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Image and Reality

Kekulé, Kopp, and the Scientific Imagination

Nineteenth-century chemists were faced with a particular problem: how to depict the atoms and molecules that are beyond the direct reach of our bodily senses. In visualizing this microworld, these scientists were the first to move beyond high-level philosophical speculations regarding the unseen. In Image and Reality, Alan Rocke focuses on the community of organic chemists in Germany to provide the basis for a fuller understanding of the nature of scientific creativity. 

Arguing that visual mental images regularly assisted many of these scientists in thinking through old problems and new possibilities, Rocke uses a variety of sources, including private correspondence, diagrams and illustrations, scientific papers, and public statements, to investigate their ability to not only imagine the invisibly tiny atoms and molecules upon which they operated daily, but to build detailed and empirically based pictures of how all of the atoms in complicated molecules were interconnected. These portrayals of “chemical structures,” both as mental images and as paper tools, gradually became an accepted part of science during these years and are now regarded as one of the central defining features of chemistry.  In telling this fascinating story in a manner accessible to the lay reader, Rocke also suggests that imagistic thinking is often at the heart of creative thinking in all fields.

Image and Reality is the first book in the Synthesis series, a series in the history of chemistry, broadly construed, edited by Angela N. H. Creager, John E. Lesch, Stuart W. Leslie, Lawrence M. Principe, Alan Rocke, E.C. Spary, and Audra J. Wolfe, in partnership with the Chemical Heritage Foundation.


“Alan Rocke’s Image and Reality does so many things vividly and convincingly: it shows how visual images led chemistry step by step to the reality of the microscopic world; how simple portrayals of the logic of substitution and combination were reified; brings to our attention the imaginative, neglected work of Williamson and Kopp; and takes a critical look at Kekule’s daydream. And it beautifully delineates the essential place the imagination has in science. A rewarding, lively picture of chemistry in formation.”

Roald Hoffmann, Nobel laureate in chemistry

“The realm of atoms and molecules has long been a battlefield among scientists: what role should mental images and visual tools play in charting the unseen? In this richly textured and closely argued study, Alan Rocke brings the nineteenth-century debates alive. From August Kekulé’s famous dream-like visions of molecular structures to Hermann Kopp’s fanciful depictions of travels within the molecular world, Rocke argues for the importance of mental imagery in nudging cutting-edge science along.”

David J. Kaiser, Massachusetts Institute of Technology

“Alan Rocke’s scholarship has fashioned our historical view of the development of chemistry in the nineteenth century. In Image and Reality, he turns to that century’s most significant achievement in chemical theory, the determination of atomic and molecular arrangement. The great challenge facing chemists in this endeavor was the conundrum of moving from the world of laboratory chemical materials and reactions to the microworld of chemical atomic and molecular reality. Rocke painstakingly depicts how chemists grappled with and overcame this challenge through the creative use of their visual imaginations. Deploying great historical erudition and sensitivity, philosophical sophistication, and immense scientific and technical knowledge, Rocke has produced the authoritative account of this great achievement and an exciting, path breaking study of the creative imagination in science.”

Seymour Mauskopf, Duke University

Image and Reality is a masterful and authoritative study by one of the profession’s most distinguished historians of chemistry.  Alan Rocke draws on his earlier work and on new sources to demonstrate the role of mental images in the thought processes that created structural chemistry in the nineteenth century. His analysis of chemists’ wit, imagination, and ‘Eureka’ moments of discovery richly informs this detailed history which extends from Alexander Williamson to August Kekulé and Hermann Kopp.”

Mary Jo Nye, Oregon State University

“In Image and Reality, Rocke covers Kekulé’s work, varied life and personality, and locates the chemist’s thinking in the context of developing ideas about chemical bonding and molecular structure. . . . Rocke’s thesis is that ‘human minds work far more visually, and less purely linguistically, than we realize.’ At every turning point, he suggests, early chemists used their imagination to visualize the constitution of the micro-world, leading the way in visual thinking. . . . Such visualization of the micro-world is now commonplace. Yet the role of visual thinking in the scientific mind is not universally accepted. ‘For scientists, mental images may seem downright embarrassing,’ suggests Rocke. But for Kekulé, as this subtle and penetrating study shows, dream images translated into chemical reality.”

Andrew Robinson | Nature

"Exciting and wide-ranging. The writing is so easy and natural that even a non-specialist can read it with delight and understanding. It inspired me."

Oliver Sacks

"A masterful account of how chemists crafted a unique visual language of the microworld....Image and Reality offers an impressive discussion of how chemistry relies on such visual thinking as well as how visual imagery contributes to the creative process in science."

Peter J. Ramberg | Science

“Highly recommended.”

R. E. Buntrock | Choice

“Rocke has produced a striking reinterpretation of the development of structure theory centered on the work of August Kekulé.”

Michael D. Gordin | Chemical Heritage

Table of Contents


List of Abbreviations

1 Ether/Or

Springtime for Chemistry?

The Education of Alexander Williamson

Interpreting Chemical Atoms

Williamson and Graham

Grasping the Ether

The Experimental Dissection of Molecules

Excursus: Isolated Radicals?

The Spread of Williamsonian Theory

The Architect of Molecules

The Education of August Kekulé

Kekulé in London

Excursus: The Road to Valence

Molecular Dreams

Building an Unseen Structure

The Start of a Teaching Career

Early Work in Heidelberg

The Theory of Polyatomic Radicals

The Theory of Atomicity of the Elements

Molecular Epistemology

A Barometer of the Science

Writing a Textbook

Formulas, Models, Reality

Excursus: A Case in Point

Erlenmeyer and Molecular Theory

Constant or Variable Atomicity?

The Heuristics of Molecular Representation




Crum Brown

Excursus: Heurism in Action

The Fate of the New Graphic Formulas

Molecules as Metaphors

Natural Types

Absolute Formulas

Excursus: Looking through the Stereoscope

Molecular Democracy or Autocracy?

The Revenge of Jupiter’s Children

Aromatic Apparitions

First Approaches to the Problem

Enter the Hexagon

Benzene through the Phenakistoscope

Excursus: Ring around the Rosie


Dimensional Molecules

Early Stereospatial Speculations

The Spiral Staircase

The Pyramid

Imagination in Science: Point/Counterpoint

Chemists, Physicists, and the Microworld

Kopp’s World

The Making of a Chemist-Historian

In amongst the Molecules

The Response

The Thirsty Chemists

Kekulé’s “Dreams”

The Festivities in Berlin

Kekulé’s Speech

The Aftermath

The Eureka Experience and the Unconscious Mind

The Scientific Image-ination

Mental Images and Science

Mental Images and History

Transdictive Images in Physics and in Chemistry



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