The [Saharan] dust particles provide nuclei for the formation of ice crystals in clouds above the rain forest and so help to enhance or maintain precipitation over the Amazon rain forests. Equally important, trace elements within the dust such as nitrates, phosphorous [sic], and potassium are a major source of plant nutrients.

Martin Williams, When The Sahara Was Green (2021)

The word in red should, of course, be phosphorus, the name of a chemical element vital to plant growth. The error will no doubt have slipped past a routine spellcheck, because phosphorous is also a word, just not the word required—it’s an adjective meaning “pertaining to, or rich in, phosphorus”. It also has a specific meaning in chemistry, in which it is contrasted with phosphoric—phosphorous acid having a different chemical formula from phosphoric acid, for example.

This is the sort of slip that would normally be picked up by a good subeditor or proofreader, but I see this misspelling of phosphorus fairly frequently. There are a few other words that fall victim to the same kind of error. Here’s one I picked up in the very next book I read:

Erosion occurring on the outside of [river] bends pulls loose the alluvial deposits. Gravel, sand and soil are transported farther downstream, creating callouses [sic] of new terrain, overlapping on the inside of bends, redirecting flow and exaggerating curvature.

Patrick Baker, The Cairngorms: A Secret History (2014)

This should be calluses, referring to the patches of hardened and thickened skin we develop in areas of frequent wear and tear. Again, callouses slips past the spellchecker because it’s a perfectly valid verb, referring to the process of causing calluses. And callous is both a verb and an adjective, so will again go undetected.

To this common misuse of phosphorous and callous, we can add mucous (for mucus) and humous (for humus). In all of these cases, an adjective ending in -ous is mistakenly used instead of a noun ending in -us. Which is actually rather odd, given that -us is a perfectly familiar ending for English words derived from Latin, whereas -ous is a vanishingly rare among English nouns. The few examples that do occur tend to be obvious foreign loanwords—there’s burnous (a hooded cloak, Arabic); nous (common sense, Greek), snous (powdered tobacco, Dutch); couscous (an alleged foodstuff, Arabic via French); houmous (Middle Eastern dip, Arabic again, often spelled hummus). The only -ous noun I can think of that looks like a conventional English word is scabious, a flowering plant with a name derived from a French adjective. So there’s not much precedent in English for -ous nouns; whereas a search of my electronic copy of the Oxford English Dictionary turns up a whopping 5640 hits for adjectives ending in -ous. Some of those will be duplicates, but still—the -ous suffix should scream ADJECTIVE! to a native English speaker.

But back to phosphorus. Before it was the name of a chemical element, it was a name for the planet Venus when it appeared in the morning sky. The name Phosphorus is Latin, but derives from Greek phosphoros, “light bearer” or “light bringer”. The latter translation seems appropriate, since when Venus rises as the morning star, the sun is not far behind.

It was also a word used by 17th century scientists to designate any plant, animal or mineral that could be induced to glow without heat—they were all called phosphori. Chemists devoted a lot of effort to producing chemical phosphori, which were often named after their discoverer. So we have Canton’s Phosphorus (calcium sulphide) and Homberg’s Phosphorus (calcium chloride), among others. The phosphorus that turned out to be the element we now call phosphorus was discovered by the alchemist Hennig Brand in 1669, after boiling a very large quantity of urine. He had managed to produce the unstable form of phosphorus called “white phosphorus”, which glows in the dark as it combines with atmospheric oxygen.

As the processes underlying the various glowing substances came to be better understood, a more precise vocabulary was built up. Anything producing light without heat was designated luminescent, and the kind of luminescence could be stipulated with a prefix—bioluminescence comes from living organisms, chemiluminescence from chemical reactions, and photoluminescence is induced by exposing a substance to light. Photoluminescence is divided into two categories, one fast and one slow. The fast one is fluorescence, in which a substance absorbs a high energy photon and then almost immediately emits a photon of somewhat lower energy—fluorescent dyes are used in, for instance, “hi-vis” safety jackets, which absorb ultraviolet light and then emit visible light. The word derives from the name of the mineral fluorite, which can be induced to glow in this way. The slow form of photoluminescence is phosphorescence, in which a substance absorbs light energy when illuminated, but releases it only slowly, so that it can glow in the dark. So the name is a reference to the various “glow-in-the-dark” phosphori of the early chemists, rather than to the element phosphorus, which is chemiluminescent.

That’s a neat modern scientific classification, but in common usage the noun phosphorescence, the adjective phosphorescent and the verb phosphoresce are all still philosophically connected to all those various unclassified phosphori of 17th century science—anything that glows in the dark can be described as phosphorescent. In particular, the glow produced in the sea by tiny bioluminescent organisms is commonly called phosphorescence. This drives marine biologist Edith Widder wild. In her book about bioluminescence, Below The Edge Of Darkness (2021), she writes:

Phosphorescence is not bioluminescence, despite how often you hear the words equated. It’s a very common misconception that has been repeated so many times it borders on a disinformation campaign.

It’s not really a misconception at all, though—just a conflict between long-standing common usage and newer scientific usage, something which is widespread in the sciences.* So I’d guess Widder was left unimpressed by the title of Julia Baird’s recent book, which takes the experience of swimming in a bioluminescent sea as a metaphor for “flashes of life in the middle of the dark, or joy in difficult times”. It was, of course, entitled Phosphorescence (2020)—the persistence of the old usage is what lets the metaphor work, while insisting on scientific precision would have killed it dead.

There’s more to write about the word phosphorus, but I’ll come back to it another day.

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* Widder undermines her position of authority a little, for me at least, by repeatedly describing bioluminescent organisms as “neon-blue”. Neon is the gas responsible for the classic orange colour of decorative gas-discharge tubes—it’s mercury that glows blue. The common usage of the word neon to indicate “a particularly vivid colour” is (ahem) a very common misconception that has been repeated so many times it borders on a disinformation campaign.