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Wednesday, July 27, 2022

Alchemy

From Wikipedia, the free encyclopedia

Depiction of an Ouroboros from the alchemical treatise Aurora consurgens (15th century), Zentralbibliothek Zürich, Switzerland

Alchemy (from Arabic: al-kīmiyā; from Ancient Greek: khumeía) is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practiced in China, India, the Muslim world, and Europe. In its Western form, alchemy is first attested in a number of pseudepigraphical texts written in Greco-Roman Egypt during the first few centuries AD.

Alchemists attempted to purify, mature, and perfect certain materials. Common aims were chrysopoeia, the transmutation of "base metals" (e.g., lead) into "noble metals" (particularly gold); the creation of an elixir of immortality; and the creation of panaceas able to cure any disease. The perfection of the human body and soul was thought to result from the alchemical magnum opus ("Great Work"). The concept of creating the philosophers' stone was variously connected with all of these projects.

Islamic and European alchemists developed a basic set of laboratory techniques, theories, and terms, some of which are still in use today. They did not abandon the Ancient Greek philosophical idea that everything is composed of four elements, and they tended to guard their work in secrecy, often making use of cyphers and cryptic symbolism. In Europe, the 12th-century translations of medieval Islamic works on science and the rediscovery of Aristotelian philosophy gave birth to a flourishing tradition of Latin alchemy. This late medieval tradition of alchemy would go on to play a significant role in the development of early modern science (particularly chemistry and medicine).

Modern discussions of alchemy are generally split into an examination of its exoteric practical applications and its esoteric spiritual aspects, despite criticisms by scholars such as Eric J. Holmyard and Marie-Louise von Franz that they should be understood as complementary. The former is pursued by historians of the physical sciences, who examine the subject in terms of early chemistry, medicine, and charlatanism, and the philosophical and religious contexts in which these events occurred. The latter interests historians of esotericism, psychologists, and some philosophers and spiritualists. The subject has also made an ongoing impact on literature and the arts.

Etymology

The word alchemy comes from old French alquemie, alkimie, used in Medieval Latin as alchymia. This name was itself adopted from the Arabic word al-kīmiyā (الكيمياء). The Arabic al-kīmiyā in turn was a borrowing of the Late Greek term khēmeía (χημεία), also spelled khumeia (χυμεία) and khēmía (χημία), with al- being the Arabic definite article 'the'. Together this association can be interpreted as 'the process of transmutation by which to fuse or reunite with the divine or original form'. Several etymologies have been proposed for the Greek term. The first was proposed by Zosimos of Panopolis (3rd–4th centuries), who derived it from the name of a book, the Khemeu. Hermanm Diels argued in 1914 that it rather derived from χύμα, used to describe metallic objects formed by casting.

Others trace its roots to the Egyptian name kēme (hieroglyphic 𓆎𓅓𓏏𓊖 khmi ), meaning 'black earth', which refers to the fertile and auriferous soil of the Nile valley, as opposed to red desert sand. According to the Egyptologist Wallis Budge, the Arabic word al-kīmiyaʾ actually means "the Egyptian [science]", borrowing from the Coptic word for "Egypt", kēme (or its equivalent in the Mediaeval Bohairic dialect of Coptic, khēme). This Coptic word derives from Demotic kmỉ, itself from ancient Egyptian kmt. The ancient Egyptian word referred to both the country and the colour "black" (Egypt was the "black Land", by contrast with the "red Land", the surrounding desert); so this etymology could also explain the nickname "Egyptian black arts".

History

Alchemy encompasses several philosophical traditions spanning some four millennia and three continents. These traditions' general penchant for cryptic and symbolic language makes it hard to trace their mutual influences and "genetic" relationships. One can distinguish at least three major strands, which appear to be mostly independent, at least in their earlier stages: Chinese alchemy, centered in China; Indian alchemy, centered on the Indian subcontinent; and Western alchemy, which occurred around the Mediterranean and whose center has shifted over the millennia from Greco-Roman Egypt to the Islamic world, and finally medieval Europe. Chinese alchemy was closely connected to Taoism and Indian alchemy with the Dharmic faiths. In contrast, Western alchemy developed its philosophical system mostly independent of but influenced by various Western religions. It is still an open question whether these three strands share a common origin, or to what extent they influenced each other.

Hellenistic Egypt

Ambix, cucurbit and retort of Zosimos, from Marcelin Berthelot, Collection des anciens alchimistes grecs (3 vol., Paris, 1887–1888).

The start of Western alchemy may generally be traced to ancient and Hellenistic Egypt, where the city of Alexandria was a center of alchemical knowledge, and retained its pre-eminence through most of the Greek and Roman periods. Following the work of André-Jean Festugière, modern scholars see alchemical practice in the Roman Empire as originating from the Egyptian goldsmith's art, Greek philosophy and different religious traditions. Tracing the origins of the alchemical art in Egypt is complicated by the pseudepigraphic nature of texts from the Greek alchemical corpus. The treatises of Zosimos of Panopolis, the earliest historically attested author (fl. c. 300 CE), can help in situating the other authors. Zosimus based his work on that of older alchemical authors, such as Mary the Jewess, Pseudo-Democritus, and Agathodaimon, but very little is known about any of these authors. The most complete of their works, The Four Books of Pseudo-Democritus, were probably written in the first century AD.

Recent scholarship tends to emphasize the testimony of Zosimus, who traced the alchemical arts back to Egyptian metallurgical and ceremonial practices. It has also been argued that early alchemical writers borrowed the vocabulary of Greek philosophical schools but did not implement any of its doctrines in a systematic way. Zosimos of Panopolis wrote in the Final Abstinence (also known as the "Final Count"). Zosimos explains that the ancient practice of "tinctures" (the technical Greek name for the alchemical arts) had been taken over by certain "demons" who taught the art only to those who offered them sacrifices. Since Zosimos also called the demons "the guardians of places" (οἱ κατὰ τόπον ἔφοροι, hoi katà tópon éphoroi) and those who offered them sacrifices "priests" (ἱερέα, hieréa), it is fairly clear that he was referring to the gods of Egypt and their priests. While critical of the kind of alchemy he associated with the Egyptian priests and their followers, Zosimos nonetheless saw the tradition's recent past as rooted in the rites of the Egyptian temples.

Mythology – Zosimos of Panopolis asserted that alchemy dated back to Pharaonic Egypt where it was the domain of the priestly class, though there is little to no evidence for his assertion. Alchemical writers used Classical figures from Greek, Roman, and Egyptian mythology to illuminate their works and allegorize alchemical transmutation. These included the pantheon of gods related to the Classical planets, Isis, Osiris, Jason, and many others.

The central figure in the mythology of alchemy is Hermes Trismegistus (or Thrice-Great Hermes). His name is derived from the god Thoth and his Greek counterpart Hermes. Hermes and his caduceus or serpent-staff, were among alchemy's principal symbols. According to Clement of Alexandria, he wrote what were called the "forty-two books of Hermes", covering all fields of knowledge. The Hermetica of Thrice-Great Hermes is generally understood to form the basis for Western alchemical philosophy and practice, called the hermetic philosophy by its early practitioners. These writings were collected in the first centuries of the common era.

Technology – The dawn of Western alchemy is sometimes associated with that of metallurgy, extending back to 3500 BC. Many writings were lost when the Roman emperor Diocletian ordered the burning of alchemical books after suppressing a revolt in Alexandria (AD 292). Few original Egyptian documents on alchemy have survived, most notable among them the Stockholm papyrus and the Leyden papyrus X. Dating from AD 250–300, they contained recipes for dyeing and making artificial gemstones, cleaning and fabricating pearls, and manufacturing of imitation gold and silver. These writings lack the mystical, philosophical elements of alchemy, but do contain the works of Bolus of Mendes (or Pseudo-Democritus), which aligned these recipes with theoretical knowledge of astrology and the classical elements. Between the time of Bolus and Zosimos, the change took place that transformed this metallurgy into a Hermetic art.

Philosophy – Alexandria acted as a melting pot for philosophies of Pythagoreanism, Platonism, Stoicism and Gnosticism which formed the origin of alchemy's character. An important example of alchemy's roots in Greek philosophy, originated by Empedocles and developed by Aristotle, was that all things in the universe were formed from only four elements: earth, air, water, and fire. According to Aristotle, each element had a sphere to which it belonged and to which it would return if left undisturbed. The four elements of the Greek were mostly qualitative aspects of matter, not quantitative, as our modern elements are; "...True alchemy never regarded earth, air, water, and fire as corporeal or chemical substances in the present-day sense of the word. The four elements are simply the primary, and most general, qualities by means of which the amorphous and purely quantitative substance of all bodies first reveals itself in differentiated form." Later alchemists extensively developed the mystical aspects of this concept.

Alchemy coexisted alongside emerging Christianity. Lactantius believed Hermes Trismegistus had prophesied its birth. St Augustine later affirmed this in the 4th & 5th centuries, but also condemned Trismegistus for idolatry. Examples of Pagan, Christian, and Jewish alchemists can be found during this period.

Most of the Greco-Roman alchemists preceding Zosimos are known only by pseudonyms, such as Moses, Isis, Cleopatra, Democritus, and Ostanes. Others authors such as Komarios, and Chymes, we only know through fragments of text. After AD 400, Greek alchemical writers occupied themselves solely in commenting on the works of these predecessors. By the middle of the 7th century alchemy was almost an entirely mystical discipline. It was at that time that Khalid Ibn Yazid sparked its migration from Alexandria to the Islamic world, facilitating the translation and preservation of Greek alchemical texts in the 8th and 9th centuries.

Byzantium

Greek alchemy is preserved in medieval Greek (Byzantine) manuscripts, and yet historians have only relatively recently begun to pay attention to the study and development of Greek alchemy in the Byzantine period.

India

The 2nd millennium BC text Vedas describe a connection between eternal life and gold. A considerable knowledge of metallurgy has been exhibited in a third-century CE text called Arthashastra which provides ingredients of explosives (Agniyoga) and salts extracted from fertile soils and plant remains (Yavakshara) such as saltpetre/nitre, perfume making (different qualities of perfumes are mentioned), granulated (refined) Sugar. Buddhist texts from the 2nd to 5th centuries mention the transmutation of base metals to gold. According to some scholars Greek alchemy may have influenced Indian alchemy but there are no hard evidences to back this claim.

The 11th-century Persian chemist and physician Abū Rayhān Bīrūnī, who visited Gujarat as part of the court of Mahmud of Ghazni, reported that they

have a science similar to alchemy which is quite peculiar to them, which in Sanskrit is called Rasāyana and in Persian Rasavātam. It means the art of obtaining/manipulating Rasa: nectar, mercury, and juice. This art was restricted to certain operations, metals, drugs, compounds, and medicines, many of which have mercury as their core element. Its principles restored the health of those who were ill beyond hope and gave back youth to fading old age.

The goals of alchemy in India included the creation of a divine body (Sanskrit divya-deham) and immortality while still embodied (Sanskrit jīvan-mukti). Sanskrit alchemical texts include much material on the manipulation of mercury and sulphur, that are homologized with the semen of the god Śiva and the menstrual blood of the goddess Devī.

Some early alchemical writings seem to have their origins in the Kaula tantric schools associated to the teachings of the personality of Matsyendranath. Other early writings are found in the Jaina medical treatise Kalyāṇakārakam of Ugrāditya, written in South India in the early 9th century.

Two famous early Indian alchemical authors were Nāgārjuna Siddha and Nityanātha Siddha. Nāgārjuna Siddha was a Buddhist monk. His book, Rasendramangalam, is an example of Indian alchemy and medicine. Nityanātha Siddha wrote Rasaratnākara, also a highly influential work. In Sanskrit, rasa translates to "mercury", and Nāgārjuna Siddha was said to have developed a method of converting mercury into gold.

Scholarship on Indian alchemy is in the publication of The Alchemical Body by David Gordon White. A modern bibliography on Indian alchemical studies has been written by White.

The contents of 39 Sanskrit alchemical treatises have been analysed in detail in G. Jan Meulenbeld's History of Indian Medical Literature. The discussion of these works in HIML gives a summary of the contents of each work, their special features, and where possible the evidence concerning their dating. Chapter 13 of HIML, Various works on rasaśāstra and ratnaśāstra (or Various works on alchemy and gems) gives brief details of a further 655 (six hundred and fifty-five) treatises. In some cases Meulenbeld gives notes on the contents and authorship of these works; in other cases references are made only to the unpublished manuscripts of these titles.

A great deal remains to be discovered about Indian alchemical literature. The content of the Sanskrit alchemical corpus has not yet (2014) been adequately integrated into the wider general history of alchemy.

Islamic world

15th-century artistic impression of Jabir ibn Hayyan (Geber), Codici Ashburnhamiani 1166, Biblioteca Medicea Laurenziana, Florence.

After the Fall of the Roman Empire, the focus of alchemical development moved to the Islamic World. Much more is known about Islamic alchemy because it was better documented: indeed, most of the earlier writings that have come down through the years were preserved as Arabic translations. The word alchemy itself was derived from the Arabic word al-kīmiyā (الكيمياء). The early Islamic world was a melting pot for alchemy. Platonic and Aristotelian thought, which had already been somewhat appropriated into hermetical science, continued to be assimilated during the late 7th and early 8th centuries through Syriac translations and scholarship.

In the late ninth and early tenth centuries, the Arabic works attributed to Jābir ibn Hayyān (Latinized as "Geber" or "Geberus") introduced a new approach to alchemy. Paul Kraus, who wrote the standard reference work on Jabir, put it as follows:

To form an idea of the historical place of Jabir's alchemy and to tackle the problem of its sources, it is advisable to compare it with what remains to us of the alchemical literature in the Greek language. One knows in which miserable state this literature reached us. Collected by Byzantine scientists from the tenth century, the corpus of the Greek alchemists is a cluster of incoherent fragments, going back to all the times since the third century until the end of the Middle Ages.

The efforts of Berthelot and Ruelle to put a little order in this mass of literature led only to poor results, and the later researchers, among them in particular Mrs. Hammer-Jensen, Tannery, Lagercrantz, von Lippmann, Reitzenstein, Ruska, Bidez, Festugière and others, could make clear only few points of detail ....

The study of the Greek alchemists is not very encouraging. An even surface examination of the Greek texts shows that a very small part only was organized according to true experiments of laboratory: even the supposedly technical writings, in the state where we find them today, are unintelligible nonsense which refuses any interpretation.

It is different with Jabir's alchemy. The relatively clear description of the processes and the alchemical apparati, the methodical classification of the substances, mark an experimental spirit which is extremely far away from the weird and odd esotericism of the Greek texts. The theory on which Jabir supports his operations is one of clearness and of an impressive unity. More than with the other Arab authors, one notes with him a balance between theoretical teaching and practical teaching, between the 'ilm and the amal. In vain one would seek in the Greek texts a work as systematic as that which is presented, for example, in the Book of Seventy.

Islamic philosophers also made great contributions to alchemical hermeticism. The most influential author in this regard was arguably Jabir. Jabir's ultimate goal was Takwin, the artificial creation of life in the alchemical laboratory, up to, and including, human life. He analyzed each Aristotelian element in terms of four basic qualities of hotness, coldness, dryness, and moistness. According to Jabir, in each metal two of these qualities were interior and two were exterior. For example, lead was externally cold and dry, while gold was hot and moist. Thus, Jabir theorized, by rearranging the qualities of one metal, a different metal would result. By this reasoning, the search for the philosopher's stone was introduced to Western alchemy. Jabir developed an elaborate numerology whereby the root letters of a substance's name in Arabic, when treated with various transformations, held correspondences to the element's physical properties.

The elemental system used in medieval alchemy also originated with Jabir. His original system consisted of seven elements, which included the five classical elements (aether, air, earth, fire, and water) in addition to two chemical elements representing the metals: sulphur, "the stone which burns", which characterized the principle of combustibility, and mercury, which contained the idealized principle of metallic properties. Shortly thereafter, this evolved into eight elements, with the Arabic concept of the three metallic principles: sulphur giving flammability or combustion, mercury giving volatility and stability, and salt giving solidity. The atomic theory of corpuscularianism, where all physical bodies possess an inner and outer layer of minute particles or corpuscles, also has its origins in the work of Jabir.

From the 9th to 14th centuries, alchemical theories faced criticism from a variety of practical Muslim chemists, including Alkindus, Abū al-Rayhān al-Bīrūnī, Avicenna and Ibn Khaldun. In particular, they wrote refutations against the idea of the transmutation of metals.

East Asia

Taoist alchemists often use this alternate version of the taijitu.

Whereas European alchemy eventually centered on the transmutation of base metals into noble metals, Chinese alchemy had a more obvious connection to medicine. The philosopher's stone of European alchemists can be compared to the Grand Elixir of Immortality sought by Chinese alchemists. In the hermetic view, these two goals were not unconnected, and the philosopher's stone was often equated with the universal panacea; therefore, the two traditions may have had more in common than initially appears.

Black powder may have been an important invention of Chinese alchemists. As previously stated above, Chinese alchemy was more related to medicine. It is said that the Chinese invented gunpowder while trying to find a potion for eternal life. Described in 9th-century texts and used in fireworks in China by the 10th century, it was used in cannons by 1290. From China, the use of gunpowder spread to Japan, the Mongols, the Muslim world, and Europe. Gunpowder was used by the Mongols against the Hungarians in 1241, and in Europe by the 14th century.

Chinese alchemy was closely connected to Taoist forms of traditional Chinese medicine, such as Acupuncture and Moxibustion. In the early Song dynasty, followers of this Taoist idea (chiefly the elite and upper class) would ingest mercuric sulfide, which, though tolerable in low levels, led many to suicide. Thinking that this consequential death would lead to freedom and access to the Taoist heavens, the ensuing deaths encouraged people to eschew this method of alchemy in favor of external sources (the aforementioned Tai Chi Chuan, mastering of the qi, etc.) Chinese alchemy was introduced to the West by Obed Simon Johnson.

Medieval Europe

"An illuminated page from a book on alchemical processes and receipts", ca. 15th century.

The introduction of alchemy to Latin Europe may be dated to 11 February 1144, with the completion of Robert of Chester's translation of the Arabic Book of the Composition of Alchemy. Although European craftsmen and technicians pre-existed, Robert notes in his preface that alchemy (though here still referring to the elixir rather than to the art itself) was unknown in Latin Europe at the time of his writing. The translation of Arabic texts concerning numerous disciplines including alchemy flourished in 12th-century Toledo, Spain, through contributors like Gerard of Cremona and Adelard of Bath. Translations of the time included the Turba Philosophorum, and the works of Avicenna and Muhammad ibn Zakariya al-Razi. These brought with them many new words to the European vocabulary for which there was no previous Latin equivalent. Alcohol, carboy, elixir, and athanor are examples.

Meanwhile, theologian contemporaries of the translators made strides towards the reconciliation of faith and experimental rationalism, thereby priming Europe for the influx of alchemical thought. The 11th-century St Anselm put forth the opinion that faith and rationalism were compatible and encouraged rationalism in a Christian context. In the early 12th century, Peter Abelard followed Anselm's work, laying down the foundation for acceptance of Aristotelian thought before the first works of Aristotle had reached the West. In the early 13th century, Robert Grosseteste used Abelard's methods of analysis and added the use of observation, experimentation, and conclusions when conducting scientific investigations. Grosseteste also did much work to reconcile Platonic and Aristotelian thinking.

Through much of the 12th and 13th centuries, alchemical knowledge in Europe remained centered on translations, and new Latin contributions were not made. The efforts of the translators were succeeded by that of the encyclopaedists. In the 13th century, Albertus Magnus and Roger Bacon were the most notable of these, their work summarizing and explaining the newly imported alchemical knowledge in Aristotelian terms. Albertus Magnus, a Dominican friar, is known to have written works such as the Book of Minerals where he observed and commented on the operations and theories of alchemical authorities like Hermes and Democritus and unnamed alchemists of his time. Albertus critically compared these to the writings of Aristotle and Avicenna, where they concerned the transmutation of metals. From the time shortly after his death through to the 15th century, more than 28 alchemical tracts were misattributed to him, a common practice giving rise to his reputation as an accomplished alchemist. Likewise, alchemical texts have been attributed to Albert's student Thomas Aquinas.

Roger Bacon, a Franciscan friar who wrote on a wide variety of topics including optics, comparative linguistics, and medicine, composed his Great Work (Latin: Opus Majus) for Pope Clement IV as part of a project towards rebuilding the medieval university curriculum to include the new learning of his time. While alchemy was not more important to him than other sciences and he did not produce allegorical works on the topic, he did consider it and astrology to be important parts of both natural philosophy and theology and his contributions advanced alchemy's connections to soteriology and Christian theology. Bacon's writings integrated morality, salvation, alchemy, and the prolongation of life. His correspondence with Clement highlighted this, noting the importance of alchemy to the papacy. Like the Greeks before him, Bacon acknowledged the division of alchemy into practical and theoretical spheres. He noted that the theoretical lay outside the scope of Aristotle, the natural philosophers, and all Latin writers of his time. The practical confirmed the theoretical, and Bacon advocated its uses in natural science and medicine. In later European legend, he became an archmage. In particular, along with Albertus Magnus, he was credited with the forging of a brazen head capable of answering its owner's questions.

Soon after Bacon, the influential work of Pseudo-Geber (sometimes identified as Paul of Taranto) appeared. His Summa Perfectionis remained a staple summary of alchemical practice and theory through the medieval and renaissance periods. It was notable for its inclusion of practical chemical operations alongside sulphur-mercury theory, and the unusual clarity with which they were described. By the end of the 13th century, alchemy had developed into a fairly structured system of belief. Adepts believed in the macrocosm-microcosm theories of Hermes, that is to say, they believed that processes that affect minerals and other substances could have an effect on the human body (for example, if one could learn the secret of purifying gold, one could use the technique to purify the human soul). They believed in the four elements and the four qualities as described above, and they had a strong tradition of cloaking their written ideas in a labyrinth of coded jargon set with traps to mislead the uninitiated. Finally, the alchemists practiced their art: they actively experimented with chemicals and made observations and theories about how the universe operated. Their entire philosophy revolved around their belief that man's soul was divided within himself after the fall of Adam. By purifying the two parts of man's soul, man could be reunited with God.

In the 14th century, alchemy became more accessible to Europeans outside the confines of Latin speaking churchmen and scholars. Alchemical discourse shifted from scholarly philosophical debate to an exposed social commentary on the alchemists themselves. Dante, Piers Plowman, and Chaucer all painted unflattering pictures of alchemists as thieves and liars. Pope John XXII's 1317 edict, Spondent quas non-exhibent forbade the false promises of transmutation made by pseudo-alchemists. In 1403, Henry IV of England banned the practice of multiplying metals (although it was possible to buy a licence to attempt to make gold alchemically, and a number were granted by Henry VI and Edward IV). These critiques and regulations centered more around pseudo-alchemical charlatanism than the actual study of alchemy, which continued with an increasingly Christian tone. The 14th century saw the Christian imagery of death and resurrection employed in the alchemical texts of Petrus Bonus, John of Rupescissa, and in works written in the name of Raymond Lull and Arnold of Villanova.

Nicolas Flamel is a well-known alchemist, but a good example of pseudepigraphy, the practice of giving your works the name of someone else, usually more famous. Although the historical Flamel existed, the writings and legends assigned to him only appeared in 1612. Flamel was not a religious scholar as were many of his predecessors, and his entire interest in the subject revolved around the pursuit of the philosopher's stone. His work spends a great deal of time describing the processes and reactions, but never actually gives the formula for carrying out the transmutations. Most of 'his' work was aimed at gathering alchemical knowledge that had existed before him, especially as regarded the philosopher's stone. Through the 14th and 15th centuries, alchemists were much like Flamel: they concentrated on looking for the philosophers' stone. Bernard Trevisan and George Ripley made similar contributions. Their cryptic allusions and symbolism led to wide variations in interpretation of the art.

Renaissance and early modern Europe

Page from alchemic treatise of Ramon Llull, 16th century
 
The red sun rising over the city, the final illustration of 16th-century alchemical text, Splendor Solis. The word rubedo, meaning "redness", was adopted by alchemists and signalled alchemical success, and the end of the great work.

During the Renaissance, Hermetic and Platonic foundations were restored to European alchemy. The dawn of medical, pharmaceutical, occult, and entrepreneurial branches of alchemy followed.

In the late 15th century, Marsilio Ficino translated the Corpus Hermeticum and the works of Plato into Latin. These were previously unavailable to Europeans who for the first time had a full picture of the alchemical theory that Bacon had declared absent. Renaissance Humanism and Renaissance Neoplatonism guided alchemists away from physics to refocus on mankind as the alchemical vessel.

Esoteric systems developed that blended alchemy into a broader occult Hermeticism, fusing it with magic, astrology, and Christian cabala. A key figure in this development was German Heinrich Cornelius Agrippa (1486–1535), who received his Hermetic education in Italy in the schools of the humanists. In his De Occulta Philosophia, he attempted to merge Kabbalah, Hermeticism, and alchemy. He was instrumental in spreading this new blend of Hermeticism outside the borders of Italy.

Philippus Aureolus Paracelsus, (Theophrastus Bombastus von Hohenheim, 1493–1541) cast alchemy into a new form, rejecting some of Agrippa's occultism and moving away from chrysopoeia. Paracelsus pioneered the use of chemicals and minerals in medicine and wrote, "Many have said of Alchemy, that it is for the making of gold and silver. For me such is not the aim, but to consider only what virtue and power may lie in medicines."

His hermetical views were that sickness and health in the body relied on the harmony of man the microcosm and Nature the macrocosm. He took an approach different from those before him, using this analogy not in the manner of soul-purification but in the manner that humans must have certain balances of minerals in their bodies, and that certain illnesses of the body had chemical remedies that could cure them. Iatrochemistry refers to the pharmaceutical applications of alchemy championed by Paracelsus.

John Dee (13 July 1527 – December, 1608) followed Agrippa's occult tradition. Although better known for angel summoning, divination, and his role as astrologer, cryptographer, and consultant to Queen Elizabeth I, Dee's alchemical Monas Hieroglyphica, written in 1564 was his most popular and influential work. His writing portrayed alchemy as a sort of terrestrial astronomy in line with the Hermetic axiom As above so below. During the 17th century, a short-lived "supernatural" interpretation of alchemy became popular, including support by fellows of the Royal Society: Robert Boyle and Elias Ashmole. Proponents of the supernatural interpretation of alchemy believed that the philosopher's stone might be used to summon and communicate with angels.

Entrepreneurial opportunities were common for the alchemists of Renaissance Europe. Alchemists were contracted by the elite for practical purposes related to mining, medical services, and the production of chemicals, medicines, metals, and gemstones. Rudolf II, Holy Roman Emperor, in the late 16th century, famously received and sponsored various alchemists at his court in Prague, including Dee and his associate Edward Kelley. King James IV of Scotland, Julius, Duke of Brunswick-Lüneburg, Henry V, Duke of Brunswick-Lüneburg, Augustus, Elector of Saxony, Julius Echter von Mespelbrunn, and Maurice, Landgrave of Hesse-Kassel all contracted alchemists. John's son Arthur Dee worked as a court physician to Michael I of Russia and Charles I of England but also compiled the alchemical book Fasciculus Chemicus.

Alchemist Sendivogius (1566–1636) by Jan Matejko, 1867

Although most of these appointments were legitimate, the trend of pseudo-alchemical fraud continued through the Renaissance. Betrüger would use sleight of hand, or claims of secret knowledge to make money or secure patronage. Legitimate mystical and medical alchemists such as Michael Maier and Heinrich Khunrath wrote about fraudulent transmutations, distinguishing themselves from the con artists. False alchemists were sometimes prosecuted for fraud.

The terms "chemia" and "alchemia" were used as synonyms in the early modern period, and the differences between alchemy, chemistry and small-scale assaying and metallurgy were not as neat as in the present day. There were important overlaps between practitioners, and trying to classify them into alchemists, chemists and craftsmen is anachronistic. For example, Tycho Brahe (1546–1601), an alchemist better known for his astronomical and astrological investigations, had a laboratory built at his Uraniborg observatory/research institute. Michael Sendivogius (Michał Sędziwój, 1566–1636), a Polish alchemist, philosopher, medical doctor and pioneer of chemistry wrote mystical works but is also credited with distilling oxygen in a lab sometime around 1600. Sendivogious taught his technique to Cornelius Drebbel who, in 1621, applied this in a submarine. Isaac Newton devoted considerably more of his writing to the study of alchemy (see Isaac Newton's occult studies) than he did to either optics or physics. Other early modern alchemists who were eminent in their other studies include Robert Boyle, and Jan Baptist van Helmont. Their Hermeticism complemented rather than precluded their practical achievements in medicine and science.

Later modern period

An alchemist, pictured in Charles Mackay's Extraordinary Popular Delusions and the Madness of Crowds.

The decline of European alchemy was brought about by the rise of modern science with its emphasis on rigorous quantitative experimentation and its disdain for "ancient wisdom". Although the seeds of these events were planted as early as the 17th century, alchemy still flourished for some two hundred years, and in fact may have reached its peak in the 18th century. As late as 1781 James Price claimed to have produced a powder that could transmute mercury into silver or gold. Early modern European alchemy continued to exhibit a diversity of theories, practices, and purposes: "Scholastic and anti-Aristotelian, Paracelsian and anti-Paracelsian, Hermetic, Neoplatonic, mechanistic, vitalistic, and more—plus virtually every combination and compromise thereof."

Robert Boyle (1627–1691) pioneered the scientific method in chemical investigations. He assumed nothing in his experiments and compiled every piece of relevant data. Boyle would note the place in which the experiment was carried out, the wind characteristics, the position of the Sun and Moon, and the barometer reading, all just in case they proved to be relevant. This approach eventually led to the founding of modern chemistry in the 18th and 19th centuries, based on revolutionary discoveries and ideas of Lavoisier and John Dalton.

Beginning around 1720, a rigid distinction began to be drawn for the first time between "alchemy" and "chemistry". By the 1740s, "alchemy" was now restricted to the realm of gold making, leading to the popular belief that alchemists were charlatans, and the tradition itself nothing more than a fraud. In order to protect the developing science of modern chemistry from the negative censure to which alchemy was being subjected, academic writers during the 18th-century scientific Enlightenment attempted, for the sake of survival, to divorce and separate the "new" chemistry from the "old" practices of alchemy. This move was mostly successful, and the consequences of this continued into the 19th, 20th and 21st centuries.

During the occult revival of the early 19th century, alchemy received new attention as an occult science. The esoteric or occultist school, which arose during the 19th century, held (and continues to hold) the view that the substances and operations mentioned in alchemical literature are to be interpreted in a spiritual sense, and it downplays the role of the alchemy as a practical tradition or protoscience. This interpretation further forwarded the view that alchemy is an art primarily concerned with spiritual enlightenment or illumination, as opposed to the physical manipulation of apparatus and chemicals, and claims that the obscure language of the alchemical texts were an allegorical guise for spiritual, moral or mystical processes.

In the 19th-century revival of alchemy, the two most seminal figures were Mary Anne Atwood and Ethan Allen Hitchcock, who independently published similar works regarding spiritual alchemy. Both forwarded a completely esoteric view of alchemy, as Atwood claimed: "No modern art or chemistry, notwithstanding all its surreptitious claims, has any thing in common with Alchemy." Atwood's work influenced subsequent authors of the occult revival including Eliphas Levi, Arthur Edward Waite, and Rudolf Steiner. Hitchcock, in his Remarks Upon Alchymists (1855) attempted to make a case for his spiritual interpretation with his claim that the alchemists wrote about a spiritual discipline under a materialistic guise in order to avoid accusations of blasphemy from the church and state. In 1845, Baron Carl Reichenbach, published his studies on Odic force, a concept with some similarities to alchemy, but his research did not enter the mainstream of scientific discussion.

In 1946, Louis Cattiaux published the Message Retrouvé, a work that was at once philosophical, mystical and highly influenced by alchemy. In his lineage, many researchers, including Emmanuel and Charles d'Hooghvorst, are updating alchemical studies in France and Belgium.

Women

Several women appear in the earliest history of alchemy. Michael Maier names Mary the Jewess, Cleopatra the Alchemist, Medera, and Taphnutia as the four women who knew how to make the philosopher's stone. Zosimos' sister Theosebia (later known as Euthica the Arab) and Isis the Prophetess also played a role in early alchemical texts.

The first alchemist whose name we know was Mary the Jewess (c. 200 A.D.). Early sources claim that Mary (or Maria) devised a number of improvements to alchemical equipment and tools as well as novel techniques in chemistry. Her best known advances were in heating and distillation processes. The laboratory water-bath, known eponymously (especially in France) as the bain-marie, is said to have been invented or at least improved by her. Essentially a double-boiler, it was (and is) used in chemistry for processes that require gentle heating. The tribikos (a modified distillation apparatus) and the kerotakis (a more intricate apparatus used especially for sublimations) are two other advancements in the process of distillation that are credited to her. Although we have no writing from Mary herself, she is known from the early-fourth-century writings of Zosimos of Panopolis.

Due to the proliferation of pseudepigrapha and anonymous works, it is difficult to know which of the alchemists were actually women. After the Greco-Roman period, women's names appear less frequently in the alchemical literature. Women vacate the history of alchemy during the medieval and renaissance periods, aside from the fictitious account of Perenelle Flamel. Mary Anne Atwood's A Suggestive Inquiry into the Hermetic Mystery (1850) marks their return during the nineteenth-century occult revival.

Modern historical research

The history of alchemy has become a significant and recognized subject of academic study. As the language of the alchemists is analyzed, historians are becoming more aware of the intellectual connections between that discipline and other facets of Western cultural history, such as the evolution of science and philosophy, the sociology and psychology of the intellectual communities, kabbalism, spiritualism, Rosicrucianism, and other mystic movements. Institutions involved in this research include The Chymistry of Isaac Newton project at Indiana University, the University of Exeter Centre for the Study of Esotericism (EXESESO), the European Society for the Study of Western Esotericism (ESSWE), and the University of Amsterdam's Sub-department for the History of Hermetic Philosophy and Related Currents. A large collection of books on alchemy is kept in the Bibliotheca Philosophica Hermetica in Amsterdam. A recipe found in a mid-19th-century kabbalah based book features step by step instructions on turning copper into gold. The author attributed this recipe to an ancient manuscript he located.

Journals which publish regularly on the topic of Alchemy include 'Ambix', published by the Society for the History of Alchemy and Chemistry, and 'Isis', published by The History of Science Society.

Core concepts

Mandala illustrating common alchemical concepts, symbols, and processes. From Spiegel der Kunst und Natur.

Western alchemical theory corresponds to the worldview of late antiquity in which it was born. Concepts were imported from Neoplatonism and earlier Greek cosmology. As such, the classical elements appear in alchemical writings, as do the seven classical planets and the corresponding seven metals of antiquity. Similarly, the gods of the Roman pantheon who are associated with these luminaries are discussed in alchemical literature. The concepts of prima materia and anima mundi are central to the theory of the philosopher's stone.

Magnum opus

The Great Work of Alchemy is often described as a series of four stages represented by colors.

Modernity

Due to the complexity and obscurity of alchemical literature, and the 18th-century disappearance of remaining alchemical practitioners into the area of chemistry, the general understanding of alchemy has been strongly influenced by several distinct and radically different interpretations. Those focusing on the exoteric, such as historians of science Lawrence M. Principe and William R. Newman, have interpreted the 'decknamen' (or code words) of alchemy as physical substances. These scholars have reconstructed physicochemical experiments that they say are described in medieval and early modern texts. At the opposite end of the spectrum, focusing on the esoteric, scholars, such as Florin George Călian and Anna Marie Roos, who question the reading of Principe and Newman, interpret these same decknamen as spiritual, religious, or psychological concepts.

New interpretations of alchemy are still perpetuated, sometimes merging in concepts from New Age or radical environmentalism movements. Groups like the Rosicrucians and Freemasons have a continued interest in alchemy and its symbolism. Since the Victorian revival of alchemy, "occultists reinterpreted alchemy as a spiritual practice, involving the self-transformation of the practitioner and only incidentally or not at all the transformation of laboratory substances", which has contributed to a merger of magic and alchemy in popular thought.

Esoteric interpretations of historical texts

In the eyes of a variety of modern esoteric and Neo-Hermeticist practitioners, alchemy is fundamentally spiritual. In this interpretation, transmutation of lead into gold is presented as an analogy for personal transmutation, purification, and perfection.

According to this view, early alchemists such as Zosimos of Panopolis (c. 300 AD) highlighted the spiritual nature of the alchemical quest, symbolic of a religious regeneration of the human soul. This approach is held to have continued in the Middle Ages, as metaphysical aspects, substances, physical states, and material processes are supposed to have been used as metaphors for spiritual entities, spiritual states, and, ultimately, transformation. In this sense, the literal meanings of 'Alchemical Formulas' were like a veil, hiding their true spiritual philosophy. In the Neo-Hermeticist interpretation, both the transmutation of common metals into gold and the universal panacea are held to symbolize evolution from an imperfect, diseased, corruptible, and ephemeral state toward a perfect, healthy, incorruptible, and everlasting state, so the philosopher's stone then represented a mystic key that would make this evolution possible. Applied to the alchemist, the twin goal symbolized their evolution from ignorance to enlightenment, and the stone represented a hidden spiritual truth or power that would lead to that goal. In texts that are held to have been written according to this view, the cryptic alchemical symbols, diagrams, and textual imagery of late alchemical works are supposed to contain multiple layers of meanings, allegories, and references to other equally cryptic works; which must be laboriously decoded to discover their true meaning.

In his 1766 Alchemical Catechism, Théodore Henri de Tschudi denotes that the usage of the metals was merely symbolic:

Q. When the Philosophers speak of gold and silver, from which they extract their matter, are we to suppose that they refer to the vulgar gold and silver?
A. By no means; vulgar silver and gold are dead, while those of the Philosophers are full of life.

Psychology

Alchemical symbolism has been important in analytical psychology and was revived and popularized from near extinction by the Swiss psychologist Carl Gustav Jung. Jung was initially confounded and at odds with alchemy and its images but after being given a copy of The Secret of the Golden Flower, a Chinese alchemical text translated by his friend Richard Wilhelm, he discovered a direct correlation or parallel between the symbolic images in the alchemical drawings and the inner, symbolic images coming up in the dreams, visions or imaginations of his patients. He observed these alchemical images occurring during the psychic process of transformation, a process that Jung called "individuation." Specifically, he regarded the conjuring up of images of gold or Lapis as symbolic expressions of the origin and goal of this "process of individuation." Together with his alchemical mystica soror (mystical sister) Jungian Swiss analyst Marie-Louise von Franz, Jung began collecting old alchemical texts, compiled a lexicon of key phrases with cross-references, and pored over them. The volumes of work he wrote brought new light into understanding the art of transubstantiation and renewed alchemy's popularity as a symbolic process of coming into wholeness as a human being where opposites are brought into contact and inner and outer, spirit and matter are reunited in the hieros gamos or divine marriage. His writings are influential in general psychology but especially to those who have an interest in understanding the importance of dreams, symbols, and the unconscious archetypal forces (archetypes) that comprise all psychic life.

Both von Franz and Jung have contributed significantly to the subject and work of alchemy and its continued presence in psychology as well as contemporary culture. Among the volumes Jung wrote on alchemy, his magnum opus is Volume 14 of his Collected Works, Mysterium Coniunctionis.

Literature

Alchemy has had a long-standing relationship with art, seen both in alchemical texts and in mainstream entertainment. Literary alchemy appears throughout the history of English literature from Shakespeare to J. K. Rowling, and also the popular Japanese manga Fullmetal Alchemist. Here, characters or plot structure follow an alchemical magnum opus. In the 14th century, Chaucer began a trend of alchemical satire that can still be seen in recent fantasy works like those of the late Sir Terry Pratchett.

Visual artists had a similar relationship with alchemy. While some of them used alchemy as a source of satire, others worked with the alchemists themselves or integrated alchemical thought or symbols in their work. Music was also present in the works of alchemists and continues to influence popular performers. In the last hundred years, alchemists have been portrayed in a magical and spagyric role in fantasy fiction, film, television, novels, comics and video games.

Science

One goal of alchemy, the transmutation of base substances into gold, is now known to be impossible by chemical means but possible by physical means. Although not financially worthwhile, Gold was synthesized in particle accelerators as early as 1941.

Computer memory

From Wikipedia, the free encyclopedia
 
Modern DDR4 SDRAM module, usually found in desktop computers.

In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term memory is often synonymous with the term primary storage or main memory. An archaic synonym for memory is store.

Computer memory operates at a high speed compared to storage that is slower but less expensive and higher in capacity. Besides storing opened programs, computer memory serves as disk cache and write buffer to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called virtual memory.

Modern memory is implemented as semiconductor memory, where data is stored within memory cells built from MOS transistors and other components on an integrated circuit. There are two main kinds of semiconductor memory, volatile and non-volatile. Examples of non-volatile memory are flash memory and ROM, PROM, EPROM and EEPROM memory. Examples of volatile memory are dynamic random-access memory (DRAM) used for primary storage, and static random-access memory (SRAM) used for CPU cache.

Most semiconductor memory is organized into memory cells each storing one bit (0 or 1). Flash memory organization includes both one bit per memory cell and multi-level cell capable of storing multiple bits per cell. The memory cells are grouped into words of fixed word length, for example, 1, 2, 4, 8, 16, 32, 64 or 128 bits. Each word can be accessed by a binary address of N bits, making it possible to store 2N words in the memory.

History

Detail of the back of a section of ENIAC, showing vacuum tubes.
 
8 GB microSDHC card on top of 8 bytes of magnetic-core memory (1 core is 1 bit).

In the early 1940s, memory technology often permitted a capacity of a few bytes. The first electronic programmable digital computer, the ENIAC, using thousands of vacuum tubes, could perform simple calculations involving 20 numbers of ten decimal digits stored in the vacuum tubes.

The next significant advance in computer memory came with acoustic delay-line memory, developed by J. Presper Eckert in the early 1940s. Through the construction of a glass tube filled with mercury and plugged at each end with a quartz crystal, delay lines could store bits of information in the form of sound waves propagating through the mercury, with the quartz crystals acting as transducers to read and write bits. Delay-line memory was limited to a capacity of up to a few thousand bits.

Two alternatives to the delay line, the Williams tube and Selectron tube, originated in 1946, both using electron beams in glass tubes as means of storage. Using cathode ray tubes, Fred Williams invented the Williams tube, which was the first random-access computer memory. The Williams tube was able to store more information than the Selectron tube (the Selectron was limited to 256 bits, while the Williams tube could store thousands) and less expensive. The Williams tube was nevertheless frustratingly sensitive to environmental disturbances.

Efforts began in the late 1940s to find non-volatile memory. Magnetic-core memory allowed for recall of memory after power loss. It was developed by Frederick W. Viehe and An Wang in the late 1940s, and improved by Jay Forrester and Jan A. Rajchman in the early 1950s, before being commercialised with the Whirlwind computer in 1953. Magnetic-core memory was the dominant form of memory until the development of MOS semiconductor memory in the 1960s.

The first semiconductor memory was implemented as a flip-flop circuit in the early 1960s using bipolar transistors. Semiconductor memory made from discrete devices was first shipped by Texas Instruments to the United States Air Force in 1961. The same year, the concept of solid-state memory on an integrated circuit (IC) chip was proposed by applications engineer Bob Norman at Fairchild Semiconductor. The first bipolar semiconductor memory IC chip was the SP95 introduced by IBM in 1965. While semiconductor memory offered improved performance over magnetic-core memory, it remain larger and more expensive and did not displace magnetic-core memory until the late 1960s.

MOS memory

The invention of the metal–oxide–semiconductor field-effect transistor (MOSFET) enabled the practical use of metal–oxide–semiconductor (MOS) transistors as memory cell storage elements. MOS memory was developed by John Schmidt at Fairchild Semiconductor in 1964. In addition to higher performance, MOS semiconductor memory was cheaper and consumed less power than magnetic core memory. In 1965, J. Wood and R. Ball of the Royal Radar Establishment proposed digital storage systems that use CMOS (complementary MOS) memory cells, in addition to MOSFET power devices for the power supply, switched cross-coupling, switches and delay-line storage. The development of silicon-gate MOS integrated circuit (MOS IC) technology by Federico Faggin at Fairchild in 1968 enabled the production of MOS memory chips. NMOS memory was commercialized by IBM in the early 1970s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s.

The two main types of volatile random-access memory (RAM) are static random-access memory (SRAM) and dynamic random-access memory (DRAM). Bipolar SRAM was invented by Robert Norman at Fairchild Semiconductor in 1963, followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but requires six transistors for each bit of data. Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for the System/360 Model 95.

Toshiba introduced bipolar DRAM memory cells for its Toscal BC-1411 electronic calculator in 1965. While it offered improved performance, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory. MOS technology is the basis for modern DRAM. In 1966, Robert H. Dennard at the IBM Thomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was possible to build capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent for a single-transistor DRAM memory cell based on MOS technology. This led to the first commercial DRAM IC chip, the Intel 1103 in October 1970. Synchronous dynamic random-access memory (SDRAM) later debuted with the Samsung KM48SL2000 chip in 1992.

The term memory is also often used to refer to non-volatile memory including read-only memory (ROM) through modern flash memory. Programmable read-only memory (PROM) was invented by Wen Tsing Chow in 1956, while working for the Arma Division of the American Bosch Arma Corporation. In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the floating gate of a MOS semiconductor device could be used for the cell of a reprogrammable ROM, which led to Dov Frohman of Intel inventing EPROM (erasable PROM) in 1971. EEPROM (electrically erasable PROM) was developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at the Electrotechnical Laboratory in 1972. Flash memory was invented by Fujio Masuoka at Toshiba in the early 1980s. Masuoka and colleagues presented the invention of NOR flash in 1984, and then NAND flash in 1987. Toshiba commercialized NAND flash memory in 1987.

Developments in technology and economies of scale have made possible so-called very large memory (VLM) computers.

Volatile memory

Various memory modules containing different types of DRAM (from top to bottom): DDR SDRAM, SDRAM, EDO DRAM, and FPM DRAM
 

Volatile memory is computer memory that requires power to maintain the stored information. Most modern semiconductor volatile memory is either static RAM (SRAM) or dynamic RAM (DRAM). DRAM dominates for desktop system memory. SRAM is used for CPU cache. SRAM is also found in small embedded systems requiring little memory.

SRAM retains its contents as long as the power is connected and may use a simpler interface, but requires six transistors per bit. Dynamic RAM is more complicated for interfacing and control, needing regular refresh cycles to prevent losing its contents, but uses only one transistor and one capacitor per bit, allowing it to reach much higher densities and much cheaper per-bit costs.

Non-volatile memory

Non-volatile memory can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, most types of magnetic computer storage devices (e.g. hard disk drives, floppy disks and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards.

Non-volatile memory technologies under development include ferroelectric RAM, programmable metallization cell, Spin-transfer torque magnetic RAM, SONOS, resistive random-access memory, racetrack memory, Nano-RAM, 3D XPoint, and millipede memory.

Semi-volatile memory

A third category of memory is semi-volatile. The term is used to describe a memory that has some limited non-volatile duration after power is removed, but then data is ultimately lost. A typical goal when using a semi-volatile memory is to provide the high performance and durability associated with volatile memories while providing some benefits of non-volatile memory.

For example, some non-volatile memory types experience wear when written. A worn cell has increased volatility but otherwise continues to work. Data locations which are written frequently can thus be directed to use worn circuits. As long as the location is updated within some known retention time, the data stays valid. After a period of time without update, the value is copied to a less-worn circuit with longer retention. Writing first to the worn area allows a high write rate while avoiding wear on the not-worn circuits.

As a second example, an STT-RAM can be made non-volatile by building large cells, but doing so raises the cost per bit and power requirements and reduces the write speed. Using small cells improves cost, power, and speed, but leads to semi-volatile behavior. In some applications, the increased volatility can be managed to provide many benefits of a non-volatile memory, for example by removing power but forcing a wake-up before data is lost; or by caching read-only data and discarding the cached data if the power-off time exceeds the non-volatile threshold.

The term semi-volatile is also used to describe semi-volatile behavior constructed from other memory types. For example, a volatile and a non-volatile memory may be combined, where an external signal copies data from the volatile memory to the non-volatile memory, but if power is removed before the copy occurs, the data is lost. Or, a battery-backed volatile memory, and if external power is lost there is some known period where the battery can continue to power the volatile memory, but if power is off for an extended time, the battery runs down and data is lost.

Management

Proper management of memory is vital for a computer system to operate properly. Modern operating systems have complex systems to properly manage memory. Failure to do so can lead to bugs, slow performance, and at worst case, takeover by viruses and malicious software.

Bugs

Improper management of memory is a common cause of bugs, including the following types:

  • In an arithmetic overflow, a calculation results in a number larger than the allocated memory permits. For example, a signed 8-bit integer allows the numbers −128 to +127. If its value is 127 and it is instructed to add one, the computer can not store the number 128 in that space. Such a case will result in an undesired operation, such as changing the number's value to −128 instead of +128.
  • A memory leak occurs when a program requests memory from the operating system and never returns the memory when it's done with it. A program with this bug will gradually require more and more memory until the program fails as it runs out.
  • A segmentation fault results when a program tries to access memory that it does not have permission to access. Generally, a program doing so will be terminated by the operating system.
  • A buffer overflow means that a program writes data to the end of its allocated space and then continues to write data to memory that has been allocated for other purposes. This may result in erratic program behavior, including memory access errors, incorrect results, a crash, or a breach of system security. They are thus the basis of many software vulnerabilities and can be maliciously exploited.

Early computer systems

In early computer systems, programs typically specified the location to write memory and what data to put there. This location was a physical location on the actual memory hardware. The slow processing of such computers did not allow for the complex memory management systems used today. Also, as most such systems were single-task, sophisticated systems were not required as much.

This approach has its pitfalls. If the location specified is incorrect, this will cause the computer to write the data to some other part of the program. The results of an error like this are unpredictable. In some cases, the incorrect data might overwrite memory used by the operating system. Computer crackers can take advantage of this to create viruses and malware.

Virtual memory

Virtual memory is a system where all physical memory is controlled by the operating system. When a program needs memory, it requests it from the operating system. The operating system then decides in what physical location to place the program's code and data.

This offers several advantages. Computer programmers no longer need to worry about where their data is physically stored or whether the user's computer will have enough memory. It also allows multiple types of memory to be used. For example, some data can be stored in physical RAM chips while other data is stored on a hard drive (e.g. in a swapfile), functioning as an extension of the cache hierarchy. This drastically increases the amount of memory available to programs. The operating system will place actively used data in physical RAM, which is much faster than hard disks. When the amount of RAM is not sufficient to run all the current programs, it can result in a situation where the computer spends more time moving data from RAM to disk and back than it does accomplishing tasks; this is known as thrashing.

Protected memory

Protected memory is a system where each program is given an area of memory to use and is not permitted to go outside that range. Use of protected memory greatly enhances both the reliability and security of a computer system.

Without protected memory, it is possible that a bug in one program will alter the memory used by another program. This will cause that other program to run off of corrupted memory with unpredictable results. If the operating system's memory is corrupted, the entire computer system may crash and need to be rebooted. At times programs intentionally alter the memory used by other programs. This is done by viruses and malware to take over computers. It may also be used benignly by desirable programs which are intended to modify other programs; in the modern age, this is generally considered bad programming practice for application programs, but it may be used by system development tools such as debuggers, for example to insert breakpoints or hooks.

Protected memory assigns programs their own areas of memory. If the operating system detects that a program has tried to alter memory that does not belong to it, the program is terminated (or otherwise restricted or redirected). This way, only the offending program crashes, and other programs are not affected by the misbehavior (whether accidental or intentional).

Protected memory systems almost always include virtual memory as well.

Lie point symmetry

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Lie_point_symmetry     ...