IELTS Academic Reading Practice Test 24

Of all the technological transformations that shaped the modern world, the development of artificial refrigeration is perhaps the least celebrated. Unlike the steam engine or the electric telegraph, the refrigerator did not typically produce a dramatic public spectacle, and its impact has been felt principally in warehouses, ships' holds, and kitchen pantries. Yet without artificial cold, the geography of food production, the structure of cities, and the practice of medicine would all be virtually unrecognisable.

For most of human history, the preservation of food relied generally on natural cold, salting, smoking, drying, and fermentation. Wealthy households in the ancient Mediterranean used insulated underground chambers, called nivieri in medieval Italian, packed with snow brought down from the mountains during the winter and used to chill drinks and preserve foods through the summer. The American colonial farmer Frederic Tudor took this principle a step further in 1806 by harvesting ice from frozen New England lakes and shipping it to the tropics, where it commanded extravagant prices. By the 1850s, the "ice trade" employed tens of thousands of workers and exported more than 150,000 tonnes of natural ice annually from American ports alone. Even so, the system was vulnerable to mild winters and could not satisfy the rising demand of growing industrial cities.

The first practical mechanical refrigerator was demonstrated in 1834 by the American engineer Jacob Perkins, then resident in London. Perkins's device exploited the principle, first noted by William Cullen of the University of Glasgow in 1755, that liquids absorb heat from their surroundings when they evaporate. By compressing a volatile fluid — ether in Perkins's case — and then allowing it to expand and evaporate inside an insulated chamber, the device could produce continuous cold without the need for natural ice. Although Perkins obtained a British patent, his apparatus was small, inefficient, and difficult to operate, and the commercial possibilities he envisaged remained unrealised for nearly fifty years.

The breakthrough came not in domestic kitchens but in the meat industry of the southern hemisphere. By the 1870s, Australia and New Zealand produced vast quantities of cheap lamb and beef for which no local market existed, while the rapidly growing cities of Britain and continental Europe suffered chronic shortages of affordable protein. The challenge was solved in 1882, when the steamship Dunedin sailed from Port Chalmers, New Zealand, to London carrying 4,909 carcasses of mutton preserved by a Bell-Coleman cold-air machine. The ninety-eight-day voyage proceeded without spoilage and the cargo sold at a substantial profit. Within two decades, frozen meat had become a staple of the British working-class diet.

Refrigeration also transformed the brewing and dairy industries. The Bavarian brewer Carl von Linde, then a professor at the Technical University of Munich, developed a more efficient ammonia compression system in 1873, which allowed the German lager industry to brew its characteristic cold-conditioned beers year-round rather than only during the winter months. Linde's company supplied refrigeration plants across Europe and the United States, and by 1900 the firm had installed more than a thousand machines. Refrigerated railcars also made it possible to deliver fresh milk over distances of several hundred miles, undermining the insanitary urban dairies of the nineteenth century.

The expansion of refrigeration into the home was a phenomenon of the twentieth century. The first electric domestic refrigerator suitable for ordinary households was the Kelvinator, introduced in 1918 and selling for around $900 — equivalent to almost two years of average industrial wages. Mass production drove the price down, and by 1950 some eighty percent of American households owned a refrigerator, although ownership in Europe and Asia lagged considerably. Domestic cold transformed shopping habits, eliminating the daily visit to the butcher and grocer, and enabled the rise of frozen and chilled convenience foods.

In medicine, too, artificial cold has proved indispensable. Vaccines, blood plasma, and many pharmaceutical compounds typically require careful temperature control, and reliable cold chains have been central to public health campaigns since the 1950s. The global eradication of smallpox, formally certified by the World Health Organization in 1980, would have been impossible without refrigerated vaccine transport, and similar requirements continue to dominate efforts against poliomyelitis and measles. The environmental cost has proved substantial. The chlorofluorocarbon refrigerants developed in the 1930s and adopted worldwide were found in the 1970s to deplete the stratospheric ozone layer, prompting the 1987 Montreal Protocol that obliged signatory nations to phase them out. Their replacements, the hydrofluorocarbons, in turn proved to be potent greenhouse gases, and are now themselves being phased down under the 2016 Kigali Amendment.

A The idea that plants possess a daily rhythm of activity independent of the sun was first proposed in 1729 by the French astronomer Jean-Jacques d'Ortous de Mairan. Curious to know whether the daily movements of the leaves of the sensitive plant, Mimosa pudica, were merely a response to sunlight, he placed several specimens in a darkened cupboard and observed them through a small aperture. The leaves continued to open at dawn and close at dusk for several days, even though no sunlight could reach them. De Mairan's brief note to the French Academy of Sciences attracted only modest attention at the time, but it foreshadowed a field of inquiry that would not fully mature for more than two centuries.

B A circadian rhythm — from the Latin "circa diem," meaning "about a day" — is an internally generated cycle of biological activity that runs at close to twenty-four hours, even in the absence of external time cues. Such rhythms are now known to be ubiquitous, occurring not only in animals and plants but also in fungi, single-celled organisms, and even certain bacteria. In plants, circadian rhythms typically govern processes including leaf movement, flower opening, the timing of photosynthesis, and the rhythmic uptake of water and mineral nutrients from the soil.

C The molecular basis of plant circadian rhythms began to be understood during the 1990s, principally through work on the model plant Arabidopsis thaliana. Researchers including Steve Kay, then at the University of Virginia, identified a small number of genes whose activity oscillates with a roughly twenty-four-hour period and whose disruption abolishes the rhythm entirely. The central mechanism is a delicate feedback loop in which proteins produced from these "clock genes" inhibit their own further production, generating a self-sustaining oscillation. Two key clock genes in Arabidopsis are known as CCA1 and TOC1.

D Plant clocks are not isolated from the outside world. They are entrained — synchronised — by environmental cues, most importantly the daily cycle of light and dark and the daily rise and fall of temperature. Without such cues, plants raised under constant conditions gradually drift out of phase with the solar day, although the rhythm itself does not disappear. The receptors responsible for sensing light are a family of proteins known as phytochromes and cryptochromes, which not only detect the presence of light but distinguish its wavelength and intensity, allowing the plant clock to set itself accurately to dawn and dusk.

E The adaptive value of an internal clock can scarcely be overstated. Daily forecasting allows a plant to prepare its physiology for predictable changes before those changes actually occur. Researchers at the University of Cambridge, led by Alex Webb, demonstrated in 2005 that Arabidopsis plants whose internal clocks were matched to their environment grew larger and produced more chlorophyll than otherwise identical plants whose clocks ran too fast or too slow. The mismatched plants survived but performed measurably worse in side-by-side trials, supporting the hypothesis that circadian rhythms apparently confer a real evolutionary advantage.

F The temporal organisation of plant defence offers a particularly striking example. Many insects, including aphids and caterpillars, feed most actively during specific hours of the day. Plants that detect the regular timing of attack can pre-position chemical defences accordingly. Studies of the cabbage thale cress, Arabidopsis lyrata, have shown that the production of glucosinolates — bitter, sulphur-containing compounds that deter insect feeding — peaks shortly before the daily activity period of its principal herbivore. Plants subjected to artificially shifted attack schedules suffered substantially greater damage, confirming that defence timing is genuinely adaptive.

G Agricultural applications of circadian research are beginning to attract commercial interest. Many post-harvest losses, particularly in leafy greens and cut flowers, are influenced by the time of day at which crops are picked. A 2013 study by researchers at Rice University showed that lettuces harvested under simulated daylight, even days after separation from the parent plant, retained higher antioxidant levels than lettuces stored in continuous darkness. By extension, careful management of light exposure during transport could reduce waste and extend shelf life. Several supermarket chains in the United Kingdom and the Netherlands have begun pilot trials of such "circadian storage" systems with reported reductions of perishable waste of as much as twenty percent.

H Despite these advances, several puzzles generally remain. The circadian systems of cereals such as wheat and rice typically differ in important respects from those of Arabidopsis, and the underlying clock genes have only partially been identified in major crop species. The interaction between the plant clock and changing climate is an active area of research, since rising average temperatures and longer growing seasons appear to disrupt the synchronisation between flowering, pollinator activity, and the timing of frost. As average global temperatures continue to rise, understanding plant circadian rhythms is likely to become a matter of increasing practical importance, both for food security and for the conservation of wild ecosystems.

A For roughly four decades in the middle of the nineteenth century, the most valuable export of Peru was the accumulated excrement of seabirds. The Chincha Islands, an archipelago of three rocky outcrops twenty kilometres off the southern coast, were home to vast colonies of guanay cormorants, Peruvian boobies, and Peruvian pelicans, whose droppings had accumulated to depths of more than fifty metres in some places. The remarkable agricultural value of this material — exceptionally high in nitrogen, phosphorus, and potassium — had been recognised by Andean peoples since the time of the Inca, when transgressions against the colonies were punished by death. By the early 1840s, European and North American farmers were beginning to grasp the same lesson.

B The scientific case for guano was made most influentially by the Prussian naturalist Alexander von Humboldt, who collected samples during his South American expedition of 1799-1804 and dispatched them to leading European chemists. The report by Antoine de Fourcroy and Louis Vauquelin demonstrated that guano contained nitrogen at concentrations far higher than any then-available manure. The rapid intensification of European agriculture in the 1830s and 1840s created an appetite for fertiliser that local sources could not satisfy. By 1841, the first commercial shipments of Chincha guano had reached Liverpool, and within five years annual exports exceeded 100,000 tonnes.

C The Peruvian state, recognising the strategic importance of the resource, nationalised guano production in 1842 and licensed exports through monopoly contracts with private firms. The most prominent of these, Antony Gibbs and Sons of London, controlled the European trade from 1849 to 1861 and accumulated immense profits. Government revenues were equally extraordinary: between 1840 and 1879, guano sales provided more than half of all state income in Peru. The country built railways, retired its independence-era debts, and abolished both indigenous tribute and African slavery using funds generated almost entirely from seabird waste.

D The labour conditions on the islands, however, were among the most brutal of the nineteenth century. Local Peruvian workers refused the dangerous, foul-smelling work, and contractors increasingly relied on indentured labour from southern China. Between 1849 and 1874, approximately ninety thousand Chinese men, sometimes referred to by the disparaging term "coolies," were transported to Peru under contracts that proved in practice indistinguishable from slavery. Mortality rates were appalling: contemporary accounts record death rates of more than fifteen percent per year. International pressure, particularly from the Chinese imperial government and the British anti-slavery movement, eventually forced an end to the trade in 1874.

E The environmental consequences of intensive extraction broadly soon began to manifest. Mining the guano cliffs disturbed the breeding habitat of the very birds whose droppings sustained the resource. The biologist Robert Cushman Murphy, who studied the islands in the early twentieth century, calculated that the bird population had fallen from an estimated sixty million birds at the height of the trade to fewer than four million by 1900. The remaining colonies were saved largely by chance: as stocks were exhausted in the 1870s, the islands ceased to be intensively exploited, allowing partial recovery.

F The end of the Peruvian guano boom was hastened by the discovery of vast deposits of inorganic nitrate in the Atacama Desert, on the disputed border between Peru, Bolivia, and Chile. The nitrate fields could be mined far more cheaply and provided nitrogen of comparable agricultural value. The War of the Pacific, fought between 1879 and 1884, was driven principally by competing national claims over these nitrate deposits, which Chile ultimately seized. Peruvian guano sales collapsed during the conflict and never fully recovered. By the early twentieth century, the synthetic ammonia process developed by the German chemists Fritz Haber and Carl Bosch had begun to provide artificial nitrogen, rendering both Peruvian guano and Chilean nitrate commercially marginal.

G Modern accounts have emphasised the role of seabird excrement in the broader history of globalisation. Historians such as Gregory Cushman of the University of Kansas have argued that the trade represented an early case of ecological imperialism, in which the natural capital of a peripheral state was rapidly converted into industrial inputs for the metropolitan economies of Europe and North America. The agricultural transformation of the British countryside in the second half of the nineteenth century depended substantially on cheap nitrogen from the Chincha Islands, and the loss of seabird populations represented an externalised cost borne by the Peruvian ecology.

H Some legacies persist today. The Peruvian state retains formal ownership of the islands and continues to harvest a modest quantity of bird droppings — perhaps fifteen thousand tonnes per year — for use in organic and high-value horticulture. The seabird populations have partially recovered to perhaps four million birds, though they remain a fraction of their pre-industrial abundance and are now threatened anew by climate-driven changes to the cold Humboldt Current that supports the anchoveta fishery on which they depend for food. The story of the Chincha Islands therefore offers a vivid example of the entangled history of ecology, economy, and empire, and a reminder that even the most arcane natural resources can briefly reshape the politics of nations.