IELTS Academic Reading Practice Test 76

The idea that heat and cold could be measured emerged slowly. Ancient natural philosophers classified bodies as hot, cold, moist and dry, but they lacked any instrument that could turn these qualities into numbers. The first step came in the late sixteenth and early seventeenth centuries, when experimenters noticed that air expands as it is warmed. A glass bulb with a long neck could be partly filled with water and inverted, so that the water rose or fell as the enclosed air changed size. This device, later called an air thermoscope, showed that temperature varied, but it could not measure it in any fixed way.

The air thermoscope is often credited to Galileo Galilei, who is said to have demonstrated such an instrument around 1593, though several of his contemporaries were working on similar ideas. Galileo's friend and colleague Santorio Santorio, a physician, may have been the first to add a numerical scale along the tube, using the instrument to monitor changes in patients' body heat. These early instruments were open to the air, which made them sensitive to atmospheric pressure as well as to temperature. A rising or falling column could therefore reflect changing weather rather than true changes in heat.

The next major advance was to seal the liquid inside the tube so that the instrument responded only to temperature. In the middle of the seventeenth century, experimenters at the court of Grand Duke Ferdinand II of Tuscany developed sealed glass thermometers containing alcohol. Because the liquid was enclosed, air pressure no longer influenced the readings. Alcohol expanded substantially when warmed, but it boiled at a relatively low temperature, so it was unsuitable for measuring great heat. Instrument makers therefore had to search for liquids that remained liquid over a useful range.

Even a sealed thermometer, however, was of limited use without an agreed scale. Different makers marked different numbers at different points, so a reading of forty degrees on one instrument meant nothing on another. Scientists gradually recognised that reliable measurement required fixed points, physical conditions that are always the same. The two most obvious were the melting point of ice and the boiling point of water, but their values vary somewhat with pressure, and early experimenters did not always control for this. Without such shared references, a physician in Rome and an instrument maker in London could not compare their observations, and useful collaboration was impossible.

The first widely adopted scale was created by Daniel Gabriel Fahrenheit, an instrument maker born in Danzig who spent much of his working life in the Dutch Republic. Fahrenheit was highly skilled at making mercury thermometers, and in 1724 he described a scale based on three reference points.

He used the temperature of a mixture of ice, water and salt as zero, the freezing point of water as 32 degrees, and the normal temperature of the human body as 96 degrees. On this scale, the boiling point of water was later fixed at 212 degrees, giving exactly 180 degrees between freezing and boiling. Fahrenheit's thermometers were exceptionally consistent, and his scale spread rapidly through the English-speaking world.

In 1742 the Swedish astronomer Anders Celsius proposed a more practical scale with one hundred degrees between the two principal fixed points. Contrary to what might be expected today, Celsius placed the boiling point of water at 0 degrees and the freezing point at 100 degrees, perhaps because he wanted to avoid negative numbers for ordinary winter temperatures. The scale was soon reversed, apparently on the advice of other scientists, so that freezing became 0 and boiling became 100. Because of its convenient interval, the centigrade scale became the standard in scientific work and, later, in everyday use across most countries.

Temperature scales based on the behaviour of water were useful, but they depended on the properties of a particular substance. In 1848 the British physicist William Thomson, later Lord Kelvin, proposed a thermodynamic scale that would not rely on any material. Kelvin argued that there must be a lowest possible temperature, absolute zero, at which molecular motion would reach its minimum. His scale used the same size of degree as the Celsius scale, but began at absolute zero, which he calculated to be about −273 degrees Celsius; the modern value is −273.15 degrees. The kelvin later became the fundamental unit of temperature in the physical sciences.

From the 1950s, the kelvin was defined by the triple point of water, the unique combination of temperature and pressure at which liquid water, ice and water vapour can coexist, and the Celsius scale was in turn defined from the kelvin. In 2019 the kelvin was redefined in terms of the Boltzmann constant, a fundamental quantity of statistical physics. This change tied the unit to nature itself rather than to any sample of water, though the practical effect on ordinary measurements was negligible.

Today three scales remain in common use. The Fahrenheit scale survives mainly in the United States and a handful of other territories, while the Celsius scale, no longer formally called centigrade, is used almost everywhere else and in many branches of science. The kelvin is reserved largely for physics and chemistry, where calculations often depend on absolute temperature. The instruments themselves have also changed: liquid-in-glass thermometers have been joined by electronic sensors, platinum resistance thermometers and radiation thermometers, but the underlying principle remains the same as it was four centuries ago—temperature is measured by observing how matter responds to heat.

Every living organism leaves traces of itself in its surroundings. Fish shed scales and mucus, mammals drop hair and faeces, plants release pollen, and even microbes leave fragments of genetic material behind. The DNA recovered from these traces is known as environmental DNA, or eDNA. It allows biologists to detect species without ever seeing or handling them. Because eDNA can be collected from water, soil and air, it has begun to transform how ecologists, conservationists and environmental managers study the living world. The term itself is relatively new, but the underlying idea has a long history in ecology and microbiology.

To sample eDNA in water, researchers typically pass a known volume through a fine filter that captures cells and free DNA. The material is then taken to a laboratory, where the DNA is extracted, purified and amplified using the polymerase chain reaction. In many studies, a short region of DNA that varies between species is sequenced and compared with reference databases. If a perfect match is found, the species can be identified. With metabarcoding, a single sample can be screened for many hundreds of taxa at once, making it possible to build a picture of an entire community from one bottle of river water. Because DNA from different species can be distinguished by its sequence, the method is highly specific.

One of the first high-profile uses of eDNA was the search for invasive species. In the United States, the spread of Asian carp through waterways near the Great Lakes prompted agencies to test whether genetic methods could detect the fish before they became established. Sampling campaigns found carp eDNA in places where conventional nets and electrofishing had found none, giving managers earlier warning than they might otherwise have had. Similar approaches have since been used to monitor zebra mussels, cane toads and other troublesome organisms in many parts of the world. Early trials in the Chicago waterways were especially influential.

eDNA has proved especially valuable for rare and elusive species. Traditional surveys for amphibians, for example, may require observers to visit ponds at night and search by torchlight, which is time-consuming and can disturb the animals. A water sample, by contrast, can reveal the presence of a protected species such as the great crested newt in a matter of days. Because collection is less invasive, it is particularly attractive for monitoring populations that are small, secretive or legally protected. In Britain, eDNA testing for great crested newts has become a standard part of planning applications. It has also been used in the deep sea, where direct observation is extremely difficult.

As a survey tool, eDNA has several clear advantages. It does not require the capture or killing of organisms, so it is less disruptive than trapping, netting or electrofishing. It can often be carried out by technicians after minimal training, whereas some traditional methods depend on expert taxonomists who are in short supply. In addition, one field sample can provide evidence for an entire community, including microbes and other groups that are easy to overlook. For large landscapes, where visiting every site would be impossible, eDNA offers a way to sample many locations rapidly and relatively cheaply.

Nevertheless, eDNA is not a magic window into nature. The DNA may degrade within hours or days depending on temperature, pH and microbial activity, so the absence of a species cannot be treated as proof of absence. Samples can also be contaminated by DNA from researchers' clothing, equipment or from water carried between sites, producing false positives. Moreover, a positive result does not demonstrate that the organism was alive at the site; genetic material may have arrived in river currents or been deposited in faeces by a predator. The probability of detection also depends on how much DNA an organism sheds, which varies between species and seasons. Finally, eDNA can rarely provide reliable information about population size or the age of individuals.

Despite these limitations, eDNA is being incorporated into routine monitoring programmes. In streams and lakes, sampling at regular intervals can track changes in fish and invertebrate communities after habitat restoration. In the marine environment, eDNA from seawater has been used to map the distribution of plankton, fish and even marine mammals. Soil samples can reveal the fungi, bacteria and invertebrates that drive ecosystem processes underground. Scientists are also developing ways to compare results across laboratories, so that data collected in different studies can be compared. This standardisation is essential if eDNA is to move from a research technique to a reliable tool for regulation.

Some researchers have begun to sample airborne eDNA, hoping to detect insects, fungi or plants from the genetic material they release into the atmosphere. Early results are promising, but the field is still young, and air currents can carry particles over long distances, complicating interpretation. Meanwhile, conservation managers are learning that eDNA works best when it is combined with other methods rather than treated as a replacement for them. A negative eDNA result, for example, may justify additional trapping, while a positive result can guide where observers should look. The true promise of eDNA is not that it makes human surveyors unnecessary, but that it extends their reach into environments that would otherwise remain invisible.

Ask a city official whether street art is a gift or a crime, and you are likely to hear a very different answer from the one an aerosol artist would give. The same painted wall can be described as vandalism, as decoration, as a political statement, or as all three at once. In my view, street art occupies a genuine cultural middle ground. It cannot be dismissed simply as illegal damage, because some of the most original visual work of recent decades has appeared on walls and trains. Yet it also cannot be celebrated without reservation, because its power depends on context, illegality and an often unpredictable relationship with the public.

Modern street art has roots in the graffiti movement that emerged in New York City in the late 1960s and early 1970s. Young writers began to spray their names, or tags, on subway cars and station walls, competing for visibility in an unregulated public space. For them, the act of writing was not primarily about producing a finished image; it was about leaving a mark that could travel across the city. By the 1980s, some artists who had emerged from this scene, including Keith Haring and Jean-Michel Basquiat, had moved into galleries, but the link between graffiti and the street remained central.

The movement soon spread. From São Paulo to Berlin, from Melbourne to Johannesburg, artists developed distinctive local styles, mixing aerosol painting with stencils, posters, stickers, tiles and yarn. Stencilling allowed images to be prepared in advance and applied quickly, and it became associated with a more overtly political form of street art. A stencilled figure or slogan could appear overnight, confront commuters in the morning, and disappear by the afternoon. That speed, and the risk of being caught, gave the work a charge that a framed print in a gallery could never possess.

The most famous street artist of this generation is the anonymous British figure known as Banksy. Because the artist's identity is unknown, attention remains fixed on the work itself. Banksy's pieces often combine dark humour with direct social comment, and their appearance regularly generates intense media coverage. One of Banksy's best-known images had earlier been voted the United Kingdom's favourite work of art; in 2018, moments after it was sold at auction for more than a million pounds, a shredder hidden in the frame partly destroyed it. That event was widely read as a protest against the commercialisation of street art, though the auction house later indicated that it had been taken by surprise.

By the early twenty-first century, street art had entered museums and commercial galleries. Collectors paid high prices, auction houses held specialised sales, and major institutions staged retrospectives of urban art. For some artists, this recognition brought financial security and a larger audience. But it also raised uncomfortable questions. Could a work created on a wall without permission still be considered street art after it had been removed, framed and sold? Once a mural was painted indoors or in a designated legal area, did it lose the confrontation with authority that had made it meaningful?

Cities have responded in contradictory ways. Many now commission large murals in an effort to brighten neglected districts, attract visitors and discourage the kind of tagging they consider ugly. Legal walls and street-art festivals are common in many cities. Yet these projects can have unintended effects. When commissioned artists are brought into a working-class neighbourhood, the murals can become signs of change, and may be followed by rising rents and the displacement of long-term residents. In such cases, street art is no longer an outsider's interruption; it becomes part of the process of gentrification.

Permission is central to any serious discussion of street art. For many artists, working without permission is precisely the point, because it challenges the official control of public space. When a work is painted illegally, it exists outside the usual systems of planning, property and commerce. That illegality, however, can harm residents who must live with unwanted images, and it can lead to costly repairs. I do not believe that every illegal painting is automatically art, nor that every legal mural is automatically worthless. The value of a work has to be judged by what it does in its particular setting, not merely by whether the painter asked for permission.

Another difficulty is that street art is inherently ephemeral. Rain, sun, building work and other painters all shorten the life of an outdoor piece. Some works survive only as photographs posted on social media, which can spread an image far beyond its original neighbourhood. Documentation has therefore become a central activity of the street-art world, with websites and books preserving pieces that no longer exist. But this preservation is incomplete. An image on a screen is not the same as a wall on a particular corner, and the sense of place that gives street art much of its meaning cannot be fully reproduced.

Street art is best understood, I think, as a conversation about who owns the city and what images are allowed to occupy it. It can be a warm, witty gift to passers-by or a territorial marking that frightens them. It can support a community's identity or signal its displacement. The very same medium can carry all these messages at once. Rather than asking whether street art is a good thing, we would do better to ask who made a particular work, where it appeared, and what it does to the people who encounter it. Those are difficult questions, but they are far more interesting than simply calling it vandalism or art.