The Golden Rule

For the 19th-century chemist, life was never simple. Chemical names, symbols, and relationships were in constant flux.

A heated and often highly theoretical debate raged over the nature of the elements, their relationship to one another, and even the language used to describe them. In the midst of all this uncertainty, the working chemist who wanted to avoid the burden of theory and extended calculations longed for a quick, reliable way to quantitatively work out the composition of salts and other products of chemical reactions. The scale of chemical equivalents (also known as a chemical-equivalents slide rule) provided such a tool, at least for a time.

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A 19th-century scale of chemical equivalency

A 19th-century scale of chemical equivalency, used to quantify products of chemical reactions.

Science History Institute/Gregory Tobias

The scale of chemical equivalents was first developed in 1814 by William Hyde Wollaston and relied on the observed fact that given compounds—the known salts of the day—will always combine in the same weight ratios during chemical reactions. (Scale distances were proportional to the logarithms of the combining quantities of the elements and compounds listed on the scale.) But life is not simple and change is inevitable. Scientists kept discovering new elements and compounds, which meant that the mathematical relationships on any given scale quickly became incorrect. The scale of chemical equivalency needed frequent revisions: many different versions were produced in the 1810s and 1820s, each incorporating the latest discovery. Given their short life span and limited practical use, few scales of chemical equivalency from this period survived into the modern era.

CHF is fortunate to have recently acquired an early scale of chemical equivalency. Our version is an English-made slide rule from approximately 1825, with the scale manufactured from boxwood by “I. Newman of 122 Regent Street, London.” The scale lists 35 elements plus water and, judging by the darkening of the wood and the stains, was well used by its owner, who signed his name, “Birch,” on the back. The scale lists hydrogen as the fixed element, to which all others were mathematically related. Many chemists developed tables of chemical equivalency during the 1820s; CHF’s scale appears to make use of a table created by William Thomas Brande and published in 1824 in Annals of Philosophy. Brande was a professor of chemistry at the Royal Institution and was elected a fellow of the Royal Society at the young age of 21.

A few other scales of chemical equivalency survive in museums and in private collections. An 1828 Wollaston Scale of Chemical Equivalency has been on display in CHF’s permanent exhibition, Making Modernity, since the museum’s opening in 2008. This scale is a part of Transylvania University’s collection and will return home to Lexington, Kentucky, this fall, to be replaced by the Brande scale.