Purple is a color intermediate between blue and red. It is similar to violet, but unlike violet, a spectral color with its own wavelength on the visible spectrum of light, purple is a secondary color made by combining red and blue; the complementary color of purple is yellow. According to surveys in Europe and North America, purple is the color most associated with rarity, magic and piety; when combined with pink, it is associated with eroticism and seduction. Purple was the color worn by Roman magistrates. In Japan, the color is traditionally associated with the Emperor and aristocracy. Purple is most favorited color preferences amongst women and girls, is symbolic of the feminist movement and women's empowerment; the word'purple' comes from the Old English word purpul which derives from Latin purpura, in turn from the Greek πορφύρα, name of the Tyrian purple dye manufactured in classical antiquity from a mucus secreted by the spiny dye-murex snail. The first recorded use of the word'purple' in the English language was in the year 975 AD.
In heraldry, the word purpure is used for purple. In the traditional color wheel used by painters and purple are both placed between red and blue. Purple occupies between crimson and violet. Violet is closer to blue, is less saturated than purple. While the two colors look similar, from the point of view of optics there are important differences. Violet is a spectral color – it occupies its own place at the end of the spectrum of light first identified by Isaac Newton in 1672, it has its own wavelength – whereas purple is a combination of two spectral colors and blue. There is no such thing as the "wavelength of purple light". See Line of purples. Monochromatic violet light cannot be produced by the red-green-blue color system, the method used to create colors on a television screen or computer display. However, the system is capable of approximating it due to the fact that the L-cone in the eye is uniquely sensitive to two different discontinuous regions in the visible spectrum – its primary region being the long wavelength light of the yellow-red region of the spectrum, a secondary smaller region overlapping with the S-cone in the shortest wavelength, violet part.
This means that when violet light strikes the eye, the S-cone should be stimulated and the L-cone stimulated weakly along with it. By lighting the red primary of the display weakly along with the blue primary, a similar pattern of sensitization can be achieved, creating an illusion, the sensation of short wavelength light using what is in fact mixed light of two longer wavelengths; the resulting color has the same hue as pure violet. One psychophysical difference between purple and violet is their appearance with an increase in luminance. Violet, as it brightens, looks more blue; the same effect does not happen with purple. This is the result of -- Brücke shift. While the scientific definitions of violet and purple are clear, the cultural definitions are more varied; the color known in antiquity as Tyrian purple ranged from crimson to a deep bluish-purple, depending upon how it was made. In France, purple is defined as "a dark red, inclined toward violet"; the color called purple by the French, contains more red and half the amount of blue of the color called purple in the United States and the U.
K. In German, this color is sometimes called Purpurrot to avoid confusion. Purple first appeared in prehistoric art during the Neolithic era; the artists of Pech Merle cave and other Neolithic sites in France used sticks of manganese and hematite powder to draw and paint animals and the outlines of their own hands on the walls of their caves. These works have been dated to between 16,000 and 25,000 BC; as early as the 15th century BC the citizens of Sidon and Tyre, two cities on the coast of Ancient Phoenicia, were producing purple dye from a sea snail called the spiny dye-murex. Clothing colored with the Tyrian dye was mentioned in both the Iliad of Homer and the Aeneid of Virgil; the deep, rich purple dye made from this snail became known as Tyrian purple. The process of making the dye was long and expensive. Thousands of the tiny snails had to be found, their shells cracked. Mountains of empty shells have been found at the ancient sites of Tyre; the snails were left to soak a tiny gland was removed and the juice extracted and put in a basin, placed in the sunlight.
There a remarkable transformation took place. In the sunlight the juice turned white yellow-green green violet a red which turned darker and darker; the process had to be stopped at the right time to obtain the desired color, which could range from a bright crimson to a dark purple, the color of dried blood. Either wool, linen or silk would be dyed; the exact hue varied between crimson and violet, but it was always rich and lasting. Tyrian purple became the color of kings, nobles and magistrates all around the Mediterranean, it was mentioned in the Old Testament. Th
United States Environmental Protection Agency
The Environmental Protection Agency is an independent agency of the United States federal government for environmental protection. President Richard Nixon proposed the establishment of EPA on July 9, 1970 and it began operation on December 2, 1970, after Nixon signed an executive order; the order establishing the EPA was ratified by committee hearings in the Senate. The agency is led by its Administrator, appointed by the President and approved by Congress; the current Administrator is former Deputy Administrator Andrew R. Wheeler, acting administrator since July 2018; the EPA is not a Cabinet department, but the Administrator is given cabinet rank. The EPA has its headquarters in Washington, D. C. regional offices for each of the agency's ten regions, 27 laboratories. The agency conducts environmental assessment and education, it has the responsibility of maintaining and enforcing national standards under a variety of environmental laws, in consultation with state and local governments. It delegates some permitting and enforcement responsibility to U.
S. states and the federally recognized tribes. EPA enforcement powers include fines and other measures; the agency works with industries and all levels of government in a wide variety of voluntary pollution prevention programs and energy conservation efforts. In 2018, the agency had 14,172 full-time employees. More than half of EPA's employees are engineers and environmental protection specialists; the Environmental Protection Agency can only act under statutes, which are the authority of laws passed by Congress. Congress must approve the statute and they have the power to authorize or prohibit certain actions, which the EPA has to implement and enforce. Appropriations statutes authorize how much money the agency can spend each year to carry out the approved statutes; the Environmental Protection Agency has the power to issue regulations. A regulation is a standard or rule written by the agency to interpret the statute, apply it in situations and enforce it. Congress allows the EPA to write regulations in order to solve a problem, but the agency must include a rationale of why the regulations need to be implemented.
The regulations can be challenged by the Courts, where the regulation is confirmed. Many public health and environmental groups advocate for the agency and believe that it is creating a better world. Other critics believe that the agency commits government overreach by adding unnecessary regulations on business and property owners. Beginning in the late 1950s and through the 1960s, Congress reacted to increasing public concern about the impact that human activity could have on the environment. Senator James E. Murray introduced a bill, the Resources and Conservation Act of 1959, in the 86th Congress; the 1962 publication of Silent Spring by Rachel Carson alerted the public about the detrimental effects on the environment of the indiscriminate use of pesticides. In the years following, similar bills were introduced and hearings were held to discuss the state of the environment and Congress's potential responses. In 1968, a joint House–Senate colloquium was convened by the chairmen of the Senate Committee on Interior and Insular Affairs, Senator Henry M. Jackson, the House Committee on Science and Astronautics, Representative George P. Miller, to discuss the need for and means of implementing a national environmental policy.
In the colloquium, some members of Congress expressed a continuing concern over federal agency actions affecting the environment. The National Environmental Policy Act of 1969 was modeled on the Resources and Conservation Act of 1959. RCA would have established a Council on Environmental Quality in the office of the President, declared a national environmental policy, required the preparation of an annual environmental report. President Nixon signed NEPA into law on January 1, 1970; the law created the Council on Environmental Quality in the Executive Office of the President. NEPA required that a detailed statement of environmental impacts be prepared for all major federal actions affecting the environment; the "detailed statement" would be referred to as an environmental impact statement. On July 9, 1970, Nixon proposed an executive reorganization that consolidated many environmental responsibilities of the federal government under one agency, a new Environmental Protection Agency; this proposal included merging antipollution programs from a number of departments, such as the combination of pesticide programs from the United States Department of Agriculture, Department of Interior, U.
S. Department of Interior. After conducting hearings during that summer, the House and Senate approved the proposal; the EPA was created 90 days before it had to operate, opened its doors on December 2, 1970. The agency's first Administrator, William Ruckelshaus, took the oath of office on December 4, 1970. In its first year, the EPA had 5,800 employees. At its start, the EPA was a technical assistance agency that set goals and standards. Soon, new acts and amendments passed by Congress gave the agency its regulatory authority. EPA staff recall that in the early days there was "an enormous sense of purpose and excitement" and the expectation that "there was this agency, going to do something about a problem, on the minds of a lot of people in this country," leading to tens of thousands of resumes from those eager to participate in the mighty effort to clean up America's environment; when EPA first began operation, members of the private sector felt that the environ
Skin is the soft outer tissue covering of vertebrates with three main functions: protection and sensation. Other animal coverings, such as the arthropod exoskeleton, have different developmental origin and chemical composition; the adjective cutaneous means "of the skin". In mammals, the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue, guards the underlying muscles, bones and internal organs. Skin of a different nature exists in amphibians and birds. All mammals have some hair on their skin marine mammals like whales and porpoises which appear to be hairless; the skin is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss, its other functions are insulation, temperature regulation and the production of vitamin D folates. Damaged skin may heal by forming scar tissue; this is sometimes depigmented. The thickness of skin varies from location to location on an organism.
In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is the thickest skin on the body; the speed and quality of wound healing in skin is promoted by the reception of estrogen. Fur is dense hair. Fur augments the insulation the skin provides but can serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is hard and thick, can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier regarding the passage of chemicals via skin and is subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.
Mammalian skin is composed of two primary layers: the epidermis, which provides waterproofing and serves as a barrier to infection. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing pathogens from entering, is a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes. Keratinocytes are the major cells, constituting 95% of the epidermis, while Merkel cells and Langerhans cells are present; the epidermis can be further subdivided into the following strata or layers: Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Stratum germinativum Keratinocytes in the stratum basale proliferate through mitosis and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to become anucleated. During that process, keratinocytes will become organized, forming cellular junctions between each other and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.
Keratinocytes from the stratum corneum are shed from the surface. The epidermis contains no blood vessels, cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis; the epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, made through the action of both tissues. The basement membrane controls the traffic of the cells and molecules between the dermis and epidermis but serves, through the binding of a variety of cytokines and growth factors, as a reservoir for their controlled release during physiological remodeling or repair processes; the dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils and elastic fibers, embedded in hyaluronan and proteoglycans. Skin proteoglycans are varied and have specific locations.
For example, hyaluronan and decorin are present throughout the dermis and epidermis extracellular matrix, whereas biglycan and perlecan are only found in the epidermis. It harbors many mechanoreceptors that provide the sense of touch and heat through nociceptors and thermoreceptors, it contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as for the epidermis; the dermis is connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, a deep thicker area known as the reticular region. The papillary region is composed of loose areolar connective tissue; this is named for its fingerlike projections called papillae. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the tw
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth; the human eye can differentiate between about 10 million colors and is capable of detecting a single photon. Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil and suppression of the hormone melatonin and entrainment of the body clock; the eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea and lens; the cornea is transparent and more curved, is linked to the larger posterior segment, composed of the vitreous, retina and the outer white shell called the sclera.
The cornea is about 11.5 mm in diameter, 1/2 mm in thickness near its center. The posterior chamber constitutes the remaining five-sixths; the cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black; the size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles. Light energy enters the eye through the cornea, through the pupil and through the lens; the lens shape is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. Dimensions differ among adults by only one or two millimetres, remarkably consistent across different ethnicities; the vertical measure less than the horizontal, is about 24 mm. The transverse size of a human adult eye is 24.2 mm and the sagittal size is 23.7 mm with no significant difference between sexes and age groups.
Strong correlation has been found between the width of the orbit. The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres, a mass of 7.5 grams.. The eyeball grows increasing from about 16–17 millimetres at birth to 22.5–23 mm by three years of age. By age 12, the eye attains its full size; the eye is made up of layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the sclera; the middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid as well as the retinal vessels; the spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid, contained in two areas: the anterior chamber between the cornea and the iris, the posterior chamber between the iris and the lens.
The lens is suspended to the ciliary body by the suspensory ligament, made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation. The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition; the approximate field of view of an individual human eye varies by facial anatomy, but is 30° superior, 45° nasal, 70° inferior, 100° temporal. For both eyes combined visual field is 200 ° horizontal, it is 13700 square degrees for binocular vision. When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle. About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, 7.5° high and 5.5° wide. The retina has a static contrast ratio of around 100:1; as soon as the eye moves to acquire a target, it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil.
Initial dark adaptation takes place in four seconds of profound, uninterrupted darkness. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; the human eye can detect a luminance range of 1014, or one hundred trillion, from 10−6 cd/m2, or one millionth of a candela per square meter to 108 cd/m2 or one hundred million candelas per square meter. This range does not include looking at the midday lightning discharge. At the low end o
The oligodynamic effect is a biocidal effect of metals heavy metals, that occurs in low concentrations. The effect was discovered by Karl Wilhelm von Nägeli. Brass silverware both exhibit this effect to an extent; the metals react with thiol or amine groups of proteins, a mode of action to which microorganisms may develop resistance. Such resistance may be transmitted by plasmids. Aluminium acetate is used as an astringent mild antiseptic. Aluminium-based antiperspirant ingredients such as aluminium chlorohydrate, activated aluminium chlorohydrates, aluminium-zirconium-glycine complexes work by forming superficial plugs in the sweat ducts, reducing the flow of perspiration. Orthoesters of diarylstibinic acids are fungicides and bactericides, used in paints and fibers. Trivalent organic antimony was used in therapy for schistosomiasis. For many decades, arsenic was used medicinally to treat syphilis, it is still used in sheep dips, rat poisons, wood preservatives, weed killers, other pesticides. Arsenic is still used for murder by poisoning, for which use it has a long and continuing history in both literature and fact.
Barium polysulfide is a acaricide used in fruit and grape growing. Bismuth compounds have been used because of their astringent, antiphlogistic and disinfecting actions. In dermatology bismuth subgallate is still used in vulnerary salves and powders as well as in antimycotics. In the past, bismuth has been used to treat syphilis and malaria. Boric acid esters derived from glycols are being used for the control of microorganisms in fuel systems containing water. Brass vessels release a small amount of copper ions into stored water, thus killing fecal bacterial counts as high as 1 million bacteria per milliliter. Copper sulfate mixed with lime is used as a antihelminthic. Copper sulfate is used chiefly to destroy green algae that grow in reservoirs, stock ponds, swimming pools, fish tanks. Copper 8-hydroxyquinoline is sometimes included in paint to prevent mildew. Paint containing copper is used on boat bottoms to prevent barnacle growth. Gold inhibits the growth of bacteria. Physicians prescribed various forms of lead to heal ailments ranging from constipation to infectious diseases such as the plague.
Lead was used to preserve or sweeten wine. Lead arsenate is used in herbicides; some organic lead compounds are used as industrial biocides: thiomethyl triphenyllead is used as an antifungal agent, cotton preservative, lubricant additive. Phenylmercuric borate and acetate were used for disinfecting mucous membranes at an effective concentration of 0.07% in aqueous solutions. Due to toxicological and ecotoxicological reasons phenylmercury salts are no longer in use. However, some surgeons use mercurochrome despite toxicological objections. Dental amalgam used in fillings inhibits bacterial reproduction. Organic mercury compounds have been used as topical disinfectants and preservatives in medical preparations and grain products. Mercury was used in the treatment of syphilis. Calomel was used in infant teething powders in the 1930s and 1940s. Mercurials are used agriculturally as insecticides and fungicides; the toxicity of nickel to bacteria and fungi differs considerably. The metabolism of bacteria is adversely affected by silver ions at concentrations of 0.01–0.1 mg/L.
Therefore less soluble silver compounds, such as silver chloride act as bactericides or germicides, but not the much less soluble silver sulfide. In the presence of atmospheric oxygen, metallic silver has a bactericidal effect due to the formation of silver oxide, soluble enough to cause it. Bactericidal concentrations are reduced by adding colloidal silver, which has a high surface area. Objects with a solid silver surface have a bactericidal effect. Silver drinking vessels were carried by military commanders on expeditions for protection against disease, it was once common to place silver foil or silver coins on wounds for the same reason. Silver sulfadiazine is used as an antiseptic ointment for extensive burns. An equilibrium dispersion of colloidal silver with dissolved silver ions can be used to purify drinking water at sea. Silver is incorporated into medical devices such as catheters. Surfacine is a new antimicrobial for application to surfaces. Silver-impregnated wound dressings have proven useful against antibiotic-resistant bacteria.
Silver nitrate is used as a hemostatic and astringent. At one time, many states required that the eyes of newborns be treated with a few drops of silver nitrate to guard against an infection of the eyes called gonorrheal neonatal ophthalmia, which the infants might have contracted as they passed through the birth canal. Silver ions are incorporated into many hard surfaces, such as plastics and steel, as a way to control microbial growth on items such as toilet seats and refrigerator doors. Among the newer products being sold are plastic food containers infused with silver nanoparticies, which are intended to keep food fresher, silver-infused athletic shirts and socks, which claim to minimize odors. Thallium compounds such as thallium
Grey or gray is an intermediate color between black and white. It is a neutral color or achromatic color, meaning that it is a color "without color," because it can be composed of black and white, it is the color of ash and of lead. The first recorded use of grey as a color name in the English language was in AD 700. Grey is the dominant spelling in European and Commonwealth English, although gray remained in common usage in the UK until the second half of the 20th century. Gray has been the preferred American spelling since 1825, although grey is an accepted variant. In Europe and North America, surveys show that grey is the color most associated with neutrality, boredom, old age and modesty. Only one percent of respondents chose it as their favorite color preferences. Grey comes from the Middle English grai or grei, from the Anglo-Saxon graeg, is related to the Dutch grauw and grijs and German grau; the first recorded use of grey as a color name in the English language was in AD 700. In antiquity and the Middle Ages, grey was the color of undyed wool, thus was the color most worn by peasants and the poor.
It was the color worn by Cistercian monks and friars of the Franciscan and Capuchin orders as a symbol of their vows of humility and poverty. Franciscan friars in England and Scotland were known as the grey friars, that name is now attached to many places in Great Britain. During the Renaissance and the Baroque, grey began to play an important role in art. Black became the most popular color of the nobility in Italy and Spain, grey and white were harmonious with it. Grey was frequently used for the drawing of oil paintings, a technique called grisaille; the painting would first be composed in grey and white, the colors, made with thin transparent glazes, would be added on top. The grisaille beneath would provide the shading, visible through the layers of color. Sometimes the grisaille was left uncovered, giving the appearance of carved stone. Grey was a good background color for gold and for skin tones, it became the most common background for the portraits of Rembrandt Van Rijn and for many of the paintings of El Greco, who used it to highlight the faces and costumes of the central figures.
The palette of Rembrandt was composed entirely of somber colors. He composed his warm greys out of black pigments made from charcoal or burnt animal bones, mixed with lead white or a white made of lime, which he warmed with a little red lake color from cochineal or madder. In one painting, the portrait of Margaretha de Geer, one part of a grey wall in the background is painted with a layer of dark brown over a layer of orange and yellow earths, mixed with ivory black and some lead white. Over this he put an additional layer of glaze made of mixture of blue smalt, red ochre, yellow lake. Using these ingredients and many others, he made greys which had, according to art historian Philip Ball, "an incredible subtlety of pigmentation." The warm and rich greys and browns served to emphasize the golden light on the faces in the paintings. Grey became a fashionable color in the 18th century, both for women's dresses and for men's waistcoats and coats, it looked luminous coloring the silk and satin fabrics worn by the nobility and wealthy.
Women's fashion in the 19th century was dominated by Paris. The grey business suit appeared in the mid-19th century in London; the clothing of women working in the factories and workshops of Paris in the 19th century was grey. This gave them the name of grisettes. "Gris" or grey meant drunk, the name "grisette" was given to the lower class of Parisian prostitutes. Grey became a common color for military uniforms. Grey was the color of the uniforms of the Confederate Army during the American Civil War, of the Prussian Army for active service wear from 1910 onwards. Several artists of the mid-19th century used different tones of grey to create memorable paintings. Whistler's arrangement of different tones of grey had an effect on the world of music, on the French composer Claude Debussy. In 1894, Debussy wrote to violinist Eugène Ysaÿe describing his Nocturnes as "an experiment in the different combinations that can be obtained from one color – what a study in grey would be in painting." In the late 1930s, grey became a symbol of war.
It was the dominant color of Pablo Picasso's celebrated painting about the horrors of the Spanish Civil War, Guernica. After the war, the grey business suit became a metaphor for uniformity of thought, popularized in such books as The Man in the Gray Flannel Suit, which became a successful film in 1956; the whiteness or darkness of clouds is a function of their depth. Small, fluffy white clouds in summer look white because the sunlight is being scattered by the tiny water droplets they contain, that white light comes to the viewer's eye. However, as clouds become larger and thicker, the white light cannot penetrate through the cloud, is reflected off the top. Clouds look darkest grey during thunderstorms, when they can be as much as 20,000 to 30,000 feet high. Stratif
Silver is a chemical element with symbol Ag and atomic number 47. A soft, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, reflectivity of any metal; the metal is found in the Earth's crust in the pure, free elemental form, as an alloy with gold and other metals, in minerals such as argentite and chlorargyrite. Most silver is produced as a byproduct of copper, gold and zinc refining. Silver has long been valued as a precious metal. Silver metal is used in many bullion coins, sometimes alongside gold: while it is more abundant than gold, it is much less abundant as a native metal, its purity is measured on a per-mille basis. As one of the seven metals of antiquity, silver has had an enduring role in most human cultures. Other than in currency and as an investment medium, silver is used in solar panels, water filtration, ornaments, high-value tableware and utensils, in electrical contacts and conductors, in specialized mirrors, window coatings, in catalysis of chemical reactions, as a colorant in stained glass and in specialised confectionery.
Its compounds are used in X-ray film. Dilute solutions of silver nitrate and other silver compounds are used as disinfectants and microbiocides, added to bandages and wound-dressings and other medical instruments. Silver is similar in its physical and chemical properties to its two vertical neighbours in group 11 of the periodic table and gold, its 47 electrons are arranged in the configuration 4d105s1 to copper and gold. This distinctive electron configuration, with a single electron in the highest occupied s subshell over a filled d subshell, accounts for many of the singular properties of metallic silver. Silver is an soft and malleable transition metal, though it is less malleable than gold. Silver crystallizes in a face-centered cubic lattice with bulk coordination number 12, where only the single 5s electron is delocalized to copper and gold. Unlike metals with incomplete d-shells, metallic bonds in silver are lacking a covalent character and are weak; this observation explains the low high ductility of single crystals of silver.
Silver has a brilliant white metallic luster that can take a high polish, and, so characteristic that the name of the metal itself has become a colour name. Unlike copper and gold, the energy required to excite an electron from the filled d band to the s-p conduction band in silver is large enough that it no longer corresponds to absorption in the visible region of the spectrum, but rather in the ultraviolet. Protected silver has greater optical reflectivity than aluminium at all wavelengths longer than ~450 nm. At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm. High electrical and thermal conductivity is common to the elements in group 11, because their single s electron is free and does not interact with the filled d subshell, as such interactions lower electron mobility; the electrical conductivity of silver is the greatest of all metals, greater than copper, but it is not used for this property because of the higher cost.
An exception is in radio-frequency engineering at VHF and higher frequencies where silver plating improves electrical conductivity because those currents tend to flow on the surface of conductors rather than through the interior. During World War II in the US, 13540 tons of silver were used in electromagnets for enriching uranium because of the wartime shortage of copper. Pure silver has the highest thermal conductivity of any metal, although the conductivity of carbon and superfluid helium-4 are higher. Silver has the lowest contact resistance of any metal. Silver forms alloys with copper and gold, as well as zinc. Zinc-silver alloys with low zinc concentration may be considered as face-centred cubic solid solutions of zinc in silver, as the structure of the silver is unchanged while the electron concentration rises as more zinc is added. Increasing the electron concentration further leads to body-centred cubic, complex cubic, hexagonal close-packed phases. Occurring silver is composed of two stable isotopes, 107Ag and 109Ag, with 107Ag being more abundant.
This equal abundance is rare in the periodic table. The atomic weight is 107.8682 u. Both isotopes of silver are produced in stars via the s-process, as well as in supernovas via the r-process. Twenty-eight radioisotopes have been characterized, the most stable being 105Ag with a half-life of 41.29 days, 111Ag with a half-life of 7.45 days, 112Ag with a half-life of 3.13 hours. Silver has numerous nuclear isomers, the most stable being 108mAg, 110mAg and 106mAg. All of the remaining radioactive isotopes have half-lives of less than an hour, the majority of these have half-lives of less than three minutes. Isotopes of silver range in relative atomic mass from 92.950 u