49. Indium: The Spectre Of Isaac

The story of indium is pretty straightforward, but its namesake is practically impossible to pin down.

Featured above: A positively psychedelic portrait of London’s greatest detective, Isaac Newton.

Show Notes

My apologies for a threadbare collection of show notes again! Often when the episode runs this long, the show notes suffer, I’m afraid.

Not To Put Too Fine A Point On It: Indium quantum dots might actually be able to produce indigo light, but I don’t think anyone’s using them for that, because they’re not well suited to the task.

It’s a difficult thing to find hard data on, especially as a non-expert. So I reached out to some experts (from Indium Corporation and Nanosys, in particular) to ask. From their responses, I get the impression that a sufficiently motivated person could make an indium phosphide quantum dot that produces an indigo color, but it wouldn’t do a particularly good job of it.

Thanks, experts!

Let’s Not Get Started On Magenta: Violet and purple are not the same color, despite most people casually conflating the two. Violet is a color that can be “expressed” by a single wavelength of light, namely in the low 400-nanometers range. Purple, however, requires a mix of wavelengths — some that are violet or blue, and some that are red.

“Spectral colors” are those that can be defined by a single wavelength; those that are mixes are “non-spectral.” Most non-spectral colors are shades rather than hues, like pink or brown (which are mixes of a spectral color and some amount of grey). Purple is non-spectral because it mixes two non-adjacent primary colors, which live on opposite ends of the spectrum.

With Apologies To Amy Ray And Emily Saliers: Outside of the elementary school classroom, most people seem pretty comfortable leaving indigo behind. For instance, you probably didn’t even notice that these two prominent rainbows only contain six colors rather than seven:

The gay pride flag


Pink Floyd's Dark Side Of The Moon


Pronunkiation: Latin lives! Or at least, it did for a long, long time. As such, there are at least three major flavors of the language: Classical Latin, Church Latin, and Diet Latin New Latin. Minor differences abound among them, so Newton’s Principia can be pronounced either as “Prin-KIP-ee-ah,” “Prin-CHIP-ee-ah,” or “Prin-SIP-ee-ah.” The last is what sounds most familiar to a modern English speaker, so I just went with it.

Not A Gold Tooth: In the early 19th century, one of Isaac Newton’s teeth sold at auction for an amount equal to ~$35,000 today. That’s at least a couple brand-new decent cars!

And, as promised, someone chewing on indium so you don’t have to:


Episode Script

[Quantum Dot Technology!]

It seems like every year, the electronics industry comes up with a brand-new buzzword to make it sound like you must upgrade your perfectly serviceable HDTV.

But quantum dots are not just hollow advertising rhetoric — they are a real thing with an actual scientific definition. A quantum dot is an extremely tiny semiconductor — usually only around a dozen atoms wide.1 Being so minuscule, the material is capable of some unique tricks enabled by the subatomic strangeness of quantum mechanics. For instance: when struck with a beam of ultraviolet light, a quantum dot will in turn emit light in the visible spectrum. The color of light it casts can be precisely tuned by varying the size of the quantum dot.

That’s good for more than just TV screens. Quantum dots could benefit medical imaging, printable electronics, solar energy, and the bizarre world of quantum computing.

Indium happens to be a versatile building block for quantum dots, creating a range of colors from deep green to light red.2

That’s quite fitting for an element whose identity is so intrinsically linked to color, and perhaps a little ironic. Just about the only color element 49 can’t produce as a quantum dot is the one it’s named for: Indigo.

You’re listening to The Episodic Table Of Elements, and I’m T. R. Appleton. Each episode, we take a look at the fascinating true stories behind one element on the periodic table.

Today, we’re touching upon indium.

In 1863, German chemist Ferdinand Reich was sifting through mineral samples in search of thallium, an element that had been discovered only a few years earlier. He thought he had finally succeeded — but when fed the sample into his spectroscope, it did not display the spectral lines characteristic of thallium.

But Reich had a bit of a disadvantage when it came to peering into the kaleidoscope of chemistry: He was color-blind. So he called upon the assistance of a colleague, one Hieronymus Theodor Richter. What Reich couldn’t see was immediately visible to Richter: A bright emission of light with a wavelength of precisely 451 nanometers.

That produced a color that Richter recognized as “indigo,” and so the element earned its name, indium. (Unlike some elements, like potassium and sodium, the word for “indigo” is the same in German as it is in English.)

It’s one of the more succinct origin stories for an element’s name. As for why Richter called that color “indigo,” well, we could talk about that for quite a while.

I’m sure you’ve heard of indigo before, and I bet I know the source: ROY G. BIV.

That is the disappointingly limp mnemonic offered to schoolchildren when memorizing the colors of the visible light spectrum, which, for their benefit, we call a “rainbow”: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Any person able to perceive those colors can easily recall what green looks like, or red, or any of the others — except for indigo. Sure, it’s wedged in there somewhere between blue and violet… but that’s a pretty loose definition. It’s certainly not as distinct as, say, yellow.

The various colors that are sometimes awkwardly assigned the designation of “indigo” are rarely in agreement with each other, and most people would more readily call them blue or purple. It’s not that indigo isn’t a real color — just that it doesn’t deserve such special recognition any more than vermilion or Kelly green does.

And yet it persists. Such is the far-reaching influence of Isaac Newton.

Isaac Newton is a towering figure in the history of science. His name is often uttered in the same breath as Leonardo da Vinci and Albert Einstein — and rightly so. But he has also attained a kind of mythical status.

Newton discovered gravity when an apple fell out of a tree and struck him in the head, or so the story goes. It’s cute, it’s neatly packaged, and you can tell it to children without stirring up controversy.

There is some truth behind the anecdote, which is better than some falsehoods we’ve had to dispel before.

Newton himself told the story like this: In 1665, Cambridge shut down due to a minor outbreak of the Black Death. It was the seventeenth-century equivalent of a snow day, but with more bleeding and gangrene.

So he returned to the countryside for a while, where he conducted some of his most important work. One day, he was contemplating the nature of gravity from the shade of an apple tree when one of the apples fell to the ground nearby.

“Why should that apple always descend perpendicularly to the ground,” he wondered. “Why should it not go sideways or upwards, but constantly to the Earth’s center?” This kicked off the chain of thought that led to his conclusions about how gravity works.

It’s a little fuzzy how honest this version of the story is. Newton had already spent decades telling the story by the time this biographer recorded it. The Royal Society’s Head of Archives explains, “Clearly it’s an anecdote Newton polished. … It wasn’t just that Newton polished it, succeeding generations put a gloss on it as well.”3

It’s a shame that the public perception of Isaac Newton begins and ends with the falling fruit, because the true facts of his life are riveting enough without embellishment.

He had a tumultuous life from the very start. His father died before he was born, and his stepfather was no peach. Isaac was banished from his own home, and later admitted that he was guilty of “threatening my mother and father … to burn them and the house over them.”4

When a schoolyard bully tormented young Isaac one time too many, he swore he’d get revenge against the boy… by receiving better grades. He did one better, actually, and became top of his class at Grantham.

He attended the venerable Trinity College at Cambridge, and became a mathematics professor there after earning his Master of the Arts degree. As part of his duties, Newton was required to give regular lectures — a task that held little interest for him. He would often just recite rough drafts of his own scientific papers, stuff that was well over the heads of his hapless undergraduate students. Predictably, attendance waned quickly. But Newton was a man who took his responsibilities very seriously. One of his proteges wrote,

so few went to hear Him, & fewer that understood him, that oftimes he did in a manner, for want of Hearers, read to the Walls.”5

Clearly, his alignment was Lawful Neutral.

It wasn’t just his university job that he took so seriously. A little later in life, he was appointed Head of the Royal Mint in London. It was meant as an honorary position more than anything, his reward for being such a smart and famous Englishman. But that wasn’t Newton’s style. He was immediately zealously devoted to his new office.

Counterfeiting was kind of a big problem in England back then, and so was a practice called “coin clipping.” People would shave thin parings of metal off the edges of their shillings, farthings, florins and groats, which they’d melt down and sell. The practice was so widespread that most of the coins in circulation at the time had lost around half their original weight.

Newton tackled the problem from within, instituting all sorts of organizational changes at the Mint, buying new equipment for his employees, and making it easier to tell when a coin had been debased. (The ridges on the edges of modern-day American quarters and British pounds are one of Newton’s anti-counterfeiting measures that survives to this day.)6 He recalled all silver coins in circulation, assessed their actual value, and melted them down to re-mint as legal coinage.7 8

But he didn’t just fight crime from behind a desk. Isaac Newton hunted his quarry vigilante-style through the smog-choked alleys and alehouses of London, wearing disguises and strong-arming criminals into giving up information. This is not a joke, nor an exaggeration. And the amazing thing is, he was really good at this. By 1699, he had personally apprehended and successfully prosecuted no fewer than twenty-eight counterfeiters — all without the help of DNA, photography, or fingerprints, or professional help. Police literally did not exist yet, and wouldn’t until 1829, when Robert Peel organized the first police department in the modern world. So Newton was a one-man operation — no Inspector Lestrade to back up this Sherlock Holmes.9

But he did have a Moriarty. Newton’s arch-nemesis was William Chaloner, a rake and a scoundrel who had used his ill-gotten gains not only to purchase an impressive estate, but also the outward appearance of propriety.

Exercising this combination of privilege and audacity, Chaloner approached the parliament and offered his services as a consultant. He promised “that there is a better, securer, and more effectual way, and with very little charge to his majesty, to prevent either casting or counterfeiting of the milled money.”10 11

Chaloner’s motives were impure, of course. He wanted to study the Mint’s machines up close, the better to craft his phony coins. Newton refused the offer, unsurprisingly, and he also took great personal offense to the suggestion that the Mint’s employees were incompetent. Chaloner shuffled off and quietly continued his criminal ways, but now he had made a rival of a very smart and powerful man.12

Newton pursued him for years, and Chaloner impressively eluded his grasp. Keeping one step ahead of his opponent, Chaloner lived a hedonistic life, accruing real wealth with fake money. When Newton finally cuffed him in 1697, Chaloner was able to bribe a key witness into skipping town, and thus slipped free.13

This only served to make Newton a more determined sleuth. In 1699, he finally made an arrest that stuck, charging Chaloner with two counts of high treason — a capital offense.14

Sensing that his luck might have run out, Chaloner attempted several desperate strategies in rapid succession: He feigned insanity, then angrily insulted each witness that took the stand, then desperately insisted upon his own innocence. Finally, he implored Newton directly in a series of written letters, culminating with,

O dear Sir no body can save me but you. O God my God I shall be murdered unless you save me O I hope God will move your heart with mercy and pitty to do this thing for me I am // Your near murdered humble servant // W. Chaloner”15

All he received in response was silence.

On March 22, 1699, Chaloner was dragged through the cold and filthy streets of London on a ramshackle sledge. As the noose was slipped ’round his neck at the Old Bailey, he didn’t even had the privilege to see the face of the man who brought him down. This may have been William Chaloner’s last day on Earth, but for Isaac Newton, it was just a Sunday like any other. He saw no reason to mark the occasion with his attendance.16

Rather astonishing, then, that behind closed doors, Newton also conducted some criminal work involving silver and gold.

In 1404, King Henry IV decreed “that none from henceforth should use [alchemy] to multiply gold or silver, or use the craft of multiplication; and if any the same do, they incur the pain of felony.”17 The reasoning behind this law was the same as for the law Newton so diligently enforced: The political regime was looking pretty shaky in 1404, and King Henry was afraid that the value of his government’s money might collapse. The last thing he needed was some sorcerer threatening the legitimacy of the crown by flooding the countryside with gold bullion.18

Now, we have previously discussed how alchemy was much more than just an intemperate search for gold, but there’s no denying that gold was a big draw for Newton. He was captivated by the idea of transmutation, as evidenced by his writings on the Philosopher’s Stone, Sophic Mercury, the Tree of Life, and other such legendary objects of desire. This interest was actually somewhat common among his contemporaries, like Robert Boyle and Robert Hooke, despite its legal status. Nonetheless, he wisely kept his alchemical research secret for his whole life. It seeped into his more official research in interesting ways, though.

One of the main precepts Newton took from his mystical predecessors was the idea that materials can be broken down into their essential substances, like salt, sulfur, and mercury.19 20

That idea was hugely influential when he turned his attention to the subject of light.21 For millennia, conventional wisdom held that “pure” light was white, and took on colors when it interacted with colored objects. In an elegant experiment, with naught but a ray of sunlight and glass prism, Newton showed that pure white light is a combination of all visible colors.22 From this simple observation blossomed one of the most important scientific texts of all time: A Treatise Of The Reflexions, Refractions, Inflexions And Colours Of Light, or simply, Opticks. He spent many decades studying, writing, and refining this work to perfection. It would have been a career-defining accomplishment if he hadn’t already published the Principia, his earlier book outlining, among other things, the laws of motion.

It was during his studies on optics that he delineated the visible spectrum of light into five main pillars: Red, yellow, green, blue, and violet. Any such classification would have been arbitrary to some degree, since this is a perfect gradient, but Newton wasn’t too shy about his biases. According to him, the number seven was an important, almost sacred number, not least because there are seven named notes in a musical scale. Newton very much wanted to draw parallels between sound and color, so he quickly revised the spectrum to consist of five colors plus two: orange and indigo.23 24 25

He likened these two colors to semitones — half-steps. On a piano, Newton would have placed orange and indigo at the places where two white keys adjoin each other with no black keys in between.26

Again, this is just an idea he liked, nothing backed up with empirical study. He even expressed some doubts about this in his later work, but chose to double down and squint a little harder. Orange and indigo remained named bands of the spectrum.27

At the time, indigo happened to be of particular cultural and economic importance. More than a color, indigo is also the name of a plant and the rich, deep blue dye that can be made from it. It originated in India, so everyone simply called it, “Indian dye” — in Greek, “indikon,” from which we take “indigo.”28

India was pretty popular in Newton’s time, primarily because the British East India Company was a dominant force of trade and politics on the subcontinent. It was a new kind of economic force, a joint-stock company, and it had the direct backing of the British crown.

The success of the British East India Company, along with its Dutch, French, and Portuguese counterparts, had global consequences. Slavery, private armies, stock exchanges, imperialism, sprawling trade networks, widespread drug addiction,29 30 famine… practically every major event in three centuries of world history could trace roots back to the East India Company in some way or another.

A little ways down on that list of repercussions is the fact that England was suddenly flooded with high-quality trade goods that were never available before. Indigo dye was among them.31

It was prized as the finest blue dye available in the world — partly because it was so vibrant, and partly because there really aren’t many blue pigments to be found in nature. Every other color is well represented in both plants and animals, but blue is exceedingly rare. When it does appear, it’s almost always due to an optical phenomenon, similar to the anodization we learned about in episode 41 — not a pigment that could be turned into paint or dye.

Just try to think of a blue food. There is none! This mystery that has so perplexed humanity over the ages that George Carlin opined on the possible reasons when he hosted the very first episode of Saturday Night Live on October 11, 1975.

Why is there no blue food? I can’t find blue food. I can’t find a flavor of blue. I mean, green is lime, yellow is lemon, orange is orange, red is cherry, what’s blue? There’s no blue. Oh, they say, ‘blueberries! Uh-uh. Blue on the vine, purple on the plate. There’s no blue food! Where is the blue food? We want the blue food! Probably bestows immortality. They’re keeping it from us!”

There actually is a theory that explains why. To simplify a bit, it requires more energy for an organism to produce blue chemicals than any other color. Since the rest of the color palette is usually more than enough to suit its needs, there’s very little incentive for an organism to invest resources in blue pigment.32

So at the time when Isaac Newton was naming the various colors of the rainbow, indigo was a trendy and valuable commodity. It makes sense that, when deciding upon a name for the color between blue and violet, he would settle upon indigo.

There’s just one problem: The color of light we call indigo doesn’t really look like indigo, the dye. Rather than a dark intermediary between navy blue and violet, indigo dye looks, well, blue. Like the color of blue jeans. Which makes sense, because indigo is the dye that’s always been used to color blue jeans.3334 35

But that blue-jean color might have actually been what Newton was describing. According to physicist Gary Waldman, “A careful reading of Newton’s work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name blue-green or cyan.”36

That adds up. “Cyan” wasn’t even a color name until the 19th century.3738 And human perception of color has been far from static over time.

For instance, in The Iliad and The Odyssey, Homer describes a world that sounds positively alien to the modern ear: Green honey and wine-colored sheep are found beneath a bronze sky.39 This wasn’t because Homer had messed-up eyes, either. A review of ancient Greek texts shows that hue was not the fundamental way they distinguished color. Lightness and darkness were more important; while red and yellow were the colors most worth mentioning.40

There’s a recognized progression of color recognition across times and cultures, which Brent Berlin and Paul Kay describe in their 1969 book, Basic Color Terms: Their Universality And Evolution. They describe it as a set of stages determined by how many color words a language has.41

All the numerous and diverse cultures they studied had at minimum two words for colors, which differentiate between dark or cool, and light or warm. Most languages recognize red as a color, followed by either green or yellow. Only after all those colors have their own names does blue receive a name of its own. Brown, pink, purple, and orange all come after that.42

It’s hard to know if the way we speak influences the way our brains process visual information. Did Homer see the sky as the same shade as metallic bronze, or is this simply a semantic difference?

Alas, we don’t have time to plumb these philosophical depths any further. What matters to us is that when Hieronymous Richter looked down the spectroscope, he saw only one colored line, and he called it indigo.

That is obviously integral43 to element 49’s identity, but that is also the full extent of work that Richter contributed to this discovery. It was rather generous when Ferdinand Reich included Richter’s name as co-author of the paper announcing the new element. So, you can imagine how upset he was when he later learned that Richter claimed to be the sole discoverer of indium.44

To our eyes, that appears to be an incredibly selfish and brazen move. But who knows? Maybe Richter just saw things a little differently.

From its discovery until 1923, you would have had great difficulty adding indium to your collection: A grand total of one gram constituted the world supply of the raw metal. Richter and Reich claimed to bring a one-pound ingot of their discovery to the 1867 World’s Fair, but they were afraid somebody might try to steal it. The bar they claimed was indium was actually lead. Nobody would come up with any commercial or industrial uses of indium for decades, so no one bothered refining any more of it.

One of the applications we eventually discovered is that indium can form a great airtight seal, especially in situations where rubber or other organic materials would degrade. That’s because indium is a very soft metal, with a fairly low melting point.

At least, that’s the conclusion that engineers drew. When a YouTube personality sees an unusually malleable metal, they might wonder, “Can I chew on this like gum?” Now, look, of all the elements, indium is relatively non-toxic. It’s not very biologically active. But nobody’s really done any studies on the long-term effects of indium ingestion. You’ll probably be fine, but there’s really no reason to risk it. Especially since there are plenty of videos you can watch online of some other fool doing this stunt so you don’t have to.

For all that talk earlier about color, the main appeal of element 49 today is that it can have none at all.

Indium-tin oxide is a material that conducts electricity very well, and is also transparent to light — an uncommon combination of traits, to say the least. And that makes it perfectly suited for touchscreens, the kind you’ll find on a phone, tablet, computer, information kiosk, car dashboard, ATM, video game console… it is ubiquitous.

So if indium is what you’re after, you could look right past it, and I don’t recommend you taste it. But without too much difficulty, you can probably reach out and touch some.

Thanks for listening to The Episodic Table of Elements. Music is by Kai Engel. To learn the going rate for one of Isaac Newton’s teeth, visit episodic table dot com slash I n.

Next time, we’ll cry over tin.

Until then, this is T. R. Appleton, reminding you that, for all his supposed brilliance, not once did Isaac Newton make mention of the fluorescent greenish-yellow purple called “octarine.”


  1. The Finite Element Method, FEM For 3D Solid Elements. G.R. Liu and S.S. Quek, 2014.
  2. Nature Communications, Continuous Injection Synthesis Of Indium Arsenide Quantum Dots Emissive In The Short-Wavelength Infrared. Daniel Franke et. al, November 11, 2016.
  3. The Guardian, Isaac Newton’s Falling Apple Tale Drops Into The Web. Alok Jha, January 17, 2010.
  4. Understanding The Properties And Behavior Of The Cosmos: A Historical Perspective, p. 61. Don Hainesworth, 2011.
  5. Personal correspondence from Humphrey Newton to John Conduitt, January 17, 1727. It’s unclear if Humphrey Newton was or was not related to Isaac, but he did serve as the latter’s amanuensis for several years.
  6. Mental Floss, Why Do Coins Have Ridges? Matt Soniak, June 22, 2011.
  7. Te Mata Koi: Auckland University Law Review, The Counterfeit Presentment Of Two Britons: Isaac Newton And Currency Crime In Modern England. Sam Hiebendaal, 2009.
  8. APS News, This Month In Physics History: March 16, 1699: William Chaloner, Counterfeiter, Hanged. March 2011. This article appears to give an erroneous date for Chaloner’s execution, but I trust the APS for background on coin clipping, at least.
  9. Science Friday On NPR News, Isaac Newton: Physicist And… Crime Fighter? Interview between Ira Flatow and Thomas Levenson, June 5, 2009.
  10. Annals Of The Coinage Of Great Britain And Its Dependencies, Volume 2, p. 53. John Hearne, 1840.
  11. Newton And The Counterfeiter: The Unknown Detective Career Of The World’s Greatest Scientist, p. 18. Thomas Levenson, 2009.
  12. ibid.
  13. Mental Floss, Isaac Newton: 17th-Century London’s Dirty Harry. Judy Dutton, November 9, 2012.
  14. Notes And Records Of The Royal Society Of London, Isaac Newton And The Counterfeiters. John Craig, December 1963.
  15. William Chaloner’s letter to Isaac Newton, no later than March 16, 1699.
  16. Executed Today, 1699: William Chaloner, Isaac Newton’s Prey. Thomas Levenson, March 22, 2009. Ordinarily I’m not sure I’d trust a website called “Executed Today,” but this column is written by Thomas Levenson, author of the well researched book Newton And The Counterfeiter.
  17. The Aspiring Adept: Robert Boyle And His Alchemical Quest, p. 105. Lawrence Principe, 2018.
  18. Forbes, The Day England Outlawed Alchemy. Kiona N. Smith, January 13, 2018.
  19. Biography, Sir Isaac Newton & The Philosopher’s Stone. Zach Pelta-Heller, last revised June 18, 2019.
  20. Discover Magazine, Isaac Newton, World’s Most Famous Alchemist. Jane Bosveld, December 28, 2010.
  21. National Geographic, Isaac Newton’s Lost Alchemy Recipe Rediscovered. Michael Greshko, April 4, 2016.
  22. Shapiro, A. (1994). Artists’ Colors and Newton’s ColorsIsis,85(4), 600-630. Retrieved from http://www.jstor.org/stable/235280
  23. Studies In The History And Philosophy Of Science, Part A, p. 269-279. David Topper, June 1990.
  24. Science Progress, Newton And The Colours Of The Spectrum. R. A. Houstoun, October 1917.
  25. Journal Of The History Of Ideas, The Role Of Musical Analogies In Newton’s Optical And Cosmological Work. Niccolò Guicciardini, January 2013.
  26. The Scientist, Newton’s Color Theory, ca. 1665. Ashley P. Taylor, March 1, 2017.
  27. Winsor & Newton, Spotlight On Indigo. March 11, 2019.
  28. To be technical, it’s more like “Indian stuff.”
  29. BBC News, How Britain’s Opium Trade Impoverished Indians. Soutik Biswas, September 5, 2019.
  30. BBC News, The Dark History Behind India And The UK’s Favorite Drink. Justin Rowlatt, July 15, 2016.
  31. Nadri G.A. (2015) The Indigo Trade of the English East India Company in the Seventeenth Century: Challenges and Opportunities. In: Berg M., Gottmann F., Hodacs H., Nierstrasz C. (eds) Goods from the East, 1600–1800. Europe’s Asian Centuries. Palgrave Macmillan, London.
  32. The Skeptical Chemist, Why Does Nature Hate The Color Blue? Sean Lim, last updated August 15, 2019.
  33. Smithsonian.com, Have Scientists Found A Greener Way To Make Blue Jeans? Emily Matchar, January 22, 2018.
  34. The Allegheny Front, Indigo Banners Bring Attention To Toxic Dyes In Blue Jeans. Amy Eddings, June 14, 2019.
  35. Reader’s Digest, Why Is Denim Blue? Marissa Laliberte.
  36. Introduction To Light: The Physics Of Light, Vision, And Color, p. 193. Gary Waldman, 2002.
  37. Merriam-Webster, Cyan.
  38. Etymonline, Cyan.
  39. Clarkesworld, The Wine-Dark Sea: Color And Perception In The Ancient World. Erin Hoffman, January 2013.
  40. Aeon, The Sea Was Never Blue. Maria Michela Sassi, July 31, 2017.
  41. Basic Color Terms: Their Universality And Evolution. Brent Berlin and Paul Kay, 1969.
  42. Gibson, Edward et al. “Color naming across languages reflects color use.” Proceedings of the National Academy of Sciences of the United States of America vol. 114,40 (2017): 10785-10790. doi:10.1073/pnas.1619666114
  43. calculus joke!
  44. Discovery of the Elements, by Mary Elvira Weeks. Search in the page for “sole discoverer” and you will find the passage.