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magnetism
The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones (possibly chunks of iron ore struck by lightning) with the power to attract iron. A steel needle stroked with such a "lodestone" became "magnetic" as well, and around 1000 the Chinese found that such a needle, when freely suspended, pointed north-south. The magnetic compass soon spread to Europe. Columbus used it when he crossed the Atlantic ocean, noting not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage. Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: the Earth itself was a giant magnet, with its magnetic poles some distance away from its geographic ones (i.e. near the points defining the axis around which the Earth turns). On Earth one needs a sensitive needle to detect magnetic forces, and out in space they are usually much, much weaker. But beyond the dense atmosphere, such forces have a much bigger role, and a region exists around the Earth where they dominate the environment, a region known as the Earth's magnetosphere. That region contains a mix of electrically charged particles, and electric and magnetic phenomena rather than gravity determine its structure. We call it the Earth's magnetosphere

Only a few of the phenomena observed on the ground come from the magnetosphere: fluctuations of the magnetic field known as magnetic storms and substorms, and the polar aurora or "northern lights," appearing in the night skies of places like Alaska and Norway. Satellites in space, however, sense much more: radiation belts, magnetic structures, fast streaming particles and processes which energize them. All these are described in the sections that follow.

Until 1821, only one kind of magnetism was known, the one produced by iron magnets. Then a Danish scientist, Hans Christian Oersted, while demonstrating to friends the flow of an electric current in a wire, noticed that the current caused a nearby compass needle to move. The new phenomenon was studied in France by Andre-Marie Ampere, who concluded that the nature of magnetism was quite different from what everyone had believed. It was basically a force between electric currents: two parallel currents in the same direction attract, in oposite directions repel. Iron magnets are a very special case, which Ampere was also able to explain. In nature, magnetic fields are produced in the rarefied gas of space, in the glowing heat of sunspots and in the molten core of the Earth. Such magnetism must be produced by electric currents, but finding how those currents are produced remains a major challenge. Magnesium, «mag NEE shee uhm or mag NEE zhee uhm», a silver-white metal, is the lightest metal that is strong enough to use in construction. It weighs only about two-thirds as much as aluminum, another widely used light metal. Magnesium was discovered in 1808 by the English chemist Sir Humphry Davy.

Magnesium is a fairly abundant metallic element. But pure magnesium does not occur in nature. Various minerals contain magnesium compounds. A few of these compounds, primarily magnesium chloride and magnesium sulfate, occur in dissolved form in seawater and in some pools of underground water. Seawater contains 0.13 percent magnesium and provides a practically unlimited source of the metal. Magnesium occurs in the minerals magnesite, brucite, and dolomite, as well as in amphibole asbestos, olivine, serpentine, talc, and in certain of the other silicate minerals (see Silicate).

Magnesium plays a vital role in the life processes of plants and animals. Chlorophyll, which green plants use in photosynthesis, contains magnesium. Plants produce carbohydrates, a class of foods essential to living things, by means of photosynthesis. Magnesium takes part in the duplication of DNA (the genetic material of life) and in the production of proteins guided by the molecule RNA. Magnesium also activates many of the enzymes that speed up chemical reactions in the human body.

Uses. Magnesium and its alloys are used in manufacturing many products. Their light weight makes them suitable for aircraft and automobile parts and for tools and equipment. Most magnesium alloys contain aluminum and zinc. These materials make magnesium alloys stronger and easier to shape. Some alloys may also contain small amounts of such elements as manganese, thorium, and zirconium that provide other properties.

Magnesium is used for a variety of nonstructural purposes because it is extremely active chemically. For example, pieces of magnesium are placed next to buried steel pipelines and water tanks. If magnesium were not present, oxygen and other chemicals in the earth would corrode the steel. Instead, the magnesium reacts with the chemicals. The pieces of magnesium can easily be replaced periodically at a cost much lower than that of replacing or repairing the steel. Protective strips of magnesium are also attached to the hulls of ships.

Steel manufacturers add magnesium to steel to remove sulfur and other impurities. In addition, magnesium is used in fireworks and flares because it burns with a brilliant white light. It also produces intense heat when it burns, making it useful for incendiary bombs.

Magnesium combines with other elements to form many useful compounds. These compounds include two commonly used medicines—milk of magnesia and Epsom salt (see Magnesia). Magnesium oxide resists heat and is used to line special types of furnaces. Also, magnesium oxide forms on the surface of magnesium metal and prevents it from corroding readily at low temperatures. If it were not for this protective layer, magnesium would not be a suitable structural material. Magnesium chloride is an important catalyst (substance that speeds up a chemical reaction) in the preparation of organic compounds. Other magnesium compounds are used in tanning leather; in dyeing textiles; and in making cement, fertilizer, and insulating materials.

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Properties. Magnesium's chemical symbol is Mg. The metal belongs to the group of elements called alkaline earth metals (see Element, Chemical [table: Table of the elements]). It has a density of 1.738 grams per cubic centimeter at 20 °C. Its atomic number (number of protons in its nucleus) is 12. Its relative atomic mass is 24.3050. An element's relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. Magnesium melts at 650 °C and boils at 1090 °C.

Magnesium never occurs in nature as a pure metal because it is so active chemically. It readily combines with most acids and with many nonmetals, including nitrogen. When heated with the salts or oxides of many metals, magnesium replaces the other metal. In this process, called reduction, the magnesium purifies the other metal, preparing it for various uses.

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How magnesium is obtained. Two major methods are used to obtain magnesium. In the Dow process, magnesium is recovered from seawater. Lime obtained from the rock dolomite is mixed with seawater, which contains magnesium chloride. The lime and the magnesium chloride react to form magnesium hydroxide and calcium chloride. The magnesium hydroxide separates from the rest of the mixture. It is filtered out and mixed with hydrochloric acid, forming magnesium chloride and water. The water evaporates, leaving highly concentrated magnesium chloride, which is melted by being heated to a temperature above 708 °C. Then an electric current is passed through the melted compound. This kind of process is called electrolysis (see Electrolysis). The current changes the magnesium chloride into magnesium and chlorine gas. The molten magnesium is poured off and cast into forms called ingots.

The other method of refining magnesium is known as the ferrosilicon process or Pidgeon process. This method, used primarily in Canada, involves heating dolomite in a vacuum with an alloy consisting of silicon and iron. The magnesium in the dolomite vaporizes, and then it condenses as crystals. The magnesium crystals are melted and cast into ingots