Buried in the preface to the seventh edition of Hardwich's Manual of Photographic Chemistry, the editors George Dawson and Edward Hadow — both of King's College, London — make a startling admission about the state of chemistry in 1864:

"Although this portion of the Book has been in a great measure re-written, old-fashioned definitions of Acids and Salts, and modes of representing chemical changes, have been retained, since later and more exact views could hardly be introduced without the frequent employment of new terms, which seem out of place in a Manual treating only of what is theoretical for practical ends. Those Atomic Weights are given which accord with the ordinary chemical names familiar to Photographers."

Translation: we know the new chemistry is better, but our readers learned the old notation, so we're sticking with it.

This is a remarkable historical fossil. The book was written at exactly the moment chemistry was tearing itself apart. The Karlsruhe Congress of 1860 — just four years earlier — had finally settled the long battle over atomic weights, with Stanislao Cannizzaro convincing a generation of chemists that Avogadro's hypothesis was correct and that water was H₂O, not HO. Before Karlsruhe, chemists couldn't even agree whether the atomic weight of oxygen was 8 or 16. Different textbooks used different formulas for the same compounds.

Hardwich's editors are telling us, in dry Victorian prose, that working photographers in the 1860s were still using the pre-Karlsruhe equivalent-weight system — the system where silver nitrate might be written as AgO,NO₅ rather than AgNO₃ — because that was the language printed on their chemical bottles and in their darkroom recipes.

What's lost to modern readers isn't a recipe or a remedy. It's the memory that modern chemistry is much younger than you think. The atomic theory you learned in high school — the one where every element has a single, agreed-upon atomic weight, and every compound has a single, unambiguous formula — was barely cementing itself when American Civil War photographers were coating wet collodion plates at Antietam.

The editors' choice has a modern echo. Consider:

• Python 2 vs Python 3: a community split over notation that persisted for over a decade because practitioners couldn't afford to rewrite their working code.
• Imperial vs metric units in American engineering: NASA still loses spacecraft to this.
• Pre-SI electrical units: the abampere, statvolt, and gauss lingered in physics textbooks well into the 1980s.

Hardwich's preface captures a universal pattern: a working profession will keep using deprecated notation long after academics have moved on, because the cost of relearning exceeds the benefit of precision. The photographers of 1864 weren't ignorant — they had made a rational economic choice. Their chemistry worked. Their pictures came out.

The "wrong" notation made beautiful tintypes.