Dmitrii Ivanovich Mendeleev hit on the idea of the Periodic Table as an organizing theme for a textbook he began writing in 1868. He did some work on refining the idea, and in 1870 presented a paper on it to a meeting of the Russian Chemical Society.
Well, actually, that's not quite true-- Mendeleev did write up a paper for Russian chemists in 1870, but he didn't present it himself. He had a friend do the presentation in his stead. Mendeleev himself was out of town, inspecting artisanal cheese-making operations for the Russian government.
I talked a bit last week about Eureka moments and their place in the stories we tell about science. Our founding myths have a big role for the dramatic moment where a scientist makes a discovery and immediately knows what they have, or presents a new result to universal acclaim. But there's another class of moments that are also kind of fascinating, namely the missed Eureka moment, the moment when a scientist makes what will later be recognized as a momentous discovery, but doesn't see the importance right away, and, well, goes off to inspect cheesemakers for the Tsar.
I've run into a lot of these, too, in researching the book-in-progress-- that is, after all, why I know Mendeleev trivia in the first place-- and elsewhere in history-of-science stuff. The book I got the James Chadwick anecdote from last week points out that while Chadwick's experiment that discovered the neutron was brilliantly done, in fact Walther Bothe and Frederic and Irene Joliot-Curie had all the information they needed to realize they had a new, neutral particle, but didn't quite put the pieces together. Chadwick and Rutherford had been thinking about and looking for neutrons for a decade, so when he saw the data from the European labs, he immediately knew what was going on, and put together the experiment that nailed it down.
In other recent reading, Vera Rubin gets a lot of credit for discovering dark matter through the study of rotation curves of distant galaxies, but in fact she started looking at this in 1962, with our own galaxy. A graduate class she was teaching at Georgetown looked at a bunch of stellar velocities in the Milky Way, and wrote a paper in which they noted that the velocity didn't fall off at large radius as you would expect. They didn't quite put this together, though-- as Rubin herself wryly notes in a review, the paper "apparently influenced no one and was ignored even by the senior author when she returned to the problem of galaxy rotation a decade later."
What's really fascinating is that Eureka moments can be unevenly distributed. The story of Watson and Crick stealing a look at Rosalind Franklin's x-ray diffraction data and using it to discover the structure of DNA is often cited as one of the all-time screw jobs in the history of science, and I expected there to be an acrimonious history of claims and counter-claims and the like. Surprisingly, though, Franklin herself doesn't seem to have been all that bothered by it-- she became friendly with both Watson and Crick after the critical events, and visited them at various points before her untimely death. Watson and Crick both recognized that they were in a race for a momentous discovery, but nobody else involved noticed the Eureka moment.
(The DNA story only blows up into something really ugly after Watson's vicious hatchet job in his autobiography (Brenda Maddox spends a good deal of time trying without much success to find an explanation for this that isn't just "James Watson is kind of a dick."). This is not to say that Franklin wasn't in a bad situation, by the way-- from all accounts, she loathed a lot of the people she worked with at that time, particularly Maurice Wilkins who shared the Nobel with Watson and Crick, and was really happy just to get away from that. They also clearly didn't give her enough credit, relegating her to footnotes, always paired with Wilkins. There doesn't seem to be much personal animosity with her chief rivals, though, which was surprising given the way the story is often told; most of the outrage over Watson and Crick's behavior comes well after her death.)
There are probably at least as many of these stories as great eureka moments in the history of science-- people who had the data to make a revolutionary discovery, but didn't quite put it together in time. This kind of story was one of the compelling things about Frank Close's the Infinity Puzzle, as there are a couple of points where people working at the same institution had all the pieces for the Higgs mechanism a decade or so before Higgs, but never talked to each other to combine their ideas.
They're not quite as uplifting as the more widely told Eureka stories, of course. Except for the ones that work out in the end-- Mendeleev might've been off inspecting cheese rather than introducing the periodic table to the world, but he ended up doing all right.
(The Mendeleev portrait in the "featured image" came from this online gallery.)
One of the more famous anti-Eureka moments involved Galileo's observations of an eighth magnitude star that he found one night. He looked a second time a few nights later, noticed that the star wasn't in the location he had previously recorded, assumed that he had misrecorded the star's location, and thought nothing further of it. The star in question turned out (after some sleuthing by late 20th century scholars) to be the planet Neptune, the existence of which was not suspected for another 200 years.
Mendeleev's triumph was in part because he used his table to predict the existence, and estimate some properties, of elements that were unknown at the time. These elements were later found, and their actual properties were close to what Mendeleev had predicted.