My Science School

In 1774, the English chemist Joseph Priestley announced that he had discovered ar element within the air. Previously it had been thought that air itself was an element. However, Priestley’s achievement is an example of something that happens quite frequently in science. Although Priestley undoubtedly did discover the presence of oxygen, he was not the first to do so. A Swedish chemist called Carl Scheele had discovered it some months before, and it was not until some months later that a French chemist, Antoine Lavoisier, used Priestley's work to explain what oxygen is and its importance in respiration and combustion. He also gave oxygen its name. The sharing of scientific knowledge moves our understanding of the world forward. No one person can put together all the pieces of the jigsaw puzzle.

Priestley entered the service of the Earl of Shelburne in 1773 and it was while he was in this service that he discovered oxygen. In a classic series of experiments he used his 12inch "burning lens" to heat up mercuric oxide and observed that a most remarkable gas was emitted. In his paper published in the Philosophical Transactions of the Royal Society in 1775 he refers to the gas as follows: "this air is of exalted nature…A candle burned in this air with an amazing strength of flame; and a bit of red hot wood crackled and burned with a prodigious rapidity, exhibiting an appearance something like that of iron glowing with a white heat, and throwing sparks in all directions. But to complete the proof of the superior quality of this air, I introduced a mouse into it; and in a quantity in which, had it been common air, it would have died in about a quarter of an hour; it lived at two different times, a whole hour, and was taken out quite vigorous."

Although oxygen was his most important discovery, Priestley also described the isolation and identification of other gases such as ammonia, sulphur dioxide, nitrous oxide and nitrogen dioxide.

The Leeds Library holds important archival material on Priestley's time there. It was while he was in Leeds that he began his most important scientific researches namely those connected with the nature and properties of gases. A bizarre consequence of this is that Priestley can claim to be the father of the soft drinks industry. He found a technique for dissolving carbon dioxide in water to produce a pleasant "fizzy" taste. Over a hundred years later Mr Bowler of Bath benefited from this when he formed his soft drinks industry.

Priestley should be included in any pantheon of scientists. The bicentenary of his death is an opportune time to reassess his life and work and several events are planned during the year. He possessed enormous scientific skills and originality of thought as well as having the courage to promote unpopular views. He was a man of rare insight and talent.

Sir Edmond Halley’s name is remembered because he was the first person to predict that the comet he saw in 1682 followed a path that would bring it within sight of the Earth again in 1758. Unfortunately, he was no longer alive at that date to see his prediction come true, but his achievement was recognized and his name attached to the comet ever afterwards. In fact, the comet can be seen from Earth every 75-79 years. Its appearance was first recorded by Chinese astronomers in 240BC. The comet, still an unexpected visitor, also appeared in 1066 and was embroidered onto the Bayeux Tapestry, which records the Norman invasion of England.

Edmond (or Edmund) Halley was an English scientist best known for predicting the orbit of the comet that was later named after him. Though he is remembered foremost as an astronomer, he also made significant discoveries in the fields of geophysics, mathematics, meteorology and physics.           

In 1704, Halley was appointed the Savilian professor of geometry at Oxford. Continuing his work in observational astronomy, Halley published "A Synopsis of the Astronomy of Comets" in 1705. In this work, he showed that comet sightings of 1456, 1531, 1607 and 1682 were so similar that they must have been the same comet returning. He predicted that it would return in 1758.

In 1716, Halley devised a method for observing transits of Venus across the disk of the sun in order to determine the distance of Earth from the sun. He also proposed two types of diving bells for exploring underwater. In 1718, by comparing star positions with data recorded by the Greek philosopher Ptolemy, he deduced the motion of stars.

In 1720, Halley succeeded Flamsteed as Astronomer Royal. He continued to make observations, such as timing the transits of the moon across the meridian, which he hoped would eventually be useful in determining longitude at sea.

Halley died Jan. 14, 1742, in Greenwich, England. He did not survive to see the return of what later was named Halley's Comet, on Christmas Day in 1758.

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At one time tens or even hundreds of years might have passed between a scientist’s discovery of a potentially useful fact or method and its use by a wide range of other people. Nowadays, the process is much quicker. This is partly because research is often very expensive and there is pressure to find a commercial use for an invention to help to pay for new research. Modem methods of mass production and global advertising also mean that new products can become popular very quickly.

Hundreds of years ago, news about new products travelled very slowly. Today, advertising is aimed at individual markets and ensures that as many people as possible are aware of what is available.

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Galileo Galilei (1564-1642) was an Italian scientist who worked on many mechanical problems but is perhaps best known for his astronomical observations. These supported the ideas developed by Nicholas Copernicus (1473-1543), a Polish scientist. He claimed that rather than the Sun orbiting the Earth, the Earth orbits the Sun. This idea went against the teachings of the Church, so Copernicus did not tell many people about it. Indeed, when Galileo spoke out in its support, he was put on trial and forced to withdraw his claim. Even today, scientific discoveries are not always popular when they go against long-held beliefs.

Italian astronomer Galileo Galilei provided a number of scientific insights that laid the foundation for future scientists. His investigation of the laws of motion and improvements on the telescope helped further the understanding of the world and universe around him. Both led him to question the current belief of the time — that all things revolved around the Earth.

The Ancient Greek philosopher, Aristotle, taught that heavier objects fall faster than lighter ones, a belief still held in Galileo's lifetime. But Galileo wasn't convinced. Experimenting with balls of different sizes and weights, he rolled them down ramps with various inclinations. His experiments revealed that all of the balls boasted the same acceleration independent of their mass. He also demonstrated that objects thrown in the air travel along a parabola.

At the same time, Galileo worked with pendulums. In his life, accurate timekeeping was virtually nonexistent. Galileo observed, however, that the steady motion of a pendulum could improve this. In 1602, he determined that the time it takes a pendulum to swing back and forth does not depend on the arc of the swing. Near the end of his lifetime, Galileo designed the first pendulum clock.

Galileo is often incorrectly credited with the creation of a telescope. (Hans Lippershey applied for the first patent in 1608, but others may have beaten him to the actual invention.) Instead, he significantly improved upon them. In 1609, he first learned of the existence of the spyglass, which excited him. He began to experiment with telescope-making, going so far as to grind and polish his own lenses. His telescope allowed him to see with a magnification of eight or nine times. In comparison, spyglasses of the day only provided a magnification of three.

It wasn't long before Galileo turned his telescope to the heavens. He was the first to see craters on the moon, he discovered sunspots, and he tracked the phases of Venus. The rings of Saturn puzzled him, appearing as lobes and vanishing when they were edge-on — but he saw them, which was more than can be said of his contemporaries.

The spinning jenny was one of the inventions that revolutionized textile production in the eighteenth century. For thousands of years, spinners were able to produce only one thread at a time, using devices such as spinning wheels. Then in 1764, James Hargreaves, an English weaver, invented a machine that could be operated by one person but spin several threads at the same time.

During the 1700s, a number of inventions set the stage for an industrial revolution in weaving. Among them were the flying shuttle, the spinning jenny, the spinning frame, and the cotton gin. Together, these new tools allowed for the handling of large quantities of harvested cotton.

Credit for the spinning jenny, the hand-powered multiple spinning machine invented in 1764, goes to a British carpenter and weaver named James Hargreaves. His invention was the first machine to improve upon the spinning wheel. At the time, cotton producers had a difficult time meeting the demand for textiles, as each spinner produced only one spool of thread at a time. Hargreaves found a way to ramp up the supply of thread.

The people who took the raw materials (such as wool, flax, and cotton) and turned them into thread were spinners who worked at home with a spinning wheel. From the raw material they created a roving after cleaning and carding it. The roving was put over a spinning wheel to be twisted tighter into thread, which collected on the device's spindle.

The original spinning jenny had eight spindles side by side, making thread from eight rovings’ across from them. All eight were controlled by one wheel and a belt, allowing for much more thread to be created at one time by one person. Later models of the spinning jenny had up to 120 spindles.

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An invention is a new method, material or machine that applies theoretical principles to a practical use. That does not mean that the inventor necessarily understands why his invention works! Inventions may be the result of hard work, or luck, or both. Very often, it is the name of the person who popularized the new idea that we remember, not the person who first thought of it.

An invention is a unique or novel device, method, composition or process. The invention process is a process within an overall engineering and product development process. It may be an improvement upon a machine or product or a new process for creating an object or a result. An invention that achieves a completely unique function or result may be a radical breakthrough. Such works are novel and not obvious to others skilled in the same field. An inventor may be taking a big step toward success or failure.

Some inventions can be patented. A patent legally protects the intellectual property rights of the inventor and legally recognizes that a claimed invention is actually an invention. The rules and requirements for patenting an invention vary by country and the process of obtaining a patent is often expensive.

Another meaning of invention is cultural invention, which is an innovative set of useful social behaviours adopted by people and passed on to others. The Institute for Social Inventions collected many such ideas in magazines and books. Invention is also an important component of artistic and design creativity. Inventions often extend the boundaries of human knowledge, experience or capability.

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In the eighteenth century, wealthy and influential men often interested themselves in more than one branch of learning. The American Benjamin Franklin was a statesman, printer, author and scientist. He left school at twelve, being the fifteenth child of seventeen, but soon made up for his lack of formal education. As well as his political work, he conducted many experiments concerning electricity. In 1752, he flew a kite in a thunder-storm, attaching a metal key to the damp string. An electrical charge ran down the string and Franklin was able to feel it jump to his finger when he approached the key. From this he concluded that lightning was an electrical spark and in 1753 launched his invention of the lightning conductor.

By 1750, in addition to wanting to prove that lightning was electricity, Franklin began to think about protecting people, buildings, and other structures from lightning. This grew into his idea for the lightning rod. Franklin described an iron rod about 8 or 10 feet long that was sharpened to a point at the end. He wrote, "The electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike..." Two years later, Franklin decided to try his own lightning experiment. Surprisingly, he never wrote letters about the legendary kite experiment; someone else wrote the only account 15 years after it took place.

In June of 1752, Franklin was in Philadelphia, waiting for the steeple on top of Christ Church to be completed for his experiment (the steeple would act as the "lightning rod"). He grew impatient, and decided that a kite would be able to get close to the storm clouds just as well. Ben needed to figure out what he would use to attract an electrical charge; he decided on a metal key, and attached it to the kite. Then he tied the kite string to an insulating silk ribbon for the knuckles of his hand. Even though this was a very dangerous experiment, some people believe that Ben wasn't injured because he didn't conduct his test during the worst part of the storm. At the first sign of the key receiving an electrical charge from the air, Franklin knew that lightning was a form of electricity. His 21-year-old son William was the only witness to the event.

Two years before the kite and key experiment, Ben had observed that a sharp iron needle would conduct electricity away from a charged metal sphere. He first theorized that lightning might be preventable by using an elevated iron rod connected to earth to empty static from a cloud. Franklin articulated these thoughts as he pondered the usefulness of a lightning rod.

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A huge range of items can be made from steel, from tiny paperclips to huge girders forming the frames for skyscrapers. One useful property of steel is that it can be recycled and used over and over again.

Steel has a long history. People in India and Sri Lanka were making small amounts of steel more than 2,500 years ago. It was very expensive and was often used to make swords and knives. In the middle Ages, steel could be made only in small amounts since the processes took a long time.

In the time since, there have been many changes to the way steel is made. In about the year 1610 steel started to be made in England, and the way it was made got better and cheaper over the next 100 years. Cheap steel helped start the Industrial Revolution in England and in Europe. The first industrial Converter (metallurgy) for making cheap steel was the Bessemer converter, followed by Siemens-Martin open-hearth process.

Today the most common way of making steel is the basic-oxygen process. The converter is a large turnip-shaped vessel. Liquid raw iron called “pig iron” is poured in and some scrap metal is added in to balance the heat. Oxygen is then blown into the iron. The oxygen burns off the extra carbon and other impurities. Then enough carbon is added to make the carbon contents as wanted. The liquid steel is then poured. It can be either cast into molds or rolled into sheets, slabs, beams and other so-called “long products”, such as railway tracks. Some special steels are made in electric arc furnaces.

Steel is most often made by machines in huge buildings called steel mills. It is a very cheap metal and is used to make many things. Steel is used in making buildings and bridges, and all kinds of machines. Almost all ships and cars are today made from steel. When a steel object is old, or it is broken beyond repair, it is called scrap. It can be melted down and re-shaped into a new object. Steel is recyclable material; that is, the same steel can be used and re-used.

Alloys of steel, in which steel is combined with other metals, can be very useful. Railway tracks are often made of an alloy of steel and manganese.

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Steel is an alloy of iron and carbon. Iron extracted from iron ore contains about 4% carbon and some other impurities. The carbon makes it hard but weakens it. Removing some of the carbon and other impurities in an oxygen furnace produces steel.

Steel is an alloy of iron and carbon containing less than 2% carbon and 1% manganese and small amounts of silicon, phosphorus, sulphur and oxygen. Steel is the world's most important engineering and construction material. It is used in every aspect of our lives; in cars and construction products, refrigerators and washing machines, cargo ships and surgical scalpels.

Steels are a large family of metals. All of them are alloys in which iron is mixed with carbon and other elements. Steels are described as mild, medium- or high-carbon steels according to the percentage of carbon they contain, although this is never greater than about 1.5%.

The properties of steel are closely linked to its composition. For example, there is a big difference in hardness between the steel in a drinks can and the steel that is used to make a pair of scissors. The metal in the scissors contains nearly twenty times as much carbon and is many times harder. Notice how the percentage of carbon in the steel items in varies. Changing the carbon content changes the properties of the steel and the way that it is used.

The heat treatment given to steel can affect its properties too. Cooling a red-hot tool steel rapidly in cold water makes it harder and more brittle. We could have made the same piece of metal softer by keeping it at red heat for longer and then cooling it slowly. Heat treatment is another method that the steelmaker uses to make the properties of the steel match the job it has to do.

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So important was the metal-working industry of Belgium that a building in Brussels called the Atomium was made in the shape of a molecule of iron — magnified 165 billion times!

Andre Waterkeyn was born 1917 in Wimbledon, London and died in Brussels in 2005 at the age of 88. In a moment of quite delightful synchronicity Double Stone Steel are building a new stainless steel colouring factory in Wimbledon.

In 1954 Waterkeyn whilst working for Fabrimetal a group of metal fabrication companies, was asked to design a building that would showcase Belgian engineering skills to the world. An iron crystal magnified 165 billion times was deemed the way to go.

Three industrial groups – the Federation of the metalworking, mechanical and electrotechnical engineering industries, the Belgian blast-furnace and steel working group and the Union on non-ferrous metals industries – joined together in a non-profit-making organisation and appointed André Waterkeyn as Managing Director.

The Atomium was a monumental image of the then new and exciting nuclear age. During the fair, the Atomium held an exhibition showing the benefits of nuclear science to mankind. This was the age where the boffins around the world were convinced that nuclear science would completely remove the need for anyone work or for any other type of power generation system. Electricity would be so cheap that it would be free to all. The world would find its Utopia at long last. The Nuclear age was going to be, safe, cheap and simple. Nuclear power would save the world. I am not sure the general public were convinced as the memory of the nuclear booming in Japan were very fresh.

The Atomium consists of nine spheres, each sphere having a diameter of 18m.The spheres are connected by twenty, 23m long metal tubes, the tubes have a diameter 3.3m. The tubes allow the visitor to move between the spheres using escalators or staircases. The structure stands on three pillars known as ‘bipods’. In 1958 the Atomium had the fastest lift in Europe, reaching speeds of 5 meters per second.

The original construction of the frame was in steel, with 10-12mm aluminium panels. The spheres’ aluminium is an alloy called ‘Peraluman 15’ which was then covered with a thin sheet of aluminium called ‘reflectal’, which was then highly polished. In 2004 the building was shut to the public to be refurbished. The original aluminium panels being replaced by polished stainless steel panels. Over 6000 honeycombed panels were fabricated in 1.2mm, grade 316L with a rock wool insulation core and a 1mm galvanised interior skin. The building looks wonderful and should stand for many many years.

“The story of the Atomium is, above all, one of love, the love that the Belgians have for an extraordinary structure symbolising a frame of mind that wittily combines aesthetic daring with technical mastery. The appearance of the Atomium is unusual and unforgettable. It has a rare quality of lifting everyone’s spirits and firing their imagination.”

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Iron is the most widely used of all metals. It is cheap and very strong, so it can be used to make the supports for huge buildings and bridges. The Industrial Revolution would not have been possible without iron to make the machinery used in new factories. Today most iron is made into steel, a metal that can be used for a wider variety of purposes than any other metal on Earth.

Iron is an incredibly useful substance. It's less brittle than stone yet, compared to wood or copper, extremely strong. If properly heated, iron is also relatively easy to shape into various forms, as well as refine, using simple tools. And speaking of those tools, unlike wood, iron can handle high temperatures, allowing us to build everything from fire tongs to furnaces out of it. In contrast to most substances, you can also magnetize iron, making it useful in the creation of electric motors and generators. Finally, there certainly aren't any iron shortages to worry about. The Earth’s crust is 5 percent iron, and in some areas, the element concentrates in ores that contain as much as 70 percent iron.

When you compare iron and steel with something like aluminum, you can see why it was so important historically. To refine aluminum, you need access to huge quantities of electricity. Furthermore, to shape aluminum, you have to either cast it or extrude it. Iron, however, is much easier to manipulate. The element has been useful to people for thousands of years, while aluminum really didn't exist in any meaningful way until the 20th century.

­Fortunately, iron can be created relatively easily with tools that were available to primitive societies. There will likely come a day when humans become so technologically advanced that iron is completely replaced by aluminum, plastics and things like carbon and glass fibers. But right now, the economic equation gives inexpensive iron and steel a huge advantage over these much more expensive alternatives.

The only real problem with iron and steel is rust. Fortunately, you can control rust by painting, galvanizing, chrome plating or coating the iron with a sacrificial anode, which corrodes faster than the stronger metal. Think of this last option as hiring a bodyguard to take a bullet for the president. The more active metal has to almost completely corrode before the less active iron or steel begins the process.

­Humans have come up with countless uses for iron, from carpentry tools and culinary equipment to complicated machinery and instruments of torture. Before iron can be put to any of these uses, however, it has to be mined from the ground.

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Smelting is what is known as a reduction reaction. It is a method of extracting iron from iron ore. Iron ore, or haematite, is a rock that contains iron and oxygen. The process of smelting takes place in a blast furnace, where iron ore, limestone and coke (a form of carbon) are heated together while hot air is blasted into the furnace. The carbon in the coke reacts with the oxygen in the air to form carbon monoxide. This is turn takes oxygen from the iron ore, leaving behind iron mixed with a little carbon.

The majority of Earth’s iron, however, exists in iron ore. Mined right out of the ground, raw ore is mix of ore proper and loose earth called gangue. The ore proper can usually be separated by crushing the raw ore and simply washing away the lighter soil. Breaking down the ore proper is more difficult, however, as it is a chemical compound of carbonates, hydrates, oxides, silicates, sulfides and various impurities.

To get to the bits of iron in the ore, you have to smelt it out. Smelting involves heating up ore until the metal becomes spongy and the chemical compounds in the ore begin to break down. Most important, it releases oxygen from the iron ore, which makes up a high percentage of common iron ores.

The most primitive facility used to smelt iron is a bloomery. There, a blacksmith burns charcoal with iron ore and a good supply of oxygen (provided by a bellows or blower). Charcoal is essentially pure carbon. The carbon combines with oxygen to create carbon dioxide and carbon monoxide (releasing lots of heat in the process). Carbon and carbon monoxide combine with the oxygen in the iron ore and carry it away, leaving iron metal.

In a bloomery, the fire doesn't get hot enough to melt the iron completely. Instead, the iron heats up into a spongy mass containing iron and silicates from the ore. Heating and hammering this mass (called the bloom) forces impurities out and mixes the glassy silicates into the iron metal to create wrought iron. Wrought iron is hardy and easy to work, making it perfect for creating tools.

Tool and weapon makers learned to smelt copper long before iron became the dominant metal. Archeological evidence suggests that blacksmiths in the Middle East were smelting iron as early as 2500 B.C., though it would be more than a thousand years before iron became the dominant metal in the region.

­To create higher qualities of iron, blacksmiths would require better furnaces. The technology gradually developed over the centuries. By the mid-1300s, taller furnaces and manually operated bellows allowed European furnaces to burn hot enough to not just soften iron, but actually melt it.

Frost is formed overnight when the air temperature drops below 0°C (32°F) and the dew freezes. Clouds in the sky act as insulation, preventing the heat from the Sun that has built up in the land, sea and air during the day from escaping. This means that the temperature is less likely to drop below freezing. When the sky is clear, the day’s heat is able to escape easily, and a frost is likely.

There are several factors that influence whether frost will form on a given day or not. These factors include temperature of the surface (e.g. grass, window) on which it forms, clearness of the sky during the evening and night, wind speed during the night and humidity levels.

Surface temperature is one main factor of which frost forms. Within a certain area, there can be a difference of temperature readings. For example, when the weather personnel read the current temperature, they are receiving these readings from an outdoor weather station or what is called the Stevenson Screen (instrument shelter). These weather stations are sensors which are placed 4 feet 11 inches (1.5 meters) above ground level. The actual ‘ground or surface temperature’ could be lower than what the local weather personnel state as the ground is almost 5 feet lower than the sensor. If the temperature of the item’s surface is below freezing [32? F (0? C)], the water droplets or dew will turn into frost.

A clear or cloudy sky also will influence the formation of frost. If the sky is clear and calm while the temperatures continue to fall into the evening, the chance for frost increases. If the sky is cloudy as the evening approaches, the clouds will help contain the heat emitted from the earth’s surface. This will keep the ground warmer making frost a less likely possibility.

Another factor assisting in the formation of frost is wind. If there is no wind, the air is still and the colder air will settle on the ground. However, if there is a slow, gentle wind, the colder air will be pushed along and not have the chance to settle on the ground making frost a bit difficult to form. The most damaging wind type is one which is very cold and has below freezing temperatures. If frost forms during this time, it can be severely damaging to plants and other surfaces.

Finally, an important factor in the formation of frost is the humidity in the air. Air holds water vapor and can contain up to 100% water vapor saturation. This is known as 100% relative humidity. At this point, the air can no longer hold any additional water vapor so the water begins to condense into a liquid form. If the temperature is above freezing, dew will form leaving droplets of water on many different types of surfaces.

Plants and animals are made up of cells, each of which is surrounded by a cell wall. Some foods contain a great deal of water. As the water freezes, it expands, breaking the cell walls. When the food is defrosted, its texture has been changed and what remains may well be just a mushy mass. It is not dangerous to eat this food, but it may not look or taste very pleasant.

Foods in the freezer — are they safe? Every year, thousands of callers to the USDA Meat and Poultry Hotline aren't sure about the safety of items stored in their own home freezers. The confusion seems to be based on the fact that few people understand how freezing protects food. Here is some information on how to freeze food safely and how long to keep it.

You can freeze almost any food. Some exceptions are canned food or eggs in shells. However, once the food (such as a ham) is out of the can, you may freeze it. Being able to freeze food and being pleased with the quality after defrosting are two different things. Some foods simply don't freeze well. Examples are mayonnaise, cream sauce and lettuce. Raw meat and poultry maintain their quality longer than their cooked counterparts because moisture is lost during cooking.

Food stored constantly at 0 °F will always be safe. Only the quality suffers with lengthy freezer storage. Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a dormant stage. Freezing preserves food for extended periods because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness.

Freezing to 0 °F inactivates any microbes — bacteria, yeasts and molds — present in food. Once thawed, however, these microbes can again become active, multiplying under the right conditions to levels that can lead to foodborne illness. Since they will then grow at about the same rate as microorganisms on fresh food, you must handle thawed items as you would any perishable food. Trichina and other parasites can be destroyed by sub-zero freezing temperatures. However, very strict government-supervised conditions must be met. Home freezing cannot be relied upon to destroy trichina. Thorough cooking, however, will destroy all parasites.

Freshness and quality at the time of freezing affect the condition of frozen foods. If frozen at peak quality, thawed foods emerge tasting better than foods frozen near the end of their useful life. So freeze items you won't use quickly sooner rather than later. Store all foods at 0° F or lower to retain vitamin content, color, flavor and texture.

Melons, which have very high water content, do not freeze successfully. Other fruits may still be edible after freezing but have a different texture.

Two things have to happen to the mixture of dairy products and flavourings that make up ice-cream: they must be frozen and they must be stirred, to prevent large ice crystals from forming. Before electrical freezing machines were available, the ice-cream mixture was put into a chum, around which a mixture of salt and ice was packed. Heat from the ice-cream mixture gradually passed into the colder ice, until the cream itself was frozen. Meanwhile, the mixture was stirred by means of a paddle connected to a handle outside the tub. This became harder work as the ice-cream froze!

There is a good summary in The National Trust Book of Sorbets, Flummeries and Fools by Colin Cooper English (published 1985). Time-consuming and costly, the old-fashioned way was to place the ingredients into a thin drum, which was then sunk into a larger container which held a mixture of ice and salt. Although water freezes at 32F (0C), milk and cream will not freeze until they are down to 20F (-6.7C). The salt melts the ice and produces a brine with a temperature around 17F (-8.3C), and it is this freezing brine which provides the refrigeration. The effort needed to produce a serving of ice-cream in an early Victorian household can be seen in this 1856 recipe: 'Break a pail of ice in pieces, add four pounds of salt and mix well; put a pewter freezing-can in an empty pail and surround it with ice; put the pudding ... into the can, and turn it very rapidly with the finger and thumb; when the pudding adheres to the sides of the can, scrape off with a spittle or spoon. When the pudding has become stiff, put it into a mould, cover it up with a lid, having put two plies of paper between; bury the mould in the ice; when wanted, take a basin of cold water and wash off the salt, take off the cover, turn it out on a dish and serve.' All this assumes that you have a handy supply of ice. Those who could afford it had ice-cellars or ice-houses built underground, in which ice from the winter could be kept, insulated by the air trapped in a layer of straw, reeds, chaff or bundles of thin wood faggots throughout the rest of the year. The idea seems to have been used first by the Chinese. At the time of Confucius (500 BC) there were accounts of ice-cellars. Alexander the Great is said to have employed slaves in relays to carry snow and ice down from the mountains. The ice-cream recipe was brought back to Venice from China by Marco Polo in 1292. By the mid-19th century a number of freezing mixtures had been devised, which did not require snow or ice to start them off. They included such lethal cocktails as a mixture of sal ammoniac, nitre and water, said to reduce the temperature from 50F to 10F; nitrate of ammonia and water (50F down to 4F); and sulphate of soda with dilute sulphuric acid (50F down to 3F).