A common chemical reagent for botanical microscopy in the latter half of the nineteenth century was chloriodide of zinc, or, in German, Chlorzinkjod. The exact name varies — chlor-zinc-iodide, iodine zinc chloride, etc. — but basically it is a test for cellulose, having replaced the more volatile iodine + sulfuric acid test of the 1830s.

But who invented it? Search the internet and you run into the problem that, not only does this stuff have a lot of different names, but it has also been attributed to a lot of different scientists. Since 2011 the English language Wikipedia has attributed it to the zoologist Max Schultze, a 2009 New Oxford Dictionary for Scientific Writers attributes it to someone named Ernst Schultze, several different chemistry websites or even chemical suppliers just say “Schultze” with no additional attribution, despite the fact that “Schultze” is a common name.

This is the rare case where it gets worse with historical research. In 1963 Annemarie Ziegler wrote, “von SCHULTZE (aus dem Jahr 1850) stammt die bekannte Chlorzinkjodprobe zum Nachweis von Zellulose,” with the citation, “SCHULTZE 1850: nach RADLKOFER, L., 1855, Lieb. Ann. 44, 332.” Great! Wilhelm Behrens’ handbook of microscopy reagents from 1883 also names “Radlkofer,” but provides a citation to Leopold Dippel’s 1867 microscopy textbook. Dippel in turn provides no actual citations, except to say: “Ein für den Pflanzenhistiologen sehr wichtiges Reagenz bildet das Jod in Verbindung mit Chlorzink als sogenannte Chlorzinkjodlösung. Diese Lösung wurde zuerst von Professor Schulze in Rostock als Reagenz auf Zellstoff empfohlen,” etc.

So is it Schultze with a T, or Schulze with a Z? In 1850 Max Schultze (with a T) was living in the Prussian university town of Greifswald, with his father the anatomist Karl Schultze—not in the Mecklenburg port city of Rostock. So let’s keep digging. If we go back to Ziegler’s article from 1963, we at least have a citation we can follow, to Justus Liebig’s Annalen der Chemie, along with a year (1855) and a page number (332). Here we hit the jackpot: an article by L. (possibly Ludwig?) Radlkofer, “Ueber die Darstellung der Chlorzinkjodlösung als Reagens auf Zellstoff für mikroscopische Untersuchungen.”

Is it a jackpot yet? Here’s the first line: “Zur Bereitung dieses von Professor Schulze in Rostock aufgefundenen Reagens giebt Schacht folgende Vorschrift,” etc. etc., along with a citation to Hermann Schacht’s textbook from 1851, Das Mikroskop und seine Anwendung, insbesondere für Pflanzen-Anatomie und Physiologie. Let’s go find Schacht’s textbook then, where on page 31 we read: “Eine Auflösung von Chlorzink, Jod und Jodkalium. Ein Tropfen dieser Mischung auf ein in wenig Wasser liegendes Präparat bewirkt dieselbe Färbung als Jod und Schwefelsäure. Diese Mischung ward erst neulich vom Prof. Schulz, gegenwärtig in Rostock, empfohlen, sie ist bequemer wie Jod und Schwefelsäure zu verwenden und leistet ungefähr dieselben Dienste,” etc.

At this point I was beside myself: we have 20th and 21st century writers citing Max Schultze or even Ernst Schultze, then we can follow citations around in a circle in the mid-19th century to Schulze and now even Schulz. I almost gave up, when I decided to just look for every Schultze, Schulze, and Schulz in Rostock in the Deutsche Biographie project—when, at last, I found the chemist Franz Ferdinand Schulze (1815–1873), professor of chemistry at the University of Rostock. (Who was, incidentally, father of Franz Eilhard Schulze, who was one of Max Schultze’s good friends. Ugh.)

If I had to guess, I would bet that: 1) Franz Ferdinand Schulze told Hermann Schacht about chlor-zinc-iodide in 1850 but never published it, 2) Schacht got the name wrong, and 3) Max Schultze became much more famous for, among other things, inventing new microscopy reagents like osmic acid. With this incredible combination of close names and a lack of an original citation, people’s imaginations for who really invented chlor-iodide of zinc has run wild for over 170 years, and I’ve finally cracked it. I hope.

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