in his novel Elective Affinities , Goethe exploited a striking chemical metaphor in his analysis of human relations. Passions, antipathies and the difficulties of establishing permanent unions were all treated by analogy with substances that, depending on their nature, would unite "like friends and acquaintances" or stand apart as "strangers side by side". In adopting the metaphor of affinities, Goethe propelled into the realm of general literate culture an interpretation of chemical combination that had fired chemists since the beginning of the 18th century.

While the interpretation, with its predominantly Newtonian underpinnings, has by no means gone unstudied, it has suffered some degree of what Mi Gyung Kim sees as a collective "historical amnesia". Lavoisier's omission of chemical affinity from the paradigm-defining Traité Elémentaire de Chimie (1789) stands as the fount of the distortion; ever since, historians' admiration for his role in the "chemical revolution" has compounded the perception of pre-Lavoisian chemistry as an empirical pursuit lacking methodological rigour and any but the crudest overarching theoretical system.

The author's aim is to put all this to rights by setting the chemical revolution within a broader transformation of chemistry from an art into a dignified component of the public culture of the late Enlightenment. An essential part of this process was the establishment of a truly philosophical chemistry, and Kim's painstaking refinement of this deceptively simple idea provides the core originality of her book. Her focus is on the "theory domains" that mediated between philosophy (concerned with the most general beliefs about the working of nature) and the very different worlds of industry and laboratory practice. The two "theory domains" that mattered for most 18th-century chemists were those of composition and affinity. Although these ranged widely, they lacked the generality of philosophy, serving instead to fashion concrete problems and investigative strategies.

Their emergence, between 1700 and 1720, marked the first of what Kim identifies as the two key "theoretical moments" of her story. One was defined by the challenge that Louis Lemery and Etienne-Francois Geoffroy launched against notions of composition founded on the existence of the five property-bearing principles of earth, water, salt, sulphur (or oil) and mercury (or acid spirit). Chemists of the generation of Lemery's father, Nicolas, had argued that any body in nature resulted from a "union and mixtion" of these and that its properties, including its affinity for other bodies, were determined by the quantities of the various principles that it contained. The new approach, by contrast, made chemical composition a consequence, rather than a cause, of the affinities, or rapports, between a body's constituents.

It is part and parcel of Kim's vision that the two "domains" - principles and affinity - coexisted as distinct but fluctuating and sometimes overlapping traditions for more than half a century. They drew on and fostered different laboratory practices, based on distillation in the case of principles and on solution in the case of affinity, and converged significantly only in the 1770s and 1780s.

Then, in the second of Kim's "theoretical moments", an interest in affinities became common intellectual terrain both for Lavoisier's quantitatively precise engagement with the problem of composition and for Guyton de Morveau's experimental investigation of the attractions that governed chemical action. From that encounter, as we know, Lavoisier's new system emerged in its full-blown form, though with affinity finally expunged from the Traité and left to undergo what might appear, in retrospect, fruitless posthumous elaboration in Claude-Louis Berthollet's Essai de Statique Chimique (1803).

From all this, what survives of the once straightforward notion of a single chemical revolution, effected by Lavoisier and hailed by his more enlightened contemporaries? Kim explicitly declines to take issue on this.

But the implications of her book are unmistakable. As she herself insists, they point to the need to introduce a strong social dimension and a new degree of complexity into any future account. Readers familiar with trends in the historiography of science over the past two or three decades will not be surprised by this or by the author's prescription that we should listen to the "multiple voices" that shaped the revolution, in particular to those of the apothecaries, physicians, mineralogists and industrial chemists who possessed diverse forms of practical knowledge on which chemists of a more philosophical disposition could draw.

Perhaps, as she suggests, we should think not of a single revolution, but rather of a cluster of revolutions, each focused on a different, limited range of phenomena. Here, her insistence that the very meaning of "revolution" should be rethought is especially cogent. She is surely right to see most of the key changes in 18th-century chemistry as decisions to strike out on new areas of research and incorporate new methods; they were rarely attempts to overthrow existing beliefs.

Such a perspective certainly makes sense of Lavoisier's famous prediction, set down in a notebook in 1773, that his work on combustion would lead to a "revolution in physics and chemistry". At that date, the revolution that Lavoisier foresaw could scarcely have rested on the rejection of phlogiston, a position that he only began to articulate the following year; it is more plausibly interpreted, with Kim, as an anticipation of the potential that lay in the emerging and still uncharted domain of pneumatic chemistry.

Affinity, that Elusive Dream is presented as an "interpretive essay designed to open new venues of historical investigation". It is that, but it is also far more. It rests on careful research, reflected in 80 pages of notes and a bibliography of almost 1,000 printed items. The resulting description is "thick", in Clifford Geertz's sense, and the argument is one of engrossing subtlety - one that future historians of 18th-century chemistry will have to take very seriously indeed.

Robert Fox is professor of the history of science, University of Oxford.

Affinity, that Elusive Dream: A Genealogy of the Chemical Revolution

Author - Mi Gyung Kim
Publisher - MIT Press
Pages - 599
Price - £35.95
ISBN - 0 262 113 6

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