Zinc is a chemical element with symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. In some respects zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state, the Zn2+ and Mg2+ ions are of similar size. Zinc has five stable isotopes; the most common zinc ore is sphalerite, a zinc sulfide mineral. The largest workable lodes are in Australia and the United States. Zinc is refined by froth flotation of the ore and final extraction using electricity. Brass, an alloy of copper and zinc in various proportions, was used as early as the third millennium BC in the Aegean, the United Arab Emirates, Kalmykia and Georgia, the second millennium BC in West India, Iran, Syria and Israel/Palestine. Zinc metal was not produced on a large scale until the 12th century in India, though it was known to the ancient Romans and Greeks; the mines of Rajasthan have given definite evidence of zinc production going back to the 6th century BC. To date, the oldest evidence of pure zinc comes from Zawar, in Rajasthan, as early as the 9th century AD when a distillation process was employed to make pure zinc.
Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow". The element was named by the alchemist Paracelsus after the German word Zinke. German chemist Andreas Sigismund Marggraf is credited with discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. Corrosion-resistant zinc plating of iron is the major application for zinc. Other applications are in electrical batteries, small non-structural castings, alloys such as brass. A variety of zinc compounds are used, such as zinc carbonate and zinc gluconate, zinc chloride, zinc pyrithione, zinc sulfide, dimethylzinc or diethylzinc in the organic laboratory. Zinc is an essential mineral, including to postnatal development. Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, diarrhea.
Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc may cause ataxia and copper deficiency. Zinc is a bluish-white, diamagnetic metal, though most common commercial grades of the metal have a dull finish, it is somewhat less dense than iron and has a hexagonal crystal structure, with a distorted form of hexagonal close packing, in which each atom has six nearest neighbors in its own plane and six others at a greater distance of 290.6 pm. The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C. Above 210 °C, the metal can be pulverized by beating. Zinc is a fair conductor of electricity. For a metal, zinc has low melting and boiling points; the melting point is the lowest of all the d-block metals aside from cadmium. Many alloys contain zinc, including brass. Other metals long known to form binary alloys with zinc are aluminium, bismuth, iron, mercury, tin, cobalt, nickel and sodium.
Although neither zinc nor zirconium are ferromagnetic, their alloy ZrZn2 exhibits ferromagnetism below 35 K. A bar of zinc generates a characteristic sound when bent, similar to tin cry. Zinc makes up about 75 ppm of Earth's crust. Soil contains zinc in 5–770 ppm with an average 64 ppm. Seawater has only 30 ppb and the atmosphere, 0.1–4 µg/m3. The element is found in association with other base metals such as copper and lead in ores. Zinc is a chalcophile, meaning the element is more to be found in minerals together with sulfur and other heavy chalcogens, rather than with the light chalcogen oxygen or with non-chalcogen electronegative elements such as the halogens. Sulfides formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. Sphalerite, a form of zinc sulfide, is the most mined zinc-containing ore because its concentrate contains 60–62% zinc. Other source minerals for zinc include smithsonite, hemimorphite and sometimes hydrozincite. With the exception of wurtzite, all these other minerals were formed by weathering of the primordial zinc sulfides.
Identified world zinc resources total about 1.9–2.8 billion tonnes. Large deposits are in Australia and the United States, with the largest reserves in Iran; the most recent estimate of reserve base for zinc was made in 2009 and calculated to be 480 Mt. Zinc reserves, on the other hand, are geologically identified ore bodies whose suitability for recovery is economically based at the time of determination. Since exploration and mine development is an ongoing process, the amount of zinc reserves is not a fixed number and sustainability of zinc ore supplies cannot be judged by extrapolating the combined mine life of today's zinc mines; this concept is well supported by data from the United States Geol
Food drying is a method of food preservation in which food is dried. Drying inhibits the growth of bacteria and mold through the removal of water. Dehydration has been used for this purpose since ancient times. C. by inhabitants of the modern Middle East and Asia regions. Water is traditionally removed through evaporation, although today electric food dehydrators or freeze-drying can be used to speed the drying process and ensure more consistent results. Many different foods can be prepared by dehydration. Meat has held a significant role. For centuries, much of the European diet depended on dried cod—known as salt cod, bacalhau, or stockfish, it formed the main protein source for the slaves on the West Indian plantations, was a major economic force within the triangular trade. Dried fish most cod or haddock, known as Harðfiskur, is a delicacy in Iceland, while dried reindeer meat is a traditional Sami food. Dried meats include prosciutto, bresaola and beef jerky. Dried fruits have been consumed due to their high sugar content and sweet taste, a longer shelf-life from drying.
Fruits may be used differently. The plum becomes the grape a raisin. Figs and dates may be transformed into different products that can either be eaten as they are, used in recipes, or rehydrated. Freeze-dried vegetables are found in food for backpackers and the military. Garlic and onion are dried. Edible mushrooms, as well as other fungi, are sometimes dried for preservation purposes or to be used as seasonings. Home drying of vegetables and meat can be carried out with electrical dehydrators or by sun-drying or by wind. Preservatives such as potassium metabisulfite, BHA, or BHT are not required. However, dried products without these preservatives may require refrigeration or freezing to ensure safe storage for a long time. Industrial food dehydration is accomplished by freeze-drying. In this case food is flash frozen and put into a reduced-pressure system which causes the water to sublimate directly from the solid to the gaseous phase. Although freeze-drying is more expensive than traditional dehydration techniques, it mitigates the change in flavor and nutritional value.
In addition, another used industrial method of drying of food is convective hot air drying. Industrial hot air dryers are simple and easy to design and maintain. More so, it is affordable and has been reported to retain most of the nutritional properties of food if dried using appropriate drying conditions. There are many different methods for drying, each with their own advantages for particular applications; these include: National Center for Home Food Preservation, drying section
Ice house (building)
Ice houses or icehouses are buildings used to store ice throughout the year used prior to the invention of the refrigerator. Some were underground chambers man-made, close to natural sources of winter ice such as freshwater lakes, but many were buildings with various types of insulation. During the winter and snow would be cut from lakes or rivers, taken into the ice house, packed with insulation, it would remain frozen for many months until the following winter, could be used as a source of ice during the summer months. The main application of the ice was the storage of foods, but it could be used to cool drinks, or in the preparation of ice-cream and sorbet desserts. During the heyday of the ice trade, a typical commercial ice house would store 2,700 tonnes of ice in a 30-by-100-foot and 14-metre-high building. A cuneiform tablet from c. 1780 BC records the construction of an icehouse by Zimri-Lim, the King of Mari, in the northern Mesopotamian town of Terqa, "which never before had any king built."
In China, archaeologists have found remains of ice pits from the 7th century BC, references suggest that these were in use before 1100 BC. Alexander the Great stored snow in pits dug for that purpose around 300 BC. In Rome, in the 3rd century AD, snow was imported from the mountains, stored in straw-covered pits, sold from snow shops; the ice that formed in the bottom of the pits sold at a higher price than the snow on top. The ice house was introduced to Britain around 1660. Various types and designs of ice house exist but British ice houses were brick-lined, domed structures, with most of their volume underground. Ice houses varied in design depending on the date and builder, but were conical or rounded at the bottom to hold melted ice, they had a drain to take away the melt-water. It is recorded that the idea for ice houses was brought to Britain by travellers who had seen similar arrangements in Italy, where peasants collected ice from the mountains and used it to keep food fresh inside caves.
Ice houses were known as ice wells, ice pits or ice mounds. Game larders and venison larders were sometimes marked on Ordnance Survey maps as ice houses. Bruce Walker, an expert on Scottish Vernacular buildings, has suggested that numerous and long-ruined ice houses on country estates have led to Scotland's many legends of secret tunnels. Ice was imported into the UK from Scandinavia until the 1920s, although from around 1900 the import of ice declined due to the development of factories in the UK where ice was made artificially. Only large mansions had purpose-built buildings to store ice. Many examples of ice houses exist in the UK. Good examples of 19th-century ice houses can be found at Ashton Court, Albrighton, Grendon, at Christchurch Mansion, Suffolk, Petworth House, Danny House, Ayscoughfee Hall, Rufford Abbey, Eglinton Country Park in Scotland, Parlington Hall in Yorkshire and Croxteth Hall Liverpool, Burghley House and Moggerhanger Park, Bedfordshire. A domed example with circular tie-access from above and side-entrance survives at Stoke Park, Berkshire.
An unusual example of an ice house, converted from a redundant brick springhead can be found in the former grounds of Norton House, Midsomer Norton, Somerset. The largest surviving ice house in the UK is the Tugnet Ice House in Spey Bay, it was built in 1830, used to store ice for packing salmon caught in the River Spey before transportation to market in London. In 2018, the large Park Crescent West ice well was discovered in Park Crescent, London, it was created for Samuel Dash in the early 1780s for commercial use before the building of the John Nash crescent was begun in 1806. This ice house is 9.5 metres deep, 7.5 metres wide, is only a few metres away from the Jubilee line on the London Underground. Used for the storage of local ice taken from the River Thames in the winter months, it was taken over in the 1820s by the ice merchant William Leftwich, who used it for storing imported ice from the frozen lakes of Norway. A pair of commercial ice wells has been preserved in London, beneath what is now the London Canal Museum at King's Cross.
They are around 30 feet in diameter and were 42 feet deep. They were built in 1863 by the Swiss entrepreneur Carlo Gatti. In 1985, a passage was discovered beneath Ardgillan Castle in Co. Dublin, Republic of Ireland; this passage was found to be the ice house, known to exist on the grounds, but whose location had not been rediscovered until this date. Ice houses allowed a trade in ice, a major part of the early economy of the New England region of the United States, which saw fortunes made by people who transported ice in straw-packed ships to the southern states and throughout the Caribbean Sea. Most notable was Frederic Tudor who formed the Tudor Ice Company in the early 19th century. In winter months, ice was chipped from a lake surface and dragged by sled to the ice house. In summer months, icemen delivered it to residences in ice-wagons; as home and business refrigeration became more commonplace, ice houses fell into disuse, the home ice delivery business declined until it had disappeared by the late 1960s.
Smaller ice houses no more than a sawdust pile covered by a makeshift roof or tarpaulin, continued to be maintained for storing ice for use in local events such as fairs. Today, most ice for daily consumption is made in a home
A pie safe called a pie chest, pie cupboard, kitchen safe, meat safe, is a piece of furniture designed to store pies and other food items. This was a normal household item before iceboxes came into regular use, it was an important part of the American household starting in the 1700s and continuing through the 1800s; the pie safe was used to store not only pies, but bread and other perishables as well, to protect them from insects and vermin. A common pie safe is made of wood, is around the same size as a large bureau, is 18 inches deep; the shelves within the storage area are perforated. The safe has two hinged doors on the front; these doors, the sides, are ordinarily ventilated either with tin plates with punched holes, or screens. The holes in the tin are punched to produce an image such as a simple shape, or something more intricate like a church scene and stars, or a Masonic emblem. A notable design is the Wythe County pie safe. Pie safes that are freestanding are ordinarily made with long legs to keep them away from the floor.
Some are suspended from the ceiling. Most have a drawer above the pie storage area, but sometimes below. In Cajun or Creole Louisiana, a pie safe is referred to a garde de manger. Pie safes from this region had doors with punched, tin panels, known in the region as tôles de panneaux, or were inlaid with baluster spaced; these items of furniture were considered utilitarian, as opposed to decorative, were coloured dull red, referred to as gros rouge. The origin of the name of chess pie may have come from the term "pie chest", another name for a pie safe. Pie safes are considered to be collectable antiques and are reproduced, they are popular pieces in the shabby chic interior design style. A notable pie safe maker was the American industrialist and founder of PPG Industries, Captain John Baptiste Ford, who made tin pie safes and sold them throughout the United States
A cooler, portable ice chest, ice box, cool box, chilly bin, or esky is an insulated box used to keep food or drink cool. Ice cubes are most placed in it to help the contents inside stay cool. Ice packs are sometimes used, as they either contain the melting water inside, or have a gel sealed inside that stays cold longer than plain ice. Coolers are taken on picnics, on vacation or holiday. Where summers are hot, they may be used just for getting cold groceries home from the store, such as keeping ice cream from melting in a hot automobile. Without adding ice, this can be helpful if the trip home will be lengthy; some coolers have built-in cupholders in the lid. They are made with interior and exterior shells of plastic, with a hard foam in between, they come in sizes from small personal ones to large family ones with wheels. Disposable ones are made from polystyrene foam about 2 cm or one inch thick. Most reusable ones have molded-in handles; the cooler has developed from just a means of keeping beverages cold into a mode of transportation with the ride-on cooler.
A thermal bag or cooler bag is similar in concept, but smaller and not rigid. The portable ice chest was invented by Richard C. Laramy of Joliet, Illinois. On February 24, 1951, Laramy filed an application with the United States Patent Office for a portable ice chest; the patent was issued December 22, 1953. The Coleman Company popularized the cooler with its initial offering of a galvanized cooler in 1954. Three years Coleman developed a process to make a plastic liner for coolers and jugs; some modern coolers are thermoelectric. Rather than using a compressor and refrigerant such as a refrigerator or other heat pump, these use the Peltier effect along with an external fan to draw away the heat. By reversing the current, this concept can heat the contents instead of cooling them, useful for keeping meals hot from a drive-through, or to keep items from freezing in cold climates. Thermoelectric coolers can drop the temperature by about 40 °F or 22 °C below ambient temperature, or can raise it by at least that much.
Some better units have digital thermostat controls. They do draw a significant amount of power and can drain a non running car's battery so much so that it cannot start. Most electric coolers have an undervoltage shutoff at around 10.5 volts to prevent this. Many come with power adapters, which use an electronic transformer to convert AC mains or line voltage down to 12 volts, with a lighter-like socket for the cooler's cord to plug in; some have a crossover-connection device to reverse the current for heating service. Rotationally molded coolers have become popular in recent years. Roto-molded coolers are manufactured using a process called rotational molding, a process by which a heated and softened material is applied into a rotating mold; the mold continues producing a thick and uniform final product. In the case of roto-molded coolers, the heated liquid plastic is applied over a thick layer of insulation; the resulting product has no seals or imperfections and is much stronger and more durable than traditional coolers.
Examples of roto-molded coolers include ORCA or Grizzly coolers. A ride-on cooler is a means of transportation that can store and cool beverages and other food products. There are two main designs for the motor in the cooler: the electric motor, the gas-powered lawnmower engine; the ride-on cooler is a combination of a low power, or small electric, engine with a go-cart frame which uses the cooler as a seat. Gas engine options include 5 and 6.5 hp Briggs engines, 6.5 hp OHV greyhound Harbor Freight engines, 6.5 hp Honda engines. These small engines can be powerful, making them suitable for homeowners, recreational users, light-duty professional users, racing enthusiasts; the fastest ride-on coolers use modified racing cart engines that produce over 14 hp and are capable of speeds exceeding 70 mph. Another option is an electric motor ranging from 250 to 2,000 watts; the electric motors can be lighter, more energy efficient, quieter than most gas motors. Without the excess weight of the fuel tank and its contents, the electric-powered coolers can be lighter and silent.
The frame has two design styles: three wheels with a single steering wheel out front, or four wheels, as in the average go-cart. The steering column of a gas-powered ride-on cooler is similar to that of a go-cart. One major difference is the position of the steering wheel. For the go-cart, the steering wheel is angled toward the driver, enabling him to sit lower to the ground and maintain control at higher speeds. For the ride-on cooler, the steering column is vertical because the seat is higher and the driver sits more vertical; the ride-on cooler has the ability to transport food and drinks short distances and can be used in a small backyard, a neighborhood, or at large outdoor parties. The ride-on cooler can be equipped with a trailer hitch, allowing it to tow an extra cooler as a trailer; the extra trailer has more holding capacity than the drivable cooler due to the lack of an engine. The electric ride-on coolers are advertised as being capable of holding 24 12-ounce cans and a bag of ice.
The trailer can hold up to a bag of ice. Larger gas-powered coolers are capable of carrying up to 72 drinks and 15
Smoking is the process of flavoring, cooking, or preserving food by exposing it to smoke from burning or smoldering material, most wood. Meat and lapsang souchong tea are smoked. In Europe, alder is the traditional smoking wood, but oak is more used now, beech to a lesser extent. In North America, mesquite, pecan, alder and fruit-tree woods, such as apple and plum, are used for smoking. Other biomass besides wood can be employed, sometimes with the addition of flavoring ingredients. Chinese tea-smoking uses a mixture of uncooked rice and tea, heated at the base of a wok; some North American ham and bacon makers smoke their products over burning corncobs. Peat is burned to smoke the barley malt used to make whisky and some beers. In New Zealand, sawdust from the native manuka is used for hot smoking fish. In Iceland, dried sheep dung is used to cold-smoke fish, lamb and whale. Farms in the Western world included a small building termed the "smokehouse," where meats could be smoked and stored; this was well-separated from other buildings both because of the fire danger and because of the smoke emanations.
Smoking can be done in four ways: cold smoking, warm smoking, hot smoking, through the employment of "liquid smoke". However, these methods of imparting smoke only affect the food surface, are unable to preserve food, smoking is paired with other microbial hurdles, such as chilling and packaging, to extend food shelf-life; the smoking of food dates back to the paleolithic era. As caves or simple huts lacked chimneys, these dwellings would have become smoky, it is supposed that early men would hang meat up to dry and out of the way of pests, thus accidentally becoming aware that meat, stored in smoky areas acquired a different flavor, was better preserved than meat that dried out. This process was combined with pre-curing the food in salt or salty brine, resulting in a remarkably effective preservation process, adapted and developed by numerous cultures around the world; until the modern era, smoking was of a more "heavy duty" nature as the main goal was to preserve the food. Large quantities of salt were used in the curing process and smoking times were quite long, sometimes involving days of exposure.
The advent of modern transportation made it easier to transport food products over long distances and the need for the time and material intensive heavy salting and smoking declined. Smoking became more of a way to flavor. In 1939 a device called; the kiln allowed for uniform mass-smoking and is considered the prototype for all modern large-scale commercial smokers. Although refinements in technique and advancements in technology have made smoking much easier, the basic steps involved remain the same today as they were hundreds if not thousands of years ago. Cold smoking differs from hot smoking in that the food remains raw, rather than cooked, throughout the smoking process. Smokehouse temperatures for cold smoking are done between 20 to 30 °C. In this temperature range, foods take on a smoked flavor, but remain moist. Cold smoking does not cook foods, as such, meats should be cured before cold smoking. Cold smoking can be used as a flavor enhancer for items such as cheese or nuts, along with meats such as chicken breasts, pork chops, salmon and steak.
The item is hung in a dry environment first to develop a pellicle it can be cold smoked up to several days to ensure it absorbs the smokey flavour. Some cold smoked foods are baked, steamed, roasted, or sautéed before eating. Cold smoking meats is not something that should be attempted at home, according to the US National Center for Home Food Preservation:"Most food scientists cannot recommend cold-smoking methods because of the inherent risks." Cold smoking meats should only be attempted by personnel certified in HACCP, or Hazard Analysis and Critical Control Points, to ensure that it is safely prepared. Warm smoking exposes foods to temperatures of 25–40 °C. Hot smoking exposes the foods to smoke and heat in a controlled environment such as a smoker oven or smokehouse. Hot smoking requires the use of a smoker which generates heat either from a charcoal base, heated element within the smoker or from a stove-top or oven, food is hot smoked by cooking and flavoured with wood smoke simultaneously.
Like cold smoking, the item may be hung first to develop a pellicle, it is smoked from 1 hour to as long as 24 hours. Although foods that have been hot smoked are reheated or further cooked, they are safe to eat without further cooking. Hams and ham hocks are cooked once they are properly smoked, they can be eaten as is without any further preparation. Hot smoking occurs within the range of 52 to 80 °C; when food is smoked within this temperature range, foods are cooked and flavorful. If the smoker is allowed to get hotter than 85 °C, the foods will shrink excessively, buckle, or split. Smoking at high temperatures reduces yield, as both moisture and fat are cooked away. Liquid smoke, a product derived from smoke compounds in water, is applied to foods through spraying or dipping. Smoke-roasting refers to any process that has the attributes of both smoking; this smoking method is sometimes referred to as pit-roasting. It may be done in a sm
Chlorofluorocarbons are halogenated paraffin hydrocarbons that contain only carbon and fluorine, produced as volatile derivative of methane and propane. They are commonly known by the DuPont brand name Freon; the most common representative is dichlorodifluoromethane. Many CFCs have been used as refrigerants and solvents; because CFCs contribute to ozone depletion in the upper atmosphere, the manufacture of such compounds has been phased out under the Montreal Protocol, they are being replaced with other products such as hydrofluorocarbons and R-134a. As in simpler alkanes, carbon in the CFCs bonds with tetrahedral symmetry; because the fluorine and chlorine atoms differ in size and effective charge from hydrogen and from each other, the methane-derived CFCs deviate from perfect tetrahedral symmetry. The physical properties of CFCs and HCFCs are tunable by changes in the number and identity of the halogen atoms. In general, they are volatile but less so than their parent alkanes; the decreased volatility is attributed to the molecular polarity induced by the halides, which induces intermolecular interactions.
Thus, methane boils at −161 °C whereas the fluoromethanes boil between −51.7 and −128 °C. The CFCs have still higher boiling points because the chloride is more polarizable than fluoride; because of their polarity, the CFCs are useful solvents, their boiling points make them suitable as refrigerants. The CFCs are far less flammable than methane, in part because they contain fewer C-H bonds and in part because, in the case of the chlorides and bromides, the released halides quench the free radicals that sustain flames; the densities of CFCs are higher than their corresponding alkanes. In general, the density of these compounds correlates with the number of chlorides. CFCs and HCFCs are produced by halogen exchange starting from chlorinated methanes and ethanes. Illustrative is the synthesis of chlorodifluoromethane from chloroform: HCCl3 + 2 HF → HCF2Cl + 2 HClThe brominated derivatives are generated by free-radical reactions of the chlorofluorocarbons, replacing C-H bonds with C-Br bonds; the production of the anesthetic 2-bromo-2-chloro-1,1,1-trifluoroethane is illustrative: CF3CH2Cl + Br2 → CF3CHBrCl + HBr The most important reaction of the CFCs is the photo-induced scission of a C-Cl bond: CCl3F → CCl2F.
+ Cl. The chlorine atom, written as Cl. behaves differently from the chlorine molecule. The radical Cl. is long-lived in the upper atmosphere, where it catalyzes the conversion of ozone into O2. Ozone absorbs UV-B radiation, so its depletion allows more of this high energy radiation to reach the Earth's surface. Bromine atoms are more efficient catalysts. CFCs and HCFCs are used in a variety of applications because of their low toxicity and flammability; every permutation of fluorine and hydrogen based on methane and ethane has been examined and most have been commercialized. Furthermore, many examples are known for higher numbers of carbon as well as related compounds containing bromine. Uses include refrigerants, blowing agents, propellants in medicinal applications and degreasing solvents. Billions of kilograms of chlorodifluoromethane are produced annually as precursor to tetrafluoroethylene, the monomer, converted into Teflon. Chlorofluorocarbons: when derived from methane and ethane these compounds have the formulae CClmF4−m and C2ClmF6−m, where m is nonzero.
Hydro-chlorofluorocarbons: when derived from methane and ethane these compounds have the formula CClmFnH4−m−n and C2ClxFyH6−x−y, where m, n, x, y are nonzero. and bromofluorocarbons have formulae similar to the CFCs and HCFCs but include bromine. Hydrofluorocarbons: when derived from methane, ethane and butane, these compounds have the respective formulae CFmH4−m, C2FmH6−m, C3FmH8−m, C4FmH10−m, where m is nonzero. A special numbering system is to be used for fluorinated alkanes, prefixed with Freon-, R-, CFC- and HCFC-, where the rightmost value indicates the number of fluorine atoms, the next value to the left is the number of hydrogen atoms plus 1, the next value to the left is the number of carbon atoms less one, the remaining atoms are chlorine. Freon-12, for example, indicates a methane derivative containing no hydrogen, it is therefore CCl2F2. Another equation that can be applied to get the correct molecular formula of the CFC/R/Freon class compounds is this to take the numbering and add 90 to it.
The resulting value will give the number of carbons as the first numeral, the second numeral gives the number of hydrogen atoms, the third numeral gives the number of fluorine atoms. The rest of the unaccounted carbon bonds are occupied by chlorine atoms; the value of this equation is always a three figure number. An easy example is that of CFC-12, which gives: 90+12=102 -> 1 carbon, 0 hydrogens, 2 fluorine atoms, hence 2 chlorine atoms resulting in CCl2F2. The main advantage of this method of deducing the molecular composition in comparison with the method described in the paragraph above is that it gives the number of carbon atoms of the molecule. Freons containing bromine are signified by four numbers. Isomers, which are common for ethane and propane derivatives, are indicated by letters following the numbers: Carbon tetrachloride was used in fire extinguishers and glass "anti-fire grenades" from the late nineteenth century until around the end of World War II. Experimentation with chloroalkanes for fire suppression on military aircraft began at least as early as the 1920s.