IELTS Academic Reading Practice Test 81

The substance now known as gunpowder was not discovered through a deliberate search for a weapon, but rather through experiments carried out by Chinese alchemists during the Tang dynasty, probably in the ninth century. These scholars were attempting to produce an elixir of immortality, and in doing so they combined saltpetre, a white mineral rich in potassium nitrate, with sulfur and charcoal. The resulting mixture, later called black powder, did not bring eternal life, but it did burn violently when ignited. Early Chinese texts describe a powder that could be used to make smoke and flames, and by the tenth century it was being employed in simple explosive devices.

Military engineers soon found ways to use gunpowder in warfare. The Chinese produced fire arrows, grenades and rockets, and by the twelfth century they had developed a primitive flamethrower known as the fire lance. This was a tube, originally of bamboo and later of metal, packed with gunpowder and fragments, which could direct a burst of flame and shrapnel at an enemy. Some historians consider the fire lance to be the ancestor of the firearm. At first, however, gunpowder was used mainly for its incendiary and psychological effects, since early mixtures were not powerful enough to burst strong metal containers.

The knowledge of how to make gunpowder travelled along the Silk Roads, and it may have reached the Islamic world by the thirteenth century. Arab writers of that period refer to saltpetre as "Chinese snow", which suggests both the substance's origin and its unusual appearance. From the Middle East the technology passed into Europe, where it began to transform the nature of warfare. By the middle of the fourteenth century, cannons were being used in European battles, and the age of castles was coming to an end, because walls that had resisted siege engines could be shattered by gunstones and iron shot.

The Mongols used gunpowder weapons in their thirteenth-century campaigns, and it is often said that they carried the technology from China to the Middle East. However, historians debate the exact route, since knowledge could equally have moved through merchants and diplomats. What is certain is that gunpowder played a growing part in siege warfare, with early bombs made from bamboo or paper cases, and that its use slowly changed the design of fortifications.

Black powder is a mechanical mixture of three ingredients. In a typical recipe, saltpetre makes up about 75 per cent, charcoal about 15 per cent and sulfur about 10 per cent, although the proportions varied considerably over time and between uses. Saltpetre supplies oxygen when it decomposes, allowing the other ingredients to burn rapidly even in a confined space. Medieval European scholars, including the English friar Roger Bacon and the German bishop Albertus Magnus, recorded versions of the recipe, often in coded language, because they were aware of the mixture's dangerous power. Their involvement gave gunpowder a learned status, but the recipes probably arrived in Europe through practical channels rather than through scholarly books.

Once powder makers had learned to purify saltpetre and mix the ingredients thoroughly, gunpowder became reliable enough to fire projectiles from hand-held weapons. The first European handguns appeared in the fourteenth century, and by the fifteenth century the matchlock had introduced a simple mechanism in which a slow-burning cord, called a match, was lowered into a pan of powder. The trigger released the mechanism, allowing the match to ignite the priming charge. Although such weapons were heavy and awkward, they required far less training than the longbow and could penetrate armour at close range, which gradually changed the balance of power on the battlefield.

The same chemistry that propels a bullet also produces the colours and noise of a firework. The origins of fireworks are usually traced to China, where bamboo filled with gunpowder was thrown into fires to produce a loud crack, and where the first true rockets were made by attaching a powder-filled tube to a stick. Italian pyrotechnicians of the Renaissance developed aerial shells, which explode high above the ground and release stars made from chemicals chosen for their behaviour when burned. Strontium compounds give a red flame, barium compounds give green, copper compounds give blue, and sodium compounds give yellow. Other additives create crackles, whistles and smoke.

Modern fireworks are built around a lift charge that thrusts the shell into the air and a bursting charge that opens the shell at the top of its flight. The delay between the two is controlled by a fuse, and the arrangement of the stars inside the shell determines the pattern that appears in the sky. Because fireworks contain explosive material, their manufacture is tightly regulated in many countries, and industrial accidents have sometimes destroyed entire workshops. In the twentieth century, large public displays were increasingly coordinated by computer, but the underlying pyrotechnic principles remain essentially the same as those used centuries ago.

Gunpowder and fireworks have had sharply contrasting reputations. The former has been associated with destruction and the centralisation of state power, since only wealthy rulers could afford the new artillery. The latter, by contrast, has become linked with celebration and public entertainment, from royal weddings and national holidays to local festivals. Yet both descend from the same chance discovery, and both illustrate how a single technical innovation can branch into very different social roles. Today black powder is still used in some specialist applications, while fireworks continue to evolve through new chemical formulations and ever more ambitious choreography.

The Moon has been a familiar companion to the Earth for most of the planet's history, yet the question of how it formed long remained one of the outstanding puzzles of planetary science. Before the space age, three main explanations competed. One held that the Earth had thrown off a piece of itself while spinning rapidly; another proposed that the Moon had formed elsewhere and been captured by Earth's gravity; a third suggested that the two bodies condensed side by side from the same cloud of material. Each idea had serious difficulties, and none could account for all the basic properties of the Earth–Moon system.

Analysis of the samples returned by the Apollo missions proved decisive. It showed that lunar rocks are extremely poor in water and other volatile substances, which counted against some older ideas. Scientists found that the Moon's overall density is much lower than the Earth's, which implies that its iron core is small relative to its size. They also discovered that the proportions of oxygen isotopes in Earth and lunar rocks are virtually identical, whereas meteorites from other parts of the solar system show different ratios. Such evidence pointed to a close genetic link between the two bodies, and it slowly narrowed the field.

In the mid-1970s, researchers put forward a radically different model. According to this giant-impact hypothesis, an object roughly the size of Mars, later named Theia, struck the newly formed Earth about 4.5 billion years ago. The collision was not a gentle merger but a violent glancing blow. The impactor's metallic core sank into the Earth, while a vast quantity of rocky debris from the outer layers of both bodies was hurled into orbit. Within a short period, perhaps only a few years or centuries, much of this material gathered together under its own gravity to form the Moon.

The model explained several observations in a single framework. Because the Moon formed mainly from the rocky mantles of the two colliding bodies rather than from their dense cores, it inherited relatively little iron, which accounts for its unusually small core. The enormous heat generated by the impact allowed volatile elements to escape into space, leaving the Moon dry and depleted of substances that would have boiled away. At the same time, the identical oxygen isotopes made sense, because the Earth and Theia would have formed from the same general region of the solar nebula. Computer models of the disk indicate that the Moon initially formed only a few Earth radii away, which is consistent with evidence that it has slowly receded.

For decades the hypothesis has been the closest thing planetary scientists have to a consensus. Computer simulations have shown that giant impacts of this kind produce satellite-forming disks, and the model naturally explains the angular momentum of the Earth–Moon system, a quantity that had defeated earlier theories. The Moon's orbit is gradually moving outward as it steals rotational energy from the Earth, a process that can be traced back to a Moon that formed close to its parent planet. Nevertheless, the theory is not without unresolved difficulties, and some scientists have questioned whether it can account for the Moon's exact bulk composition.

One long-standing problem concerns the great isotopic similarity between Earth and the Moon. In the original model, the Moon should contain a substantial proportion of Theia, which was likely to have had a different isotopic signature from the early Earth. Yet lunar and terrestrial oxygen isotopes are almost indistinguishable. Some scientists have argued that Theia happened to be made of material almost identical to Earth's, but others regard that as an unlikely coincidence. This has stimulated alternative versions of the theory, in which the impact was so energetic that the two bodies mixed thoroughly before the Moon formed, or in which multiple smaller impacts contributed to the satellite.

More recent analyses have tested these ideas with increasingly precise measurements of lunar samples and meteorites. Small differences in titanium, tungsten and other elements have been reported, and these may preserve a trace of the original impactor. At the same time, researchers continue to examine the possibility that the Moon lost some volatile elements after it formed, rather than during the impact alone, as volcanic activity and subsequent bombardment may have played a role. Recent work on water trapped in lunar minerals has also softened the once simple picture of a completely dry Moon. The precise mechanism that produced the Moon remains the subject of intense study, and several modified versions of the giant-impact concept now exist. The debate remains active, but it is conducted within the broad framework of a giant collision.

The Moon's violent origin may have had consequences for the Earth itself. The impact is thought to have tilted the Earth's axis, giving rise to the seasons, and it may have delivered water and other elements that later helped life to begin. As new spacecraft visit the Moon and return further samples, scientists hope to learn not only how our satellite was born, but also how the Earth became the planet it is today. A single catastrophic event, therefore, may have shaped the conditions in which complex life eventually emerged. Moreover, the same impact may have altered the length of the day, although this effect is less certain.

Modern humans spend the overwhelming majority of their time inside buildings, yet our senses and physiology evolved in natural environments. This mismatch has become a focus of research in environmental psychology and architecture. The term biophilic design describes an approach to building that deliberately incorporates elements of the natural world—daylight, plants, water, natural materials and views of living things—into the places where people live and work. Its advocates argue that such design is not decorative but functional, reducing stress, improving concentration and even speeding recovery from illness.

The intellectual roots of the movement lie in the biophilia hypothesis, proposed by the American biologist Edward O. Wilson in the 1980s. Wilson suggested that humans possess an innate tendency to affiliate with nature because our ancestors depended on reading natural signals for survival. Although the hypothesis is broad and hard to test directly, it encouraged designers to think of contact with nature as a basic human need rather than a luxury. Later writers, notably the social ecologist Stephen Kellert, translated the idea into practical principles for buildings, and the concept has since spread into architecture, interior design and urban planning.

Some of the most influential evidence comes from health care. In a famous 1984 study, the environmental psychologist Roger Ulrich compared patients recovering from gallbladder surgery in a suburban hospital. Some patients looked out on a small stand of trees, while others faced a brick wall. Those with the tree view needed fewer doses of strong painkillers and were discharged about a day earlier. Ulrich's study was small, but it helped establish the idea that even a passive view of vegetation could improve medical outcomes, and it has been widely cited ever since.

Since then, researchers have widened the evidence base, though the quality of studies varies considerably. Experiments in offices and schools have reported that indoor plants are associated with lower self-reported stress and modest improvements in attention, while access to natural daylight has been linked to better sleep and stronger performance on cognitive tasks. Yet many of these studies are short-term, use small samples or rely on subjective ratings. It is often impossible to separate the effects of nature from those of better lighting, reduced noise or simply novelty, and results sometimes fail to replicate in larger trials.

A further complication is that the field has developed its own vocabulary and certification schemes. The consulting firm Terrapin Bright Green has identified fourteen patterns of biophilic design, ranging from visual connection with nature and dynamic light to biomorphic forms and mystery. Such frameworks are useful for practitioners, but they can also encourage the idea that adding a few plants to a room is enough. Critics argue that a green wall in a windowless office may do little more than signal an organisation's environmental credentials, and that genuine contact with nature requires more than decoration.

In my assessment, the core claim—that some contact with nature improves well-being—is plausible and supported by a growing body of research. However, biophilic design is not a precise medical intervention. A view of trees is not the same as a forest walk, and a potted plant cannot compensate for poor ventilation, excessive noise or a culture of overwork. Too often, biophilic elements are treated as ornaments, while the deeper lesson is that spaces should support natural rhythms of attention and rest. This means designing rooms that alternate busy areas with calm ones, and that provide opportunities for the eye to rest on living forms.

There are also environmental and financial arguments. Buildings that use more daylight and natural ventilation can reduce energy consumption, and views of nature may make workplaces more attractive to staff. But these benefits are hard to generalise. A design that works in a mild, temperate climate may fail where summers are hot or winters are dark, and an office that is healthy for one group of workers may not suit another. Moreover, the benefits of biophilic design tend to be greatest for people who currently have the least access to green space, yet the premium associated with parkside apartments and landscaped offices suggests that the market does not distribute them equally.

The COVID-19 pandemic intensified interest in healthy buildings, and the term biophilic design began to appear in corporate brochures and government guidance. Some health authorities now mention access to nature as one factor in mental well-being, while university courses in architecture increasingly include the subject.

At the same time, architects caution that a checklist approach can produce spaces that look natural in photographs but do not feel natural to inhabit. The most successful projects, in my view, are those in which nature is not simply added but becomes part of the building's logic—shaping its orientation, materials and sequence of rooms, so that a walk through the building feels varied and connected to the outside.

In the end, biophilic design should be judged not by the number of plants it specifies but by whether it helps people live and work more comfortably. The evidence is not yet precise enough to say exactly which elements matter, in what doses, or for whom, and some commercial claims certainly outrun the data. But the direction of travel is clear: buildings that entirely exclude nature tend to be less healthy, less pleasant and less sustainable than those that invite it in, carefully and without exaggeration.