Mining has taken place since the Stone Age, when flints were mined for making stone tools. By 3500BC, people were mining for copper, which they soon combined with tin to harden it and make tools and weapons. Today we mine minerals, diamonds, metals, coal and rock for building material, using a variety of techniques.
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Cameras work by focusing light in order to produce an image on a light-sensitive surface. Some cameras work by forming the image on light-sensitive film. Sometimes film uses dyes that change colour when exposed to light, as happens with film used to make coloured slides or prints. Earlier film or glass plates used a silver compound as the light-sensitive material. To produce a photographic print the image is projected onto the surface of light-sensitive paper. It is treated with chemicals to make the print visible and prevent further changes when exposed to light.
Digital cameras are a recent development. They convert the image they receive into electrical signals that are stored. These signals can then be read by a computer and used to produce a picture on screen, which can then be printed out.
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Printing is the process by which many copies of words and pictures can be reproduced on paper or other materials. Simple wood blocks with carved letters covered with ink were used to print until around 1440, when Johannes Gutenberg developed movable type (a separate piece of metal type for each character). A machine pressed the inked type against paper.
Most modern printing is carried out by offset printing, or lithography. Images are placed on metal printing plates by a photographic process, and a greasy ink is applied that sticks to the areas where the image will be. The inked image is then transferred to a rubber roller and applied onto paper. Some of the fastest newspaper presses can print at a speed of 900 m of paper per minute.
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Explosives are substances that release a very large amount of energy in a short time when they burn. They all contain a fuel plus an oxidizer, which is a substance containing sufficient oxygen to burn the fuel. Explosives do not need air to explode, because they contain their own oxygen, and so they can burn in confined spaces or even underwater. Some explosives, such as those in a cartridge detonator, explode violently when hit or heated. Others only cause a violent explosion when confined in a gun barrel or a hole drilled in rock for blasting.
Gunpowder, the first ever explosive, was invented in China over 1,000 years ago. Large amounts of modern explosives are commonly used in quarrying and mining, as well as for munitions.
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Plastics are synthetic materials that consist of long chains of molecules called polymers. When the chains are positioned in long rows the plastic is rigid. When the polymer molecules are tangled together, the plastic is soft and flexible. All plastics can be moulded under pressure into complicated shapes. Thermoplastics melt when they are heated, and this means that they can be remelted at any time. There are other forms of plastic known as thermosetting plastics, which cannot be remelted.
The majority of plastics are made from synthetic resins, which are usually derived from oil. Other types come from sources such as coal or wood. As they do not conduct electricity, most plastics are excellent insulators, so are widely used in electrical wiring. The main disadvantage of plastics is that they are very slow to break down, causing serious environmental problems when discarded.
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Natural rubber comes from a kind of tree that originally grew in South America. Its liquid sap, called latex, is drained from the tree by making cuts in the bark. The latex is collected and processed into rubber. Natural rubber is very soft and is used for making the soles of shoes and sandals.
In 1839 Charles Goodyear, an American inventor, found that heating rubber together with sulphur made it much harder. This process, known as vulcanization, made it possible to use rubber for vehicle tyres. Rubber is now used for electrical insulation, motor tyres, cushions, golf balls and many other applications.
Since World War II most of the rubber we use has been produced synthetically. Rubber’s properties are based on the way its molecules link up into long chains, making giant molecules called polymers.
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