How to See a Diagram: A Visual Anthropology of Chemical Affinity

In 1766, Thomas Cochrane entered the Edinburgh classroom of Joseph Black (1728-99) to learn chemistry for the first time. Cochrane was studying medicine, and, like so many of Black's students, he dutifully recorded several diagrams in his notebooks. These visualizations were not complex. They were, in fact, simple. One of them, reproduced in this essay, was a single "X" a chiasm. Black used it to illustrate ratios of chemical attraction. This diagram is particularly important for the history of chemistry because it is often held to be the first chemical formula, and, as such, historians have endeavored to explain why it was unique and how Black invented it. In this essay, I wish to turn the foregoing premise on its head by arguing that Black's chiasm was neither visually unique nor invented by him. I do this by approaching a number of his diagrams via a visual anthropology that allows me to examine how students learned to attach meaning to patterns that were already familiar to them. In the end, we will see that Black's diagrams were successful because their visual simplicity and familiarity made them ideally suited to represent the chemical theories that he so skillfully attached to them.