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Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H−, and is used when hydrogen forms a compound with a more electropositive element. The existence of the hydride anion, suggested by Gilbert N. Lewis in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 by the electrolysis of molten lithium hydride (LiH), producing a stoichiometry quantity of hydrogen at the anode. For hydrides other than group I and II metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is BeH 2, which is polymeric. In lithium aluminium hydride, the AlH− 4 anion carries hydridic centers firmly attached to the Al(III).
Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminium hydride. Binary indium hydride has not yet been identified, although larger complexes exist.
In inorganic chemistry, hydrides can also serve as bridging ligands that link two metal centers in a coordination complex. This function is particularly common in group 13 elements, especially in boranes (boron hydrides) and aluminium complexes, as well as in clustered carboranes.
Oxidation of hydrogen removes its electron and gives H+, which contains no electrons and a nucleus which is usually composed of one proton. That is why H+ is often called a proton. This species is central to discussion of acids. Under the Bronsted-Lowry theory, acids are proton donors, while bases are proton acceptors.
A bare proton, H+, cannot exist in solution or in ionic crystals, because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the electron clouds of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or cationic hydrogen attached to other species in this fashion, and as such is denoted "H+" without any implication that any single protons exist freely as a species.
To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "hydronium ion" (H 3O+). However, even in this case, such solvated hydrogen cations are more realistically conceived as being organized into clusters that form species closer to H 9O+ 4. Other oxonium ions are found when water is in acidic solution with other solvents.
Although exotic on Earth, one of the most common ions in the universe is the H+ 3 ion, known as protonated molecular hydrogen or the trihydrogen cation.
Hydrogen has three naturally occurring isotopes, denoted 1H, 2H and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature.
Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritium. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for phosphorus and thus is not available for protium. In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry allows any of D, T, 2H, and 3H to be used, although 2H and 3H are preferred.
In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas. In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "flammable air". He speculated that "flammable air" was in fact identical to the hypothetical substance called "phlogiston" and further finding in 1781 that the gas produces water when burned. He is usually given credit for its discovery as an element. In 1783, Antoine Lavoisier gave the element the name hydrogen (from the Greek ὑδρο- hydro meaning "water" and -γενής genes meaning "creator") when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.
Lavoisier produced hydrogen for his experiments on mass conservation by reacting a flux of steam with metallic iron through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. He produced solid hydrogen the next year. Deuterium was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck. Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932. François Isaac de Rivaz built the first de Rivaz engine, an internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819. The Döbereiner's lamp and limelight were invented in 1823.
The first hydrogen-filled balloon was invented by Jacques Charles in 1783. Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard. German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called Zeppelins; the first of which had its maiden flight in 1900. Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war.
The first non-stop transatlantic crossing was made by the British airship R34 in 1919. Regular passenger service resumed in the 1920s and the discovery of helium reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, H2 was used in the Hindenburg airship, which was destroyed in a midair fire over New Jersey on 6 May 1937. The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the aluminized fabric coating by static electricity. But the damage to hydrogen's reputation as a lifting gas was already done.
In the same year the first hydrogen-cooled turbogenerator went into service with gaseous hydrogen as a coolant in the rotor and the stator in 1937 at Dayton, Ohio, by the Dayton Power & Light Co.; because of the thermal conductivity of hydrogen gas, this is the most common type in its field today.
The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2). For example, the ISS, Mars Odyssey and the Mars Global Surveyor are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009, more than 19 years after launch, and 13 years over their design life.
Because of its simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure. Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H+ 2 allowed fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.
One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogen, half a century before full quantum mechanical theory arrived. Maxwell observed that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.
Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms (most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as dark matter and dark energy). This element is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction and the CNO cycle nuclear fusion.
Throughout the universe, hydrogen is mostly found in the atomic and plasma states whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. Hydrogen is found in the neutral atomic state in the interstellar medium. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the Universe up to redshift z=4.
Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H2. However, hydrogen gas is very rare in the Earth's atmosphere (1 ppm by volume) because of its light weight, which enables it to escape from Earth's gravity more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface, mostly in the form of chemical compounds such as hydrocarbons and water. Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, as is methane, itself a hydrogen source of increasing importance.
A molecular form called protonated molecular hydrogen (H+ 3) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from cosmic rays. This charged ion has also been observed in the upper atmosphere of the planet Jupiter. The ion is relatively stable in the environment of outer space due to the low temperature and density. H+ 3 is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium. Neutral triatomic hydrogen H3 can only exist in an excited form and is unstable. By contrast, the positive hydrogen molecular ion (H+ 2) is a rare molecule in the universe.
H 2 is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the hydrogenation of unsaturated substrates; and in nature as a means of expelling reducing equivalents in biochemical reactions.
The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is in the range 80–94%.
An alloy of aluminium and gallium in pellet form added to water can be used to generate hydrogen. The process also produces alumina, but the expensive gallium, which prevents the formation of an oxide skin on the pellets, can be re-used. This has important potential implications for a hydrogen economy, as hydrogen can be produced on-site and does not need to be transported.
Hydrogen can be prepared in several different ways, but economically the most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (1000–1400 K, 700–1100 °C or 1300–2000 °F), steam (water vapor) reacts with methane to yield carbon monoxide and H 2.
This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 MPa, 20 atm or 600 inHg). This is because high-pressure H 2 is the most marketable product and Pressure Swing Adsorption (PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and related compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:
Consequently, steam reforming typically employs an excess of H 2O. Additional hydrogen can be recovered from the steam by use of carbon monoxide through the water gas shift reaction, especially with an iron oxide catalyst. This reaction is also a common industrial source of carbon dioxide:
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia, hydrogen is generated from natural gas. Electrolysis of brine to yield chlorine also produces hydrogen as a co-product.
There are more than 200 thermochemical cycles which can be used for water splitting, around a dozen of these cycles such as the iron oxide cycle, cerium(IV) oxide–cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricity. A number of laboratories (including in France, Germany, Greece, Japan, and the USA) are developing thermochemical methods to produce hydrogen from solar energy and water.
Under anaerobic conditions, iron and steel alloys are slowly oxidized by the protons of water concomitantly reduced in molecular hydrogen (H 2). The anaerobic corrosion of iron leads first to the formation of ferrous hydroxide (green rust) and can be described by the following reaction:
In its turn, under anaerobic conditions, the ferrous hydroxide (Fe(OH) 2 ) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:
In the absence of atmospheric oxygen (O 2), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H 2) is produced during the process of serpentinization by the anaerobic oxidation by the water protons (H+) of the ferrous (Fe2+) silicate present in the crystal lattice of the fayalite (Fe 2SiO 4, the olivine iron-endmember). The corresponding reaction leading to the formation of magnetite (Fe 3O 4), quartz (SiO 2) and hydrogen (H 2) is the following:
From all the fault gases formed in power transformers, hydrogen is the most common and is generated under most fault conditions; thus, formation of hydrogen is an early indication of serious problems in the transformer's life cycle.
Large quantities of H 2 are needed in the petroleum and chemical industries. The largest application of H 2 is for the processing ("upgrading") of fossil fuels, and in the production of ammonia. The key consumers of H 2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocracking. H 2 has several other important uses. H 2 is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and oils (found in items such as margarine), and in the production of methanol. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H 2 is also used as a reducing agent of metallic ores.
Hydrogen is highly soluble in many rare earth and transition metals and is soluble in both nanocrystalline and amorphous metals. Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice. These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is a metallurgical problem, contributing to the embrittlement of many metals, complicating the design of pipelines and storage tanks.
Apart from its use as a reactant, H 2 has wide applications in physics and engineering. It is used as a shielding gas in welding methods such as atomic hydrogen welding. H2 is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas. Liquid H2 is used in cryogenic research, including superconductivity studies. Because H 2 is lighter than air, having a little more than 1⁄14 of the density of air, it was once widely used as a lifting gas in balloons and airships.
In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called forming gas) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries. Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.
Hydrogen's rarer isotopes also each have specific applications. Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions. Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects. Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs, as an isotopic label in the biosciences, and as a radiation source in luminous paints.
Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low density, low viscosity, and the highest specific heat and thermal conductivity of all gases.
Hydrogen is not an energy resource, except in the hypothetical context of commercial nuclear fusion power plants using deuterium or tritium, a technology presently far from development. The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth. Elemental hydrogen from solar, biological, or electrical sources require more energy to make it than is obtained by burning it, so in these cases hydrogen functions as an energy carrier, like a battery. Hydrogen may be obtained from fossil sources (such as methane), but these sources are unsustainable.
The energy density per unit volume of both liquid hydrogen and compressed hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher. Nevertheless, elemental hydrogen has been widely discussed in the context of energy, as a possible future carrier of energy on an economy-wide scale. For example, CO 2 sequestration followed by carbon capture and storage could be conducted at the point of H 2 production from fossil fuels. Hydrogen used in transportation would burn relatively cleanly, with some NOx emissions, but without carbon emissions. However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial. Fuel cells can convert hydrogen and oxygen directly to electricity more efficiently than internal combustion engines.
Hydrogen is employed to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon that helps stabilizing material properties. It is also a potential electron donor in various oxide materials, including ZnO, SnO2, CdO, MgO, ZrO2, HfO2, La2O3, Y2O3, TiO2, SrTiO3, LaAlO3, SiO2, Al2O3, ZrSiO4, HfSiO4, and SrZrO3.
H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water. The natural cycle of hydrogen production and consumption by organisms is called the hydrogen cycle.
Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast. Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen. Efforts have also been undertaken with genetically modified alga in a bioreactor.
Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form. In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids. Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement, leading to cracks and explosions. Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.
Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the parahydrogen/orthohydrogen ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given). Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.
A web browser (commonly referred to as a browser) is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier (URI/URL) and may be a web page, image, video or other piece of content. Hyperlinks present in resources enable users easily to navigate their browsers to related resources.
Although browsers are primarily intended to use the World Wide Web, they can also be used to access information provided by web servers in private networks or files in file systems.
The first web browser was invented in 1990 by Sir Tim Berners-Lee. Berners-Lee is the director of the World Wide Web Consortium (W3C), which oversees the Web's continued development, and is also the founder of the World Wide Web Foundation. His browser was called WorldWideWeb and later renamed Nexus.
In 1993, browser software was further innovated by Marc Andreessen with the release of Mosaic, "the world's first popular browser", which made the World Wide Web system easy to use and more accessible to the average person. Andreesen's browser sparked the internet boom of the 1990s. The introduction of Mosaic in 1993 – one of the first graphical web browsers – led to an explosion in web use. Andreessen, the leader of the Mosaic team at National Center for Supercomputing Applications (NCSA), soon started his own company, named Netscape, and released the Mosaic-influenced Netscape Navigator in 1994, which quickly became the world's most popular browser, accounting for 90% of all web use at its peak (see usage share of web browsers).
Microsoft responded with its Internet Explorer in 1995, also heavily influenced by Mosaic, initiating the industry's first browser war. Bundled with Windows, Internet Explorer gained dominance in the web browser market; Internet Explorer usage share peaked at over 95% by 2002.
Opera debuted in 1996; it has never achieved widespread use, having less than 2% browser usage share as of February 2012 according to Net Applications. Its Opera-mini version has an additive share, in April 2011 amounting to 1.1% of overall browser use, but focused on the fast-growing mobile phone web browser market, being preinstalled on over 40 million phones. It is also available on several other embedded systems, including Nintendo's Wii video game console.
In 1998, Netscape launched what was to become the Mozilla Foundation in an attempt to produce a competitive browser using the open source software model. That browser would eventually evolve into Firefox, which developed a respectable following while still in the beta stage of development; shortly after the release of Firefox 1.0 in late 2004, Firefox (all versions) accounted for 7% of browser use. As of August 2011, Firefox has a 28% usage share.
Apple's Safari had its first beta release in January 2003; as of April 2011, it had a dominant share of Apple-based web browsing, accounting for just over 7% of the entire browser market.
The most recent major entrant to the browser market is Chrome, first released in September 2008. Chrome's take-up has increased significantly year by year, by doubling its usage share from 8% to 16% by August 2011. This increase seems largely to be at the expense of Internet Explorer, whose share has tended to decrease from month to month. In December 2011, Chrome overtook Internet Explorer 8 as the most widely used web browser but still had lower usage than all versions of Internet Explorer combined. Chrome's user-base continued to grow and in May 2012, Chrome's usage passed the usage of all versions of Internet Explorer combined. By April 2014, Chrome's usage had hit 45%.
Internet Explorer, on the other hand, was bundled free with the Windows operating system (and was also downloadable free), and therefore it was funded partly by the sales of Windows to computer manufacturers and direct to users. Internet Explorer also used to be available for the Mac. It is likely that releasing IE for the Mac was part of Microsoft's overall strategy to fight threats to its quasi-monopoly platform dominance - threats such as web standards and Java - by making some web developers, or at least their managers, assume that there was "no need" to develop for anything other than Internet Explorer. In this respect, IE may have contributed to Windows and Microsoft applications sales in another way, through "lock-in" to Microsoft's browser.
In January 2009, the European Commission announced it would investigate the bundling of Internet Explorer with Windows operating systems from Microsoft, saying "Microsoft's tying of Internet Explorer to the Windows operating system harms competition between web browsers, undermines product innovation and ultimately reduces consumer choice." Microsoft Corp v Commission
Safari and Mobile Safari were likewise always included with OS X and iOS respectively, so, similarly, they were originally funded by sales of Apple computers and mobile devices, and formed part of the overall Apple experience to customers.
Today, most commercial web browsers are paid by search engine companies to make their engine default, or to include them as another option. For example, Google pays Mozilla, the maker of Firefox, to make Google Search the default search engine in Firefox. Mozilla makes enough money from this deal that it does not need to charge users for Firefox. In addition, Google Search is also (as one would expect) the default search engine in Google Chrome. Users searching for websites or items on the Internet would be led to Google's search results page, increasing ad revenue and which funds development at Google and of Google Chrome.
The primary purpose of a web browser is to bring information resources to the user ("retrieval" or "fetching"), allowing them to view the information ("display", "rendering"), and then access other information ("navigation", "following links").
This process begins when the user inputs a Uniform Resource Locator (URL), for example http://en.wikipedia.org/, into the browser. The prefix of the URL, the Uniform Resource Identifier or URI, determines how the URL will be interpreted. The most commonly used kind of URI starts with http: and identifies a resource to be retrieved over the Hypertext Transfer Protocol (HTTP). Many browsers also support a variety of other prefixes, such as https: for HTTPS, ftp: for the File Transfer Protocol, and file: for local files. Prefixes that the web browser cannot directly handle are often handed off to another application entirely. For example, mailto: URIs are usually passed to the user's default e-mail application, and news: URIs are passed to the user's default newsgroup reader.
In the case of http, https, file, and others, once the resource has been retrieved the web browser will display it. HTML and associated content (image files, formatting information such as CSS, etc.) is passed to the browser's layout engine to be transformed from markup to an interactive document, a process known as "rendering". Aside from HTML, web browsers can generally display any kind of content that can be part of a web page. Most browsers can display images, audio, video, and XML files, and often have plug-ins to support Flash applications and Java applets. Upon encountering a file of an unsupported type or a file that is set up to be downloaded rather than displayed, the browser prompts the user to save the file to disk.
Information resources may contain hyperlinks to other information resources. Each link contains the URI of a resource to go to. When a link is clicked, the browser navigates to the resource indicated by the link's target URI, and the process of bringing content to the user begins again.
Available web browsers range in features from minimal, text-based user interfaces with bare-bones support for HTML to rich user interfaces supporting a wide variety of file formats and protocols. Browsers which include additional components to support e-mail, Usenet news, and Internet Relay Chat (IRC), are sometimes referred to as "Internet suites" rather than merely "web browsers".
All major web browsers allow the user to open multiple information resources at the same time, either in different browser windows or in different tabs of the same window. Major browsers also include pop-up blockers to prevent unwanted windows from "popping up" without the user's consent.
A browser extension is a computer program that extends the functionality of a web browser. Every major web browser supports the development of browser extensions.
Most web browsers can display a list of web pages that the user has bookmarked so that the user can quickly return to them. Bookmarks are also called "Favorites" in Internet Explorer. In addition, all major web browsers have some form of built-in web feed aggregator. In Firefox, web feeds are formatted as "live bookmarks" and behave like a folder of bookmarks corresponding to recent entries in the feed. In Opera, a more traditional feed reader is included which stores and displays the contents of the feed.
Most browsers support HTTP Secure and offer quick and easy ways to delete the web cache, download history, form and search history, cookies, and browsing history. For a comparison of the current security vulnerabilities of browsers, see comparison of web browsers.
Early web browsers supported only a very simple version of HTML. The rapid development of proprietary web browsers led to the development of non-standard dialects of HTML, leading to problems with interoperability. Modern web browsers support a combination of standards-based and de facto HTML and XHTML, which should be rendered in the same way by all browsers.
Web browsers consist of a user interface, layout engine, rendering engine, JavaScript interpreter, UI backend, networking component and data persistence component. These components achieve different functionalities of a web browser and together provide all capabilities of a web browser.
Boston (pronounced i/ˈbɒstən/) is the capital and largest city of the Commonwealth of Massachusetts in the United States. Boston also served as the historic county seat of Suffolk County until Massachusetts disbanded county government in 1999. The city proper covers 48 square miles (124 km2) with an estimated population of 655,884 in 2014, making it the largest city in New England and the 24th largest city in the United States. The city is the economic and cultural anchor of a substantially larger metropolitan area called Greater Boston, home to 4.7 million people and the tenth-largest metropolitan statistical area in the country. Greater Boston as a commuting region is home to 8.1 million people, making it the sixth-largest combined statistical area in the United States.
One of the oldest cities in the United States, Boston was founded on the Shawmut Peninsula in 1630 by Puritan settlers from England. It was the scene of several key events of the American Revolution, such as the Boston Massacre, the Boston Tea Party, the Battle of Bunker Hill, and the Siege of Boston. Upon American independence from Great Britain, the city continued to be an important port and manufacturing hub, as well as a center for education and culture. Through land reclamation and municipal annexation, Boston has expanded beyond the original peninsula. Its rich history attracts many tourists, with Faneuil Hall alone drawing over 20 million visitors per year. Boston's many firsts include the United States' first public school, Boston Latin School (1635), and first subway system (1897).
The area's many colleges and universities make Boston an international center of higher education and medicine, and the city is considered to be a world leader in innovation. Boston's economic base also includes finance, professional and business services, biotechnology, information technology, and government activities. Households in the city claim the highest average rate of philanthropy in the United States; businesses and institutions rank amongst the top in the country for environmental sustainability and investment. The city has one of the highest costs of living in the United States, though it remains high on world livability rankings.
Boston's early European settlers had first called the area Trimountaine (after its "three mountains"—only traces of which remain today) but later renamed it Boston after Boston, Lincolnshire, England, the origin of several prominent colonists. The renaming, on September 7, 1630 (Old Style),[b] was by Puritan colonists from England, who had moved over from Charlestown earlier that year in quest of fresh water. Their settlement was initially limited to the Shawmut Peninsula, at that time surrounded by the Massachusetts Bay and Charles River and connected to the mainland by a narrow isthmus. The peninsula is known to have been inhabited as early as 5000 BC.
In 1629, the Massachusetts Bay Colony's first governor, John Winthrop, led the signing of the Cambridge Agreement, a key founding document of the city. Puritan ethics and their focus on education influenced its early history; America's first public school was founded in Boston in 1635. Over the next 130 years, the city participated in four French and Indian Wars, until the British defeated the French and their native allies in North America. Boston was the largest town in British North America until Philadelphia grew larger in the mid 18th century.
Many of the crucial events of the American Revolution—the Boston Massacre, the Boston Tea Party, Paul Revere's midnight ride, the battles of Lexington and Concord and Bunker Hill, the Siege of Boston, and many others—occurred in or near Boston. After the Revolution, Boston's long seafaring tradition helped make it one of the world's wealthiest international ports, with rum, fish, salt, and tobacco being particularly important.
The Embargo Act of 1807, adopted during the Napoleonic Wars, and the War of 1812 significantly curtailed Boston's harbor activity. Although foreign trade returned after these hostilities, Boston's merchants had found alternatives for their capital investments in the interim. Manufacturing became an important component of the city's economy, and by the mid-19th century, the city's industrial manufacturing overtook international trade in economic importance. Until the early 20th century, Boston remained one of the nation's largest manufacturing centers and was notable for its garment production and leather-goods industries. A network of small rivers bordering the city and connecting it to the surrounding region facilitated shipment of goods and led to a proliferation of mills and factories. Later, a dense network of railroads furthered the region's industry and commerce.
During this period Boston flourished culturally as well, admired for its rarefied literary life and generous artistic patronage, with members of old Boston families—eventually dubbed Boston Brahmins—coming to be regarded as the nation's social and cultural elites.
Boston was an early port of the Atlantic triangular slave trade in the New England colonies, but was soon overtaken by Salem, Massachusetts and Newport, Rhode Island. Eventually Boston became a center of the abolitionist movement. The city reacted strongly to the Fugitive Slave Law of 1850, contributing to President Franklin Pierce's attempt to make an example of Boston after the Anthony Burns Fugitive Slave Case.
In 1822, the citizens of Boston voted to change the official name from "the Town of Boston" to "the City of Boston", and on March 4, 1822, the people of Boston accepted the charter incorporating the City. At the time Boston was chartered as a city, the population was about 46,226, while the area of the city was only 4.7 square miles (12 km2).
In the 1820s, Boston's population grew rapidly, and the city's ethnic composition changed dramatically with the first wave of European immigrants. Irish immigrants dominated the first wave of newcomers during this period, especially following the Irish Potato Famine; by 1850, about 35,000 Irish lived in Boston. In the latter half of the 19th century, the city saw increasing numbers of Irish, Germans, Lebanese, Syrians, French Canadians, and Russian and Polish Jews settled in the city. By the end of the 19th century, Boston's core neighborhoods had become enclaves of ethnically distinct immigrants—Italians inhabited the North End, Irish dominated South Boston and Charlestown, and Russian Jews lived in the West End. Irish and Italian immigrants brought with them Roman Catholicism. Currently, Catholics make up Boston's largest religious community, and since the early 20th century, the Irish have played a major role in Boston politics—prominent figures include the Kennedys, Tip O'Neill, and John F. Fitzgerald.
Between 1631 and 1890, the city tripled its area through land reclamation by filling in marshes, mud flats, and gaps between wharves along the waterfront. The largest reclamation efforts took place during the 19th century; beginning in 1807, the crown of Beacon Hill was used to fill in a 50-acre (20 ha) mill pond that later became the Haymarket Square area. The present-day State House sits atop this lowered Beacon Hill. Reclamation projects in the middle of the century created significant parts of the South End, the West End, the Financial District, and Chinatown. After The Great Boston Fire of 1872, workers used building rubble as landfill along the downtown waterfront. During the mid-to-late 19th century, workers filled almost 600 acres (2.4 km2) of brackish Charles River marshlands west of Boston Common with gravel brought by rail from the hills of Needham Heights. The city annexed the adjacent towns of South Boston (1804), East Boston (1836), Roxbury (1868), Dorchester (including present day Mattapan and a portion of South Boston) (1870), Brighton (including present day Allston) (1874), West Roxbury (including present day Jamaica Plain and Roslindale) (1874), Charlestown (1874), and Hyde Park (1912). Other proposals, for the annexation of Brookline, Cambridge, and Chelsea, were unsuccessful.
By the early and mid-20th century, the city was in decline as factories became old and obsolete, and businesses moved out of the region for cheaper labor elsewhere. Boston responded by initiating various urban renewal projects under the direction of the Boston Redevelopment Authority (BRA), which was established in 1957. In 1958, BRA initiated a project to improve the historic West End neighborhood. Extensive demolition was met with vociferous public opposition.
The BRA subsequently reevaluated its approach to urban renewal in its future projects, including the construction of Government Center. In 1965, the first Community Health Center in the United States opened, the Columbia Point Health Center, in the Dorchester neighborhood. It mostly served the massive Columbia Point public housing complex adjoining it, which was built in 1953. The health center is still in operation and was rededicated in 1990 as the Geiger-Gibson Community Health Center. The Columbia Point complex itself was redeveloped and revitalized into a mixed-income community called Harbor Point Apartments from 1984 to 1990. By the 1970s, the city's economy boomed after 30 years of economic downturn. A large number of high rises were constructed in the Financial District and in Boston's Back Bay during this time period. This boom continued into the mid-1980s and later began again. Hospitals such as Massachusetts General Hospital, Beth Israel Deaconess Medical Center, and Brigham and Women's Hospital lead the nation in medical innovation and patient care. Schools such as Boston College, Boston University, the Harvard Medical School, Northeastern University, Wentworth Institute of Technology, Berklee College of Music and Boston Conservatory attract students to the area. Nevertheless, the city experienced conflict starting in 1974 over desegregation busing, which resulted in unrest and violence around public schools throughout the mid-1970s.
Boston is an intellectual, technological, and political center but has lost some important regional institutions, including the acquisition of The Boston Globe by The New York Times, and the loss to mergers and acquisitions of local financial institutions such as FleetBoston Financial, which was acquired by Charlotte-based Bank of America in 2004. Boston-based department stores Jordan Marsh and Filene's have both been merged into the Cincinnati–based Macy's. Boston has experienced gentrification in the latter half of the 20th century, with housing prices increasing sharply since the 1990s. Living expenses have risen, and Boston has one of the highest costs of living in the United States, and was ranked the 129th most expensive major city in the world in a 2011 survey of 214 cities. Despite cost of living issues, Boston ranks high on livability ratings, ranking 36th worldwide in quality of living in 2011 in a survey of 221 major cities.
On April 15, 2013, two Chechen Islamist brothers exploded two bombs near the finish line of the Boston Marathon, killing three people and injuring roughly 264.
Boston has an area of 89.6 square miles (232.1 km2)—48.4 square miles (125.4 km2) (54.0%) of land and 41.2 square miles (106.7 km2) (46.0%) of water. The city's official elevation, as measured at Logan International Airport, is 19 ft (5.8 m) above sea level. The highest point in Boston is Bellevue Hill at 330 feet (100 m) above sea level, and the lowest point is at sea level. Situated onshore of the Atlantic Ocean, Boston is the only state capital in the contiguous United States with an oceanic coastline.
Boston is surrounded by the "Greater Boston" region and is contiguously bordered by the cities and towns of Winthrop, Revere, Chelsea, Everett, Somerville, Cambridge, Newton, Brookline, Needham, Dedham, Canton, Milton, and Quincy. The Charles River separates Boston from Watertown and the majority of Cambridge, and the mass of Boston from its own Charlestown neighborhood. To the east lie Boston Harbor and the Boston Harbor Islands National Recreation Area (which includes part of the city's territory, specifically Calf Island, Gallops Island, Great Brewster Island, Green Island, Little Brewster Island, Little Calf Island, Long Island, Lovells Island, Middle Brewster Island, Nixes Mate, Outer Brewster Island, Rainsford Island, Shag Rocks, Spectacle Island, The Graves, and Thompson Island). The Neponset River forms the boundary between Boston's southern neighborhoods and the city of Quincy and the town of Milton. The Mystic River separates Charlestown from Chelsea and Everett, and Chelsea Creek and Boston Harbor separate East Boston from Boston proper.
Boston is sometimes called a "city of neighborhoods" because of the profusion of diverse subsections; the city government's Office of Neighborhood Services has officially designated 23 neighborhoods.
More than two-thirds of inner Boston's modern land area did not exist when the city was founded, but was created via the gradual filling in of the surrounding tidal areas over the centuries, notably with earth from the leveling or lowering of Boston's three original hills (the "Trimountain", after which Tremont Street is named), and with gravel brought by train from Needham to fill the Back Bay. Downtown and its immediate surroundings consists largely of low-rise (often Federal style and Greek Revival) masonry buildings, interspersed with modern highrises, notably in the Financial District, Government Center, and South Boston. Back Bay includes many prominent landmarks, such as the Boston Public Library, Christian Science Center, Copley Square, Newbury Street, and New England's two tallest buildings—the John Hancock Tower and the Prudential Center. Near the John Hancock Tower is the old John Hancock Building with its prominent illuminated beacon, the color of which forecasts the weather. Smaller commercial areas are interspersed among areas of single-family homes and wooden/brick multi-family row houses. The South End Historic District is the largest surviving contiguous Victorian-era neighborhood in the US. The geography of downtown and South Boston was particularly impacted by the Central Artery/Tunnel Project (known unofficially as the "Big Dig"), which allowed for the removal of the unsightly elevated Central Artery and the incorporation of new green spaces and open areas.
Boston has a continental climate with some maritime influence, and using the −3 °C (27 °F) coldest month (January) isotherm, the city lies within the transition zone from a humid subtropical climate (Köppen Cfa) to a humid continental climate (Köppen Dfa), although the suburbs north and west of the city are significantly colder in winter and solidly fall under the latter categorization; the city lies at the transition between USDA plant hardiness zones 6b (most of the city) and 7a (Downtown, South Boston, and East Boston neighborhoods). Summers are typically warm to hot, rainy, and humid, while winters oscillate between periods of cold rain and snow, with cold temperatures. Spring and fall are usually mild, with varying conditions dependent on wind direction and jet stream positioning. Prevailing wind patterns that blow offshore minimize the influence of the Atlantic Ocean. The hottest month is July, with a mean temperature of 73.4 °F (23.0 °C). The coldest month is January, with a mean of 29.0 °F (−1.7 °C). Periods exceeding 90 °F (32 °C) in summer and below freezing in winter are not uncommon but rarely extended, with about 13 and 25 days per year seeing each, respectively. The most recent sub-0 °F (−18 °C) reading occurring on February 14, 2016 when the temperature dipped down to −9 °F (−23 °C), the coldest reading since 1957. In addition, several decades may pass between 100 °F (38 °C) readings, with the most recent such occurrence on July 22, 2011 when the temperature reached 103 °F (39 °C). The city's average window for freezing temperatures is November 9 through April 5.[c] Official temperature records have ranged from −18 °F (−28 °C) on February 9, 1934, up to 104 °F (40 °C) on July 4, 1911; the record cold daily maximum is 2 °F (−17 °C) on December 30, 1917, while, conversely, the record warm daily minimum is 83 °F (28 °C) on August 2, 1975.
Boston's coastal location on the North Atlantic moderates its temperature, but makes the city very prone to Nor'easter weather systems that can produce much snow and rain. The city averages 43.8 inches (1,110 mm) of precipitation a year, with 43.8 inches (111 cm) of snowfall per season. Snowfall increases dramatically as one goes inland away from the city (especially north and west of the city)—away from the moderating influence of the ocean. Most snowfall occurs from December through March, as most years see no measurable snow in April and November, and snow is rare in May and October. There is also high year-to-year variability in snowfall; for instance, the winter of 2011–12 saw only 9.3 in (23.6 cm) of accumulating snow, but the previous winter, the corresponding figure was 81.0 in (2.06 m).[d]
Fog is fairly common, particularly in spring and early summer, and the occasional tropical storm or hurricane can threaten the region, especially in late summer and early autumn. Due to its situation along the North Atlantic, the city often receives sea breezes, especially in the late spring, when water temperatures are still quite cold and temperatures at the coast can be more than 20 °F (11 °C) colder than a few miles inland, sometimes dropping by that amount near midday. Thunderstorms occur from May to September, that are occasionally severe with large hail, damaging winds and heavy downpours. Although downtown Boston has never been struck by a violent tornado, the city itself has experienced many tornado warnings. Damaging storms are more common to areas north, west, and northwest of the city. Boston has a relatively sunny climate for a coastal city at its latitude, averaging over 2,600 hours of sunshine per annum.
In 2010, Boston was estimated to have 617,594 residents (a density of 12,200 persons/sq mile, or 4,700/km2) living in 272,481 housing units— a 5% population increase over 2000. The city is the third most densely populated large U.S. city of over half a million residents. Some 1.2 million persons may be within Boston's boundaries during work hours, and as many as 2 million during special events. This fluctuation of people is caused by hundreds of thousands of suburban residents who travel to the city for work, education, health care, and special events.
In the city, the population was spread out with 21.9% at age 19 and under, 14.3% from 20 to 24, 33.2% from 25 to 44, 20.4% from 45 to 64, and 10.1% who were 65 years of age or older. The median age was 30.8 years. For every 100 females, there were 92.0 males. For every 100 females age 18 and over, there were 89.9 males. There were 252,699 households, of which 20.4% had children under the age of 18 living in them, 25.5% were married couples living together, 16.3% had a female householder with no husband present, and 54.0% were non-families. 37.1% of all households were made up of individuals and 9.0% had someone living alone who was 65 years of age or older. The average household size was 2.26 and the average family size was 3.08.
The median household income in Boston was $51,739, while the median income for a family was $61,035. Full-time year-round male workers had a median income of $52,544 versus $46,540 for full-time year-round female workers. The per capita income for the city was $33,158. 21.4% of the population and 16.0% of families are below the poverty line. Of the total population, 28.8% of those under the age of 18 and 20.4% of those 65 and older were living below the poverty line.
In 1950, whites represented 94.7% of Boston's population. From the 1950s to the end of the 20th century, the proportion of non-Hispanic whites in the city declined; in 2000, non-Hispanic whites made up 49.5% of the city's population, making the city majority-minority for the first time. However, in recent years the city has experienced significant gentrification, in which affluent whites have moved into formerly non-white areas. In 2006, the US Census Bureau estimated that non-Hispanic whites again formed a slight majority. But as of 2010, in part due to the housing crash, as well as increased efforts to make more affordable housing more available, the minority population has rebounded. This may also have to do with an increased Latino population and more clarity surrounding US Census statistics, which indicate a Non-Hispanic White population of 47 percent (some reports give slightly lower figures).
People of Irish descent form the largest single ethnic group in the city, making up 15.8% of the population, followed by Italians, accounting for 8.3% of the population. People of West Indian and Caribbean ancestry are another sizable group, at 6.0%, about half of whom are of Haitian ancestry. Over 27,000 Chinese Americans made their home in Boston city proper in 2013, and the city hosts a growing Chinatown accommodating heavily traveled Chinese-owned bus lines to and from Chinatown, Manhattan. Some neighborhoods, such as Dorchester, have received an influx of people of Vietnamese ancestry in recent decades. Neighborhoods such as Jamaica Plain and Roslindale have experienced a growing number of Dominican Americans. The city and greater area also has a growing immigrant population of South Asians, including the tenth-largest Indian community in the country.
The city has a sizable Jewish population with an estimated 25,000 Jews within the city and 227,000 within the Boston metro area; the number of congregations in Boston is estimated at 22. The adjacent communities of Brookline and Newton are both approximately one-third Jewish.

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