A In the closing years of the nineteenth century, physics was gripped by a series of unexpected discoveries. In late 1895 Wilhelm Röntgen in Germany announced that a cathode-ray tube produced penetrating rays capable of revealing the bones inside a living hand, and within weeks X-rays had become a public sensation. A few months later, the French physicist Henri Becquerel reported something stranger still: salts of uranium, left in a drawer on photographic plates wrapped in black paper, fogged the plates even though they had not been exposed to sunlight. Yet Becquerel's rays were faint, and they could not produce the dramatic images that had made Röntgen famous. Most researchers soon turned their attention elsewhere. When a young Polish-born physicist, Marie Curie, was looking for a subject for her doctoral thesis at the end of 1897, it was partly this neglect that attracted her: the field was almost empty, with little published work to master.

B What distinguished her approach from the start was a commitment to precise measurement. Rather than relying on photographic plates, which showed only whether radiation was present, she used an instrument that could quantify it. Uranium rays make the air through which they pass conduct electricity, and the tiny current involved could be measured with an electrometer that Pierre Curie, her husband, had developed years earlier with his brother Jacques. Within weeks she had established that the intensity of the radiation depended solely on the quantity of uranium in a sample; whether the element was wet or dry, powdered or solid, pure or combined with other substances made no difference. This apparently modest observation had radical implications. If the rays were unaffected by the chemical state of the material, they could not be the product of reactions between molecules; they had to originate inside the atom itself, at a time when many scientists still regarded the atom as indivisible.

C Curie then extended her survey to every element and mineral she could obtain. Only one other element, thorium, proved to be active, a result that the German chemist Gerhard Schmidt had reached independently at almost exactly the same time. The more puzzling finding concerned pitchblende, a dark ore from which uranium was extracted: some samples were several times as active as their uranium content could explain. Curie reasoned that the ore must contain a small quantity of an unknown substance far more powerful than uranium. The hypothesis was compelling enough for Pierre to set aside his own research on crystals and join her in the search.

D In July 1898 the couple announced the first of their new elements, which they named polonium in honour of Marie's native country. In December they reported a second, radium, whose activity was far greater still. In the first of these papers they used the word 'radioactive' to describe substances that emitted such rays, coining a term that has been in use ever since. Not everyone was persuaded. Chemists of the period accepted a new element only when it could be isolated in visible quantities and its atomic weight determined, and the Curies had identified theirs purely through the radiation they gave off. Polonium proved too scarce and short-lived to be separated in this way; radium, it seemed, might be possible.

E Obtaining it required years of punishing physical labour. The Austrian government agreed to supply residues from the uranium mines at Joachimsthal in Bohemia, and several tonnes eventually arrived in Paris. Since no proper laboratory was available, the Curies worked in a leaking shed at the School of Physics and Chemistry where Pierre taught. Marie took on much of the chemical processing herself, boiling down the ore in large cauldrons and stirring it with an iron rod nearly as tall as she was, before refining the products through thousands of successive crystallisations. In 1902, after almost four years, she had obtained about one tenth of a gram of pure radium chloride, enough to measure its atomic weight. The couple were already noticing the effects of what they handled: their fingers became cracked and sore, and Pierre once strapped a sample of radium to his arm for ten hours in order to study the burn it produced.

F Recognition followed quickly, though not without complications. The Nobel committee originally intended to award the 1903 physics prize to Becquerel and Pierre alone; only after Pierre was warned by a sympathetic member of the Swedish Academy, and insisted that his wife's role be acknowledged, was Marie included. The couple also took the deliberate decision not to patent their method of purifying radium, even though it was soon being exploited by a lucrative industry, on the grounds that scientific knowledge should be freely available. After Pierre was killed in a road accident in 1906, Marie was appointed to his chair at the Sorbonne, becoming the university's first female professor, and in 1911 she received a second Nobel Prize, this time in chemistry.

G Her later career turned increasingly towards the application of radiation. When war broke out in 1914, she recognised that X-ray equipment, then largely confined to city hospitals, could save lives if it were taken closer to the front. She equipped around twenty vehicles as mobile radiography units, soon nicknamed 'petites Curies', and trained women to operate them; by the end of the war, more than a million examinations of wounded soldiers are estimated to have been carried out. Curie died in 1934 of aplastic anaemia, a failure of the bone marrow almost certainly caused by decades of exposure to radiation.

A Coral reefs are the largest structures on Earth built by living organisms. The Great Barrier Reef, a system of some 2,900 individual reefs, stretches for more than 2,300 kilometres off the coast of Queensland. Yet the animals responsible are tiny. A reef-building coral is a colony of polyps, most only a few millimetres across. For much of the nineteenth century the existence of these vast formations in clear tropical seas posed two puzzles. The first was biological: how could so much life flourish in water so poor in nutrients that the open ocean around a reef has often been compared to a desert? The second was geological: why should reefs, which can grow only near the surface, rise from ocean floors thousands of metres deep?

B The answer to the first puzzle lies inside the coral's tissues. Each polyp harbours vast numbers of single-celled algae, which photosynthesise and pass much of the sugar they produce to their host, in some species enough to meet around 90 per cent of its energy requirements. In exchange the algae receive shelter and a supply of the nitrogen and phosphorus contained in the coral's waste, so that scarce nutrients are recycled within the partnership rather than lost to the surrounding water. The arrangement also powers construction. Polyps extract calcium and carbonate ions from seawater and deposit them as aragonite, a crystalline form of calcium carbonate, beneath their bodies, and experiments have shown that this process runs considerably faster in the light than in darkness. The dependence on sunlight explains why vigorous reef growth is largely restricted to the upper few tens of metres of the ocean.

C Rates of growth vary enormously. Branching species such as staghorn coral can extend by ten centimetres or more in a single year, while massive, boulder-shaped colonies typically add only about a centimetre. A reef as a whole, however, accumulates far more slowly than the corals living on it, often by only a few millimetres annually. Much of what corals produce is broken off by storms or ground down into sand and rubble, and this loose material becomes part of the structure only after it has been bound together. Crustose coralline algae, pink, rock-like organisms that spread as a thin layer over dead surfaces, play an essential role here, cementing fragments into a solid framework capable of withstanding the force of the waves.

D The second puzzle was addressed by Charles Darwin, who published his explanation in 1842, after the voyage of HMS Beagle. Darwin proposed that reefs begin as fringing reefs, growing in shallow water close to the shore of a volcanic island. As the volcano becomes extinct, the island and the sea floor beneath it slowly sink. Because the corals can grow upwards fast enough to remain in sunlit water, the reef keeps pace with this subsidence, while the island it surrounds steadily shrinks. Eventually a stretch of open water appears between the coast and the reef, which is then classed as a barrier reef. When the last of the volcanic peak disappears beneath the waves, all that remains is a roughly circular ring of reef enclosing a central lagoon. This final form is known as an atoll.

E For more than a century Darwin's theory could not be tested directly, and rival explanations had their supporters. The decisive evidence came in 1952, when American geologists drilled into Enewetak Atoll in the Marshall Islands in preparation for nuclear weapons tests. Their boreholes passed through more than 1,200 metres of reef limestone before striking the volcanic basalt beneath, just as the subsidence model predicted. Darwin's account has since been refined rather than overturned. In 1915 the Canadian-born geologist Reginald Daly argued that the repeated rise and fall of sea level during the ice ages had also shaped reefs, and both mechanisms are now thought to have operated together.

F Whether a reef grows or shrinks today depends on what scientists call its carbonate budget: the balance between the calcium carbonate added by corals and coralline algae and the amount removed by physical and biological erosion. Many reef animals are, in effect, demolition agents. Sponges and worms bore into the framework, sea urchins scrape at its surface, and parrotfish bite off chunks of coral with their beak-like teeth, grinding them up and excreting the remains as fine sand. In a healthy reef this erosion is more than offset by new growth. A 2013 survey of reefs across the Caribbean, led by Chris Perry of the University of Exeter, found, however, that many were producing far less carbonate than in the past, and that a significant proportion had tipped into net erosion, losing material faster than it was being replaced.

G The outlook depends partly on chemistry. Roughly a quarter of the carbon dioxide released by human activity is absorbed by the oceans, where it forms carbonic acid and reduces the concentration of the carbonate ions that corals need to build their skeletons. Since the start of the industrial era, the average pH of surface seawater has fallen by about 0.1 units, equivalent to a rise in acidity of around 30 per cent. Meanwhile, bleaching caused by marine heatwaves is killing the corals that drive construction. The danger is not merely that reefs will lose their colour, but that, as their carbonate budgets turn negative, they will be unable to keep up with rising sea levels, the very feat that, in Darwin's account, allowed them to survive for millions of years.

A Few questions in educational research have generated as many headlines as whether reading on a screen is inferior to reading on paper. The stakes are considerable. Schools in many countries have replaced printed textbooks with tablets and laptops, often for reasons of cost and convenience, and universities increasingly assign readings that exist only in digital form. If the medium really does affect how well we understand what we read, these decisions deserve far more scrutiny than they have generally received. Yet the debate has too often been conducted in terms of nostalgia on one side and enthusiasm for technology on the other, and neither is a reliable guide to the evidence.

B That evidence has accumulated quickly. In 2018 a team of Spanish and Israeli researchers led by Pablo Delgado published a meta-analysis that combined the results of 54 studies conducted between 2000 and 2017, involving more than 170,000 participants. Overall, readers understood informational texts better when they read them on paper, a finding the authors described as a 'screen inferiority effect'. The advantage was modest, but it was consistent, and two further details stood out. The gap was larger when participants were working under time pressure, and, contrary to the widespread assumption that a generation raised on digital devices would adapt, it had not narrowed over the period studied. If anything, it had grown.

C The effect is not universal, however. In the same analysis, no difference between the two media was found for narrative texts such as stories, which readers tend to follow from beginning to end. Experiments by the Norwegian researcher Anne Mangen nonetheless suggest that even narrative comprehension may suffer in subtler ways. In one widely reported study, adults who read a short mystery story on an e-reader performed as well as those given a paperback on most questions, but were noticeably worse at reconstructing the order in which events had occurred. Mangen has proposed that the physical book, whose weight shifts from one hand to the other as the reader progresses, offers a sense of position in the text that a screen does not.

D Why should a screen make a difference? One explanation, associated with the cognitive psychologist Rakefet Ackerman, concerns not perception but judgement. In her experiments, people reading on screens were more likely to overestimate how well they had understood a text and, consequently, to stop studying it sooner than they should have. Ackerman links this to the habits that screens encourage: most of what we read on them, from messages to news alerts, is brief and disposable, and readers may unconsciously carry the same rapid, superficial approach into material that demands sustained attention. The neuroscientist Maryanne Wolf, author of Reader, Come Home, goes further, warning that a lifetime of skimming may erode the capacity for what she calls 'deep reading': the slow, inferential and reflective processing on which critical thinking depends.

E These are serious claims, but I think they need to be treated with some caution. Wolf's argument rests partly on the plasticity of the brain, and it is easy to slide from the uncontroversial point that reading circuits are shaped by experience to the far more speculative suggestion that digital reading is causing lasting damage. Nor should the advantages of screens be dismissed. For some readers with dyslexia, for instance, e-readers that display short lines of enlarged text appear to improve both speed and comprehension, and digital texts can be searched, annotated and translated in ways that paper cannot. A wholesale return to print would sacrifice these benefits for the sake of a gain that, on average, is small.

F What the research does show, in my view, is that the problem lies less in the device than in the way we use it. The finding that the screen inferiority effect is strongest under time pressure is telling: it suggests that readers handle the two media differently when they are in a hurry, rather than that glass and pixels are intrinsically harder to learn from. The fact that the gap has widened even as screens have become sharper and more comfortable to read points in the same direction, since a purely technical problem ought to have diminished as the technology improved. Some studies have found, moreover, that simple interventions, such as asking students to list the key words of a passage before answering questions on it, reduce or even eliminate the difference.

G The practical lesson, then, is neither to banish screens from classrooms nor to pretend that the medium is irrelevant. Schools that adopt digital materials should allow more time for difficult texts, and teach students explicitly to recognise when a piece of writing requires slow, deliberate attention rather than a quick scan. Printed copies may still be preferable for the most demanding study, and many students say they favour them: surveys by the linguist Naomi Baron found that a large majority of university students in several countries chose print when reading for serious academic purposes. But preferences are not destiny. Reading has always been a skill that is learned rather than a capacity we are born with, and there is no reason why careful reading on a screen cannot be learned too.