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I never found out who started the game of hunting for treasure, that summer when I nearly died, but I’ve always assumed it was Great Uncle Søren himself.

For a man who claimed to have travelled little further than Bordeaux, my great uncle possessed an extraordinary collection of antiquated explorers’ paraphernalia, and we children were fascinated by it all. Every summer we looked forward to the annual visit when we might play at mountaineers in the rugged chalk pits beyond the woods, or prospect for gold in the willow-shaded stream at the bottom of the garden.

He lived in a former Victorian schoolhouse, distinguished by a giant araucaria tree that quite dominated the front lawn. The sign on the gatepost read Villa Nova but he just called it Wyndhams, so we did too, without question. Apart from his housekeeper and a gardener-cum-handyman, he received no visitors but us. He was said to be in correspondence with people as far away as Japan and Mozambique, yet for most of the time his only company was a large, ill-tempered cat called Otto – an English Blue I think – who held himself rather grandly, and was inexplicably known to us as The King of Siam.

Great Uncle Søren was actually our mother’s uncle by marriage. Her favourite aunt married Søren’s younger brother on the eve of World War Two, and she grew up close to her uncle Jens as well as his extraordinary elder sibling. Tall, flame-haired and with the eyes of a man whose ancestors sailed the great northern oceans, Søren must have stood out in any crowd. Among our Celtic family he was an oak in a forest of pale birches. For many years he worked as an industrial chemist, and is credited with inventing several industrial processes. He never married and, as far as anybody knew, we were by then his only surviving family. By the time I got to know him, this giant man had become stooped and grey, and he had long since retired to the peculiar seclusion of Wyndhams.

The house itself smelled strange inside, like a greenhouse full of old books. It was in eternal gloom, but the grounds were enormous and full of things to fascinate healthy children who dreamt of adventure. In the summer holidays we’d stay for a fortnight in that wonderful, musty wreck, passing long, humid afternoons in the overgrown orchard where we played recklessly at jungle explorers with machetes and pith helmets, searching for the treasure that Uncle Søren had quite naturally accrued during his daring overseas exploits; yet the most we ever uncovered was The King of Siam sunning himself on the old chicken coop.

Søren was, for us children, ancient and unchanging as a glacier-carved valley. We could not imagine him ever having been a young man. Still, we excitedly believed he had lived a different life long ago, between the wars. Unconvinced by his protestation of lifelong suburban exile, we were sure he’d travelled far and wide in his youth. It was obvious! The evidence was lying about his house: the cleated boots; the ash-handled ice-axe almost as tall as I was; the stout coils of rope; canvas dinghy and sealskin coat that was food for moths; the ancient tropical suit; butterfly nets and curious contraptions that filled a long-abandoned box room. We conjured him as an explorer in Conan Doyle’s Lost World or one of Jules Verne’s adventurers. In that house, anything – and everything – seemed possible.

The fact of hidden treasure was similarly not in doubt for us. Why he had hidden it was equally obvious: treasure is always hidden from pirates, bandits and foreign spies. Wherever he had concealed it, in the house or grounds, it was surely just waiting there for us to rediscover one bright summer’s day. While our mother was simply glad of time to bake or read without five energetic children under her feet, Søren had a strange attitude to our adventures. Sometimes he would laugh and join in until he fell on his back and demanded a stretcher-bearer but at other times he would grow serious and tell us we would never find treasure playing those sort of games.

According to our mother, there was no mystery about Søren’s past. He was a travelling salesman for a brewery in the 1920s, before turning to industrial chemistry after some great disappointment, which was understood to be a broken engagement from which he never fully recovered. I would love to have learned the truth, but uncle Søren died of a rare tropical disease before I was old enough to get a straightforward answer from him. Unusual, I'd say, for a Nordic chemist who lived in Hertfordshire.

Wyndhams then lay empty for many years. Though the gardener and housekeeper still visited occasionally to keep the weeds and cobwebs at bay (presumably out of loyalty, as Søren left no money), my family no longer went there. I was too young to go to the funeral but I remember being told that it was surprisingly well attended. Sadly, I don’t know what happened to Otto.

The story of Wyndhams would have ended there, if I hadn’t chanced upon a solicitor’s letter while going through my late mother’s papers. Uncle Søren had left the house and contents to us in his will. To us… the children. The condition was that it skipped a generation. No explanation was given, but it had been held in trust all this time. Wyndhams was waiting for us. The telephone number was out of date, and the partnership was not listed online, so I wrote to the solicitor and was surprised to receive a reply by return of post. Wyndhams really was ours. It took a while to settle the terms of the trust but in the meantime we managed to engage a local firm of surveyors to report on the condition. It wasn’t exactly positive.

Almost a year later, on a bright May morning, my siblings and I were gathered under the great araucaria tree waiting for the gardener to let us in to Wyndhams for the first time in 35 years. When he arrived on his old green scooter, the gardener looked much as I remembered him: mid forties, bearded, wind-weathered face… He didn’t make conversation, but as he opened the black front door he nodded in a half-familiar way. I smiled back like the small boy I had once been, then suddenly felt foolish as I realized a generation had passed since I was here and he must be the son of the gardener I knew, or perhaps his nephew. Before I could ask, he was gone.

The others were already inside when I entered the hallway. Nothing had changed. It was dustier and more decrepit, certainly, but it looked like Great Uncle Søren had just gone out for a walk. The brass umbrella stand, the oak chest, the hall rug, all were there. And the smell – it was exactly as I remembered it. We must have stood marvelling for 10 minutes, literally breathing in nostalgia, before we could talk. When we did, our speech was peppered with gulps, as if we wanted to eat the scented air.

As we explored the rest of the house, our spirits dropped. Everything looked decayed. The books in the library were mildewed and wormy, the tables and chairs were warped, the curtains were faded and torn, the wall clocks were falling apart. Nothing was worth salvaging. Nothing was as we remembered it. Our hearts were breaking. The room with the explorers’ paraphernalia was the greatest disappointment. Rain had got in through a broken window pane and everything was ruined. We tried to make an inventory of salvageable items but there seemed little point. What did we expect after decades of neglect… I felt foolish.

Lunch was cheese, a shared flask of soup and a box of crackers. We weren’t here for a picnic. We hadn’t even known if there would be water or power at the old house, but the kitchen was surprisingly clean and even had teabags and a few cans of food in the cupboards that were more or less in date. If the gardener had been brewing himself a cuppa every now and again, we weren’t going to begrudge him that.

After lunch, we pulled out the loft ladder. None of us had been up there as children, though we had tried of course. It wasn’t technically out of bounds like the study, but none of us could reach the latch to pull the ladder down. Besides, our uncle told us there was nothing up there but ghosts. We said we didn’t believe him but we didn’t want to risk it; perhaps we didn’t try too hard to get up there.

The loft space was large, dry, and mostly empty. But in a way, it did contain ghosts.

In the furthest corner of the dry attic stood a dark green trunk, which had neither been seen nor opened in four decades. One of us said it looked important so we gathered round and popped the catches together, almost ritually, at an unspoken signal the way siblings sometimes do. Inside the trunk, unaffected by the miasma that permeated most of the house, our uncle had stored personal documents that I knew at a glance would reopen for me the mystery of his youthful adventures: a flying log, old canvas-backed maps and sea charts, books that appeared to contain scientific research notes, a number of chemistry textbooks in English, German and Danish, and some official correspondence. It also contained copies of Life magazine recording the major events of the twentieth century from 1923 to 1964. 

My siblings were transfixed by Life’s iconic images of World War II, JFK, Norma Jean Baker and Martin Luther King, and were carefully leafing through them when I ran my hand over the calico bottom of this battered steamer trunk. It felt wrong. It was soft and unstable. The trunk had a divider, a false bottom. I allowed myself a flash of child-like excitement and pulled it up.

Underneath was a flat bundle of dark blue oilcloth. Calling my siblings over, I carefully unwrapped it to reveal two slim notebooks in tan-leather covers. On the first page of each, my great uncle’s name was carefully inscribed: Søren Peder Arnaldus von Carlsberg. Beyond that, they were very hard to fathom. One was handwritten in a cipher that I have not been able to penetrate. The other – apparently older, and certainly more decayed – was written in a mixture of English, shorthand and obscure personal abbreviations.

Later that afternoon, as we stood in the old school kitchen drinking milkless tea, I turned the notebooks over in my hands as if by doing so I could somehow unwind time. This was as close as any of us had ever been to our enigmatic great uncle’s thoughts, and he had chosen to obscure them. But why? The reality was that we knew so little about him; we had been too young and too excited by dreams of treasure to seek real answers. Now there was nobody left to ask except, perhaps, Søren himself.

My siblings soon lost interest and departed to their spouses and sofas and their 1.4 children in comfortable new towns. I had chosen a different path, and was at liberty to stay at Wyndhams without fear of missing a breakfast meeting with the head of finance or being late for the school run, and I alone had retained a childlike wonder at his mysterious past. So I resolved to find the truth in the books.

I spent the next day in the warm, damp study eating the last of the biscuits we’d brought, while trying to make sense of the older and possibly simpler book. The study’s mustiness provoked my asthma again, as it had done so dangerously in childhood, but spending time in that room, now all but stripped of books, was an indulgence I was determined to allow myself this last time – for nostalgia’s sake, if nothing else.

The first occasion I had been allowed into the study I was only four. I felt very privileged – his study was strictly out of bounds. I remember uncle Søren saying he had a very important job for me but I don’t recall what it was because his playfully stern lecture was interrupted by increasingly frantic wheezing on my part, as my lungs strained against ever-narrowing airways. I distinctly remember thinking that I was going to die, and my very first asthma attack would almost certainly have been my last had Søren not produced a jar of some vile-smelling herbal ointment that restored my breath to me. Nonetheless I was rushed to the local doctor. I was diagnosed with acute asthma and kept in the local hospital overnight. They said I’d had a lucky escape.

When I got back everybody was playing a new game outside: find Søren’s treasure. I wanted to dig beneath the monkey puzzle tree but instead we followed Otto into the long grass. The grass pollen didn’t bother me despite my new diagnosis, but I’ve kept an inhaler with me at all times ever since, although I find rarely need it. I’ll doubt I’ll ever discover what was in Søren’s herbal ointment that they say saved my life, but its distinctive smell came back to my mind as I reached for my inhaler, sitting again in the study, that last day at Wyndhams.

The house clearance was a matter of urgency. Wyndhams had been sold to property developers; it was soon to be demolished and the grounds used to build the Villa Nova science park. I objected to the destruction of the house and gardens where we’d spent so may childhood summers but mine was a lone voice. My siblings were less sentimental about the past and more pragmatic about the future. Perhaps they were right. The surveyor’s report was unequivocal; the wooden beams and panelling were rotting away from within, infected with the spores of some primal decay that could not be treated. It would be cheaper and safer to build a completely new structure than to repair and convert old Wyndhams to the standards of a modern research centre. You cannot stand in the way of progress.

Uncle Søren had been an avid solver of cryptic crosswords and I soon spotted that some of the classic conventions, which he long ago tried to teach me, had been deployed in the notebook. I laboured intensely at first, fruitlessly following dead ends and falling for linguistic traps and decoys, all the while trying to get a sense of the puzzle-setter’s style. Straining away at the clues and codes of Søren’s notebook, I found myself muttering at my decades-deceased great uncle there in his mouldering study. Initially I cursed him for his obscurity and deviousness. Hours later I acknowledged him for his craft and guile. But finally I found myself calling him a clever old bugger, out loud and, in a strange way, I distinctly felt he was with me in that room and, the way I imagined him, he was smiling at me.

Slowly, with time and mental exertion, the task became easier; I gained familiarity with my adversary, his tricks and twists, finding that one solution was often the key to the next clue and each time I solved a riddle – identifying the two halves of the clue, piecing together the meaning of one half and matching it with the other to reveal a word or phrase – I sensed I was slowly bringing something into the light. I worked like a palaeontologist, tapping an interesting rock to see if it will split this way or that to reveal something hidden for millennia, something primal and compelling. I felt something else too. I sensed that I was once again on the trail of Great Uncle Søren’s treasure – not the hidden gold of childhood summer orchards but a treasure of knowledge, of all the stories he never told us as children.

Eventually the low sun moved to the far side of the house and the study became too dark to read. Great Uncle Søren had never been one for bright illumination at the best of times. The small study window was shaded by a large monkey puzzle tree, yet the only lamp in the whole room was an aged Bakelite pendant whose brown fabric flex was tinged with alarming hints of copper.

The dim bulb hung over an enormous glass bottle filled with mosses, lichens and ferns, which had fascinated me as a child. Indeed, “Terrarium” was probably the first polysyllabic word I learned and I loved it for sounding both scientific and adventurous and, as a seven year old, I employed it precociously at every opportunity. Decades later, I still cringe with embarrassment if I recall how many junior school adventure stories I clumsily shoehorned the word into, to my teachers’ mounting dismay.

Terrarium was also the answer to the first cryptic crossword clue I ever solved on my own. My uncle never quite understood children; he’d clearly been disappointed that I was unable to grasp cryptic crosswords when he first introduced me to them at eight years old. Sadly, he died soon after – long before I was old enough to discover that I actually enjoy them – but with quite some foresight he’d left me a little book, published by the Daily Telegraph, which set out how to solve cryptic clues. It wasn’t until I got to university that I read it, understood how cryptic puzzles work and discovered that uncle Søren had set me a little graduation test at the end of the book:


Try this one, my boy: “Terry endlessly has a rum – taking one in the garden in a bottle” (9)

As any solver knows, most cryptic crossword clues are divided into two parts: a synonym of the solution (usually disguised) plus a method of constructing that same word or phrase. Any apparent sentence structure or grammar in the clue is almost always a red herring and should be carefully ignored; it is only there to mislead you. Nonetheless, the most elegant clues also reward the solver with their grammatical resonance once solved.

I identified the synonym part first: ‘the garden in a bottle’. This was easy for me as bottle garden is another name for terrarium – which has nine letters – so Terrarium would most probably be the solution. But how to construct terrarium from the other part of the clue? A half-solved cryptic clue is not solved at all. Few are as benevolently constructed as Søren’s first simple test for me and an incorrectly guessed synonym is like a desert mirage over quicksand: a false hope if you’re lucky and an unforgiving trap if you’re not.

I looked at it again: “Terry endlessly has a rum taking one in / the garden in a bottle”

The solution began to fall into place once I saw that ‘Terry endlessly’ could be Terr.

So ‘has a rum’ means I add ‘a rum’ to the end of Terr, making Terrarum.
Then ‘taking one in’ means Terrarum gains the letter I somewhere (i and 1 are interchangeable, it’s just the convention, there’s no fighting it…) finally making terrarium.

And so I had solved the first of Great Uncle Søren’s clues while I was at university, a dozen years after he had set it for me. Another dozen years later I was sitting beside that very same terrarium gazing into its peculiar foliage with another of his riddles in my hands. I was enjoying this.

I had never been allowed to touch the bottle as a child, merely to look through its misty glass sides and wonder at the primeval world within, and it still amazed me how he’d seeded a whole self-contained world through that small aperture. It had been planted, watered and sealed before I was born, existing as a self-sustaining ecosystem requiring nothing but a little gentle light and heat – the world in miniature – but now the decades-old cork had finally decayed and crumbled into the bosky interior. Chancing to sniff at the open top, I found the acrid, musty smell permeating the house was concentrated a hundredfold within the terrarium and, despite the condensation on the inside of the glass, I supposed that the terrarium’s flora was finally dying from years of neglect. I reached for my inhaler again.

I worked by the terrarium’s weak light until my eyes ached. Many passages of the journal were unfathomable and some pages appeared to have been deliberately erased, some had even been removed, but I did my best to transcribe my great uncle’s notes into a legible form. There was nothing left in the house to sustain me but a bottle of his finest malt whisky and it was well past midnight by the time I reached the end. But what a picture emerged! The book was an account of his remarkable exploits just after the Great War. In all our childhood imaginings we could never have bettered great uncle Søren’s real-life adventures.

This worn leather volume, with foxed pages and smudged pencil jottings seemed to be a research log book-cum-travel journal, written in preparation for some formal presentation. He had evidently mounted an extensive expedition with significant institutional backing, but the very presence of this diary, hidden in the family attic, and his closing comments lead me to believe that no formal report was ever delivered.

Shortly after the end of the First World War, Søren had launched an expedition in the grand style of Victorian adventurers. The aim was to bring back knowledge and materials from the lesser-known quarters of the world that might serve King and Country though industry, science and commerce – but ultimately disaster struck and Søren barely made it back with his life.

In much of what I transcribed it is hard to pick fact from fiction. Pages of field notes with detailed observations are interspersed with fantastical accounts of the places he travelled, the peoples he encountered and the things he witnessed. At times it reads as if Lemuel Gulliver and Charles Darwin had swapped adventures.

I read it through again and, as dawn broke, I finally fell asleep to dream fretful imaginings of my great uncle’s youthful adventures upon stormy seas.

7th May 1922.

"In the lands east of the equator [?] is a peninsula that, to all intents and purposes constitutes an island. There is no communication by land due to the impassable cordillera, and only in the calmest weather does the sea allow small boats to sail between Lit [?] and the isthmus of pH’.

The people of this region have no sense of colour. 

That is not strictly correct, of course – though they are entirely colour blind in the sense that they have no optical colour perception, they do discern colour, just not with their eyes. What nature, through isolated evolution or chance mutation and misfortune, has denied them via their retinal function, they have developed a means to discern through other, heightened senses and olfactory indicators.

Indeed, the full colours of the rainbow are known to them through their complex associations with anthoc..j... [?] …

[Here the smudged manuscript becomes illegible for several paragraphs but the narrative picks up again on the next page...]

...  not as might normally be expected in such strange circumstances!

On the isthmus are fifteen natural ponds, fed from deep below the surface. In the middle of each pond grows a tree so covered in Orchella Weed (a local lichen variety which I believe to be unique) that its natural shape can barely be discerned. What makes these ponds truly remarkable is their intense range of colours. From deepest violet to the most vibrant reds, these fifteen ponds sit, rainbow-fashion, in a forest clearing.

The ponds seem to cover most of the visible spectrum as if lit by a strange, divine prism. The first is deepest rose red, the second, a more fiery tone, the third, slightly orange, with the fourth, fifth and sixth progressing though yellow and lime towards the bright green of young tropical foliage. From the seventh to the tenth, the ponds move through blue-green to blue and then the intense colour of African Violets. Beyond here the last few ponds go through the colours of foxgloves to hibiscus and finally to a pink-mauve found only otherwise in some tulips.

The people of the isthmus refer to these ponds as The Universal and revere them greatly. Their language is generally hard on the western ear and I have learned little of it so far but it seems to be based on combinations of discreet symbolic units rather than complex word structures.

[ ... unreadable … ]

In each pond swim two varieties of fish. The smaller Hej.plas which swim exclusively clockwise and the larger Hej.minnows which swim anticlockwise.

Both varieties of fish are extremely aggressive, attacking anything that should fall into the water, yet they seem to have an affinity for each other that results in a peaceful symbiosis.

The aggressiveness of one species of fish is diminished by presence of the other and the degree of aggressiveness seems to be in proportion to the balance of numbers. Where there are more Hej.plas than Hej.minnows, the latter is relatively placid but the former more violent. Where there are fewer Hej.plas than Hej.minnows, the former is relatively placid but the latter is more aggressive.

When their numbers are in balance they can be seen nudging up to one another as if sharing the invisible electric field that surrounds every fish. In this state they become placid, and, though they make no permanent bond, they always pass onwards, one to another in a state of calm balance.

The deepest mauve pond at one end of the clearing contains many more Hej.minnows than Hej.plas and is considered highly dangerous to man and beast. It is used for the tanning of hides. At the other end of the clearing is the dark red pond in which swim mostly Hej.plas and their aggression knows no bounds – this also is deadly to any animal bar the fish themselves, but it too has its uses such as the disposal of animal bones. 

In the blue-green pond at the very middle, the two varieties of fish live harmoniously in equal numbers and the water is not only safe to swim in but also safe to drink.

When something organic like an animal falls into one of the outer ponds, the fish that are in greater number will attack it with a zeal reminiscent of the mythical piranha hordes of the Amazon. Yet, here something quite bizarre happens; although the poor victim is largely consumed, it is not actually eaten by the fish as such – not in the normal sense. I cannot explain it as a scientist but, as the victim is consumed, the proportion of the two fish species seems to be slightly redressed, the aggressiveness diminishes and, in extreme cases, I am told the colour of the pond can even change temporarily to reflect this, before it is replenished by some underground source. How this happens is one of the mysteries of the inhabitants of pH’ and I intend to stay with them with the hope of being initiated into such mysteries.

It is not known what the fish eat or how they breed but the pH’ have a myth that they come from the very water itself and will return to it in the end.

[Again the manuscript becomes illegible for several paragraphs, as the graphite writing has smudged. It appears to have been erased in places down to the bottom of the page. On close inspection, the next page appears to have been carefully removed with a razor but when the narrative picks up again it is clear my great uncle was still on the isthmus of pH’ ]

I have made further discoveries as I have become more proficient in the language of the region. It seems the Hej.plas live in symbiosis with another creature known locally as the Hej.tuu’oh. This appears to be not a fish at all but a kind of two-tentacled cephalopod, though it is hard to be certain, as their language is a symbolic one. At times I wish I were a naturalist or an anthropologist, not a chemist.

The problem, however, is that I cannot actually see the Hej.tuu’ohs, although the pH’ assure me that they are present in all the water, in all the pools. Whether this is an elemental spirit of local legend or a genuine, physical creature it is beyond my capacity to discern and I am beginning to wonder whether there is as clear a distinction on this strange isthmus as my rational upbringing would expect in any other land. Nonetheless the pH’ assure me that Hej.plas have a particular affinity for the Hej.tuu’oh and form loose bundles like the mating balls of certain snakes (for want of a better metaphor – and I can think of none) with one or several Hej.tuu’ohs clustered around and joined with each Hej.plas.

This condition they name h’d.Dron’ium and I am told, as I am drawn deeper into the mysteries of the pH’, that I should never imagine a Hej.plas as a single fish but rather as the symbiotic consort of one or more of these squid-like Hej.tuu’oh...

Over the page, the notes have been deliberately erased and nothing more is legible until the very final entry in my great uncle’s journal:

4th September 1922

I came seeking knowledge to share with the world but I cannot now present my discoveries to any of the eminent societies who sponsored my trip. Not to the Royal Society, for I would be laughed at. Nor can I tell my tales in the Explorer’s Club for nobody would believe me. And though the Royal Geographical Society would surely welcome my discoveries east of the equator [?] I cannot divulge the location of the isthmus of pH’ for fear that its people suffer from contact with our own society. 

What then?

My one last hope was to present certain aspects and principles of The Universal to the Royal Institution on Albemarle Street where demonstration of an optical indicator for acidity and alkalinity might be well received. However, since the loss of my samples and equipment in the South […?] Sea I have nothing to show and nothing to analyse. 

Not wishing to dwell on my own misfortune, I feel I have at least earned a kinship with poor, dear Wallace. The sinking of the Helen and the loss of his precious specimens must have seemed a mortal blow to that benighted pioneer. Yet, he recovered from this great setback, from witnessing his moneys burn and his birds drown, from the loss of four years Amazonian collecting and the destruction of nearly all his notes… Brave Wallace set forth again and ultimately gave us, with Darwin, the great Theory… why then should I not also recover from shipwreck and disaster?

At least I am neither racked with malaria nor adrift on the ocean in a leaky boat thanks to the kindness and hospitality of the […?] to whom I owe my life many times over.

And so, in the end I am simply grateful to have escaped with my health and my notebooks – those and the few precious spores of Orchella Weed, which I hope might survive the mild Hertfordshire climate. Perhaps, even, they will establish themselves and thrive at Wyndhams if I can only create a suitably hermetic habitat."

I was woken too soon by heavy machinery preparing an access road through the grounds of Wyndhams. Down in the meadow where we had played as children, workmen were already pegging out plots where the first foundations would be laid. Otto would not have been amused.

I packed the journal alongside my own notes and let the pain behind my eyes slip away as I walked out into the day. I took only what I could carry in my arms. Whisky and tiredness had taken their toll, and, as I turned out the study light for the last time I thought for a moment that I saw strange fish swimming in the terrarium. Shaking my head, I closed the door and left Wyndhams behind.

The timbers of the old house creaked in the sunshine, and in them I heard Uncle Søren’s hearty Nordic laugh.

---

Universal Indicator

Given the importance of acids and bases (alkalis) to human activity – from metallurgy and food production to tanning and soap-making – it is surprising that there was no agreed scientific scale for the strength of acids before 1909.

In fact, little was understood about the physical chemistry of acidity until Swedish polymath Svante Arrhenius proposed his Ionic Theory in 1884. Arrhenius suggested that acidity is caused by the activity of hydrogen ions (H+) within a solution, and alkalinity by the activity of hydroxide ions (OH-). These are of course the Hej.plas and O’Hej.minnows in the story.

Arrhenius later received the Nobel Prize in Chemistry for his theory of electrolytic dissociation, which stated that "the molecules of an electrolyte undergo spontaneous dissociation to form positive and negative ions" in water – an insight that came directly from his work on acids. An ion is simply an atom or molecule with an electric charge, and Arrhenius acids are solutions with an excess of H+ ions. Alkalis are also known as Arrhenius bases because they dissociate to form OH- ions in water as his theory states. Not all bases are alkalis, but all bases are alkaline.

The simplest Arrhenius acid is hydrochloric acid (HCl) comprising just hydrogen and chlorine. Pure HCl is a gas, but it is highly soluble in water where each HCl molecule dissociates to form a H+ ion and a Cl- ion. HCl is a strong electrolyte and a strong acid, as is the sulphuric acid in car batteries.

In 1909, Danish Chemist Søren Sørensen devised the pH scale of acidity/alkalinity while working at the Carlsberg Laboratory in Copenhagen. The laboratory had been established by the founder of the famous Carlsberg brewery "for the advancement of biochemical knowledge", so without brewing, we might not have the pH scale today.

In simple terms, pH defines how strongly acidic or alkaline a solution is. The scale runs from 0 to 14, where pH 0 is extremely acidic and pH 14 is extremely alkaline. In the middle is pH 7 (or "pH neutral" as any student of shampoo advertising will know). The pH scale is logarithmic, so each whole number represents a tenfold change in strength. For example, pH 4 is ten times more acidic than pH 5, and 100 times more acidic than pH 6.

A strong acid, by Arrhenius' definition, contains more free hydrogen ions than a weak one. Sørensen's pH scale describes the concentration of these "solvated" hydrogen ions and their potential to form new bonds. According to the Carlsberg Foundation, pH stands for "power of Hydrogen".

This was the state of knowledge when Great Uncle Søren visited the isthmus of pH' in 1922.

In 1923, Brønsted in Denmark and Lowry in England both eliminated the role of the hydroxide ion (OH-) by defining acids as donors of H+ ions, and bases as acceptors of H+ ions. A hydrogen atom is simply one proton and one electron, so an H+ ion is a proton. That same year, Gilbert Lewis developed a theory in which acids and bases swap electrons, instead of protons. Both definitions work equally well with Sørensen's pH scale.

By describing acidity without referring to ions, our understanding of acids has been extended beyond the simple electrolytic solutions Arrhenius started with. This doesn't mean he was wrong, merely that his definition was specific to the Arrhenius acids we encounter on an everyday basis. It was certainly a good enough definition for a brewery chemist in Copenhagen, when Sørensen devised the pH scale that is still in use.

For what it's worth, the pH of a good beer is around 4–4.5 and the pH of healthy human blood is 7.4. Neutral pH 7 is the theoretical pH of distilled water at 25°C, containing nothing but liquid H₂O – except that liquid water comprises more than H₂O molecules. Water itself partially auto-dissociates into ions, as though it were its own solvent. When water dissociates it forms OH- and H+ ions in equal number, so remains pH neutral by any definition of acidity.

But water is a peculiar substance. Instead of dissociating and recombining to form H₂O again, an ion called hydronium is formed. Free H+ ions eschew the obvious step of bonding with free OH- ions to form a water molecule, instead bonding loosely with existing H₂O molecules. Three H+ ions now share the one oxygen ion to form H₃O+. The chemical formula of water may be H₂O, but it also contains a mixture of hydronium and protons. Not only is hydronium found in the ponds of the pH' in the story, but it is abundant in the interstellar medium of outer space and in the plasma tails of comets.

The pH scale is a human invention. The numbers refer to a set of standard solutions whose pH value is recognized by international agreement: one standard solution for each pH number that Sørensen established in the Carlsberg lab. Technically, pH is an electrochemical analysis, yet most of us know pH from the coloured paper strips in a school chemistry lab or gardeners’ soil testing kit. These strips are known as indicators.

Many plants contain compounds that change colour with pH, of which red cabbage is the best-known example. The anthocyanins that produce this effect are widespread in the plant kingdom, and homemade indicators are a fun kitchen chemistry project. The best-known indicator has a long heritage and its name has passed into common usage: Litmus.

The first recorded use of litmus was around 1300 by the Spanish alchemist Arnaldus de Villa Nova. Alchemy and alkali are words of Arabic origin, and Villa Nova translated Arabic scientific texts, so his knowledge of litmus probably derives from them. Litmus is a simple indicator: red for strongish acids and blue for alkalis. It uses the same family of anthocyanins found in red cabbage, but for litmus they are extracted from lichen.

Cloth dye was commonly made from lichen in medieval Europe, so it was widely available. Red archil dye was made by exposing lichen extract to urine; if potash was added, the result was dark blue litmus. Litmus paper uses these chromatic changes to indicate acidity. The word itself derives from Old Norse, lit mus, meaning coloured moss, so Great Uncle Søren's Viking ancestors would have known about the dye and how it was produced.

The lichen used for archil and litmus dyes was known as Orchella Weed and was once the chief export of Cape Verde. In Mauritius, Orchella Weed was used to treat chest and lung conditions, perhaps due to the presence of the natural phenolic antiseptic, orcinol.

Demand for a more accurate yet simple to read test prompted the search for a universal indicator. Modern Universal Indicator is a chemical solution of synthetic compounds that together achieve this through a rainbow of colour changes, smoothly transitioning across the pH scale. It can be used as a liquid or the more familiar prepared paper strips.

The colour gamut spans from red at pH 0 through orange and yellow (mildly acidic) to green for pH 7. Mild alkalinity is indicated by blues, and finally, deep purples indicate the strongest alkaline at pH14, just as the ponds in the story do. The Japanese Yamada Corporation filed a commercial patent for universal indicator in 1933. It would have been considered a wonder by Great Uncle Søren in 1922.

While Litmus is still widely available, it has been superseded for most practical purposes by Universal Indicator.