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History of Chemistry
I INTRODUCTION
History of Chemistry, history of the study of the composition, structure, and properties of material substances, of the interactions between substances, and of the effects on substances of the addition or removal of energy in any of its several forms. From the earliest recorded times, humans have observed chemical changes and have speculated as to their causes. By following the history of these observations and speculations, the gradual evolution of the ideas and concepts that have led to the modern science of chemistry can be traced.
II ANCIENT TECHNOLOGY AND PHILOSOPHY
The first known chemical processes were carried out by the artisans of Mesopotamia, Egypt, and China. At first the smiths of these lands worked with native metals such as gold or copper, which sometimes occur in nature in a pure state, but they quickly learned how to smelt metallic ores (primarily metallic oxides and sulfides) by heating them with wood or charcoal to obtain the metals. The progressive use of copper, bronze, and iron gave rise to the names that have been applied to the corresponding ages by archaeologists. A primitive chemical technology also arose in these cultures as dyers discovered methods of setting dyes on different types of cloth, and as potters learned how to prepare glazes, and, later, to make glass.
Most of these craftspeople were employed in temples and palaces, making luxury goods for priests and nobles. In the temples, the priests especially had time to speculate on the origin of the changes they saw in the world about them. Their theories often involved magic, but they also developed astronomical, mathematical, and cosmological ideas, which they used in attempts to explain some of the changes that are now considered chemical.
III GREEK NATURAL PHILOSOPHY
The first culture to consider these ideas scientifically was that of the Greeks. From the time of Thales, about 600 bc, Greek philosophers were making logical speculations about the physical world rather than relying on myth to explain phenomena. Thales himself assumed that all matter was derived from water, which could solidify to earth or evaporate to air. His successors expanded this theory into the idea that four elements composed the world: earth, water, air, and fire. Democritus thought that these elements were composed of atoms, minute particles moving in a vacuum. Others, especially Aristotle, believed that the elements formed a continuum of mass and therefore a vacuum could not exist. The atomic idea quickly lost ground among the Greeks, but it was never entirely forgotten. When it was revived during the Renaissance, it formed the basis of modern atomic theory (see Atom).
Aristotle became the most influential of the Greek philosophers, and his ideas dominated science for nearly two millennia after his death in 323 bc. He believed that four qualities were found in nature: heat, cold, moisture, and dryness. The four elements were each composed of pairs of these qualities; for example, fire was hot and dry, water was cold and moist, air was hot and moist, and earth was cold and dry. These elements with their qualities combined in various proportions to form the components of the earthly planet. Because it was possible for the amounts of each quality in an element to be changed, the elements could be changed into one another; thus, it was thought possible also to change the material substances that were built up from the elements—lead into gold, for example.
IV ALCHEMY: RISE AND DECLINE
Aristotle's theory was accepted by the practical artisans, especially at Alexandria, Egypt, which after 300 bc became the intellectual center of the ancient world. They thought that metals in the earth sought to become more and more perfect and thus gradually changed into gold. It seemed to them that they should be able to carry out the same process more rapidly in their own workshops and so artificially to transmute common metals into gold. Beginning about ad100 this idea dominated the minds of the philosophers as well as the metalworkers, and a large number of treatises were written on the art of transmutation, which became known as alchemy. Although no one ever succeeded in making gold, a number of chemical processes were discovered in the search for the perfection of metals.
At almost the same time, and probably independently, a similar alchemy arose in China. Here, also, the aim was to make gold, although not because of the monetary value of the metal. The Chinese believed that gold was a medicine that could confer long life or even immortality on anyone who consumed it. As did the Egyptians, the Chinese gained practical chemical knowledge from incorrect theories.
A Dispersal of Greek Thought
After the decline of the Roman Empire, Greek writings were less openly studied in western Europe, and even in the eastern Mediterranean they were largely neglected. In the 6th century, however, a sect of Christians known as the Nestorians, whose language was Syriac, spread their influence throughout Asia Minor. They established a university at Edessa in Mesopotamia and translated a large number of Greek philosophical and medical writings into Syriac for use among scholars.
In the 7th and 8th centuries Arab conquerors spread Islamic culture over much of Asia Minor, North Africa, and Spain. The caliphs at Baghd?d became active patrons of science and learning. The Syriac translation of Greek texts were again translated, this time into Arabic, and along with the rest of Greek learning the ideas and practice of alchemy once again flourished.
The Arabic alchemists were also in contact with China in the East, thus receiving the concept of gold as a medicine, as well as the Greek idea of gold as a perfect metal. A specific agent, the philosopher's stone, was thought to stimulate transmutation, and this became the object of the alchemists' search. The alchemists now had an added incentive to study chemical processes, for they might lead not only to wealth but also to health. The study of chemicals and chemical apparatus made steady progress. Such important reagents as the caustic alkalis (see Alkali Metals) and ammonium salts (see Ammonia) were discovered, and distillation apparatus was steadily improved. An early realization of the need for more quantitative methods also appeared in some Arabic recipes, where specific instructions were given regarding the amounts of reagents to be employed.
B The Late Middle Ages
A great intellectual reawakening began in western Europe in the 11th century. This was stimulated in part by the cultural exchanges between Arabs and Western scholars in Sicily and Spain. Schools of translators were established, and their translations transmitted Arabic philosophical and scientific ideas to European scholars. Thus, knowledge of Greek science, passed through the intermediate languages of Syriac and Arabic, was disseminated in the scholarly tongue of Latin and so eventually came to all parts of Europe. Many of the manuscripts most eagerly read were those concerning alchemy.
These manuscripts were of two types: Some were almost purely practical, and some attempted to apply theories of the nature of matter to alchemical problems. Among the practical subjects discussed was distillation. The manufacture of glass had been greatly improved, particularly in Venice, and it now became possible to construct even better distillation apparatus than the Arabs had made and to condense the more volatile products of distillation. Among the important products obtained in this way were alcohol and the mineral acids: nitric, aqua regia (a mixture of nitric and hydrochloric), sulfuric, and hydrochloric. Many new reactions could be carried out using these powerful reagents. Word of the Chinese discovery of nitrates and the manufacture of gunpowder also came to the West through the Arabs. The Chinese at first used gunpowder for fireworks, but in the West it quickly became a major part of warfare. An effective chemical technology existed in Europe by the end of the 13th century.
The second type of alchemical manuscript transmitted by the Arabs was concerned with theory. Many of these writings reveal a mystical character that contributed little to the advancement of chemistry, but others sought to explain transmutation in physical terms. The Arabs had based their theories of matter on Aristotle's ideas, but their thinking tended to be more specific than his. This was especially true of their ideas concerning the composition of metals. They believed that metals consisted of sulfur and mercury—not the familiar substances with which they were perfectly well acquainted, but rather the “principle” of mercury, which conferred the property of fluidity on metals, and the “principle” of sulfur, which made substances combustible and caused metals to corrode. Chemical reactions were explained in terms of changes in the amounts of these principles in material substances.
C The Renaissance
During the 13th and 14th centuries the influence of Aristotle on all branches of scientific thought began to weaken. Actual observation of the behavior of matter cast doubt on the relatively simple explanations Aristotle had given; such doubts spread rapidly after the invention around 1450 of printing with movable type. After 1500 printed alchemical works appeared in increasing numbers, as did works devoted to technology. The result of this increasing knowledge became apparent in the 16th century.
C1 The Rise of Quantitative Methods
Among the influential books that appeared at this time were practical works on mining and metallurgy. These treatises devoted much space to assaying ores for their content of valuable metals, work that required the use of the laboratory balance, or scale, and the development of quantitative methods (see Chemical Analysis). Workers in other fields, especially medicine, began to recognize the need for greater precision. Physicians, some of whom were alchemists, needed to know the exact weight or volume of the doses they administered. Thus, they used chemical methods for preparing medicines.
These methods were combined and forcefully promoted by the eccentric Swiss physician Theophrastus von Hohenheim, generally called Paracelsus. He grew up in a mining region and became familiar with the properties of metals and their compounds, which he believed were superior to the herbal remedies used by orthodox physicians. He spent most of his life in violent quarrels with the medical establishment of the day, and in the process he founded the science of iatrochemistry (the use of chemical medicines), the forerunner of pharmacology. He and his followers discovered many new compounds and chemical reactions. He modified the old sulfur-mercury theory of the composition of metals by adding a third component, salt, the earthy part of all substances. He declared that when wood burns “that which burns is sulfur, that which vaporizes is mercury, and that which turns to ashes is salt.” As with the sulfur-mercury theory, these were principles and not the material substances. His emphasis on combustible sulfur was important for the later development of chemistry. The iatrochemists who followed Paracelsus modified some of his wilder ideas and collected his and their own recipes for preparing chemical remedies. Finally, at the end of the 16th century, Andreas Libavius published his Alchemia, which organized the knowledge of the iatrochemists and is frequently called the first textbook of chemistry.
In the first half of the 17th century a few men began to study chemical reactions experimentally, not because they were useful in other disciplines, but rather for their own sake. Jan Baptista van Helmont, a physician who left medical practice to devote himself to the study of chemistry, used the balance in an important experiment to show that a definite quantity of sand could be fused with excess alkali to form water glass, and that when this product was treated with acid, it regenerated the original amount of sand (silica). Thus were laid the foundations of the law of conservation of mass. Van Helmont also showed that in a number of reactions an aerial fluid was liberated. He called this substance “gas.” A new class of substances with its own physical properties was shown to exist.
C2 Revival of Atomic Theory
In the 16th century experimenters discovered how to create a vacuum, something that Aristotle had declared impossible. This called attention to the ancient theory of Democritus, who had assumed that his atoms moved in a void. The French philosopher and mathematician René Descartes and his followers developed a mechanical view of matter in which the size, shape, and motion of minute particles explained all observed phenomena. Most natural philosophers and iatrochemists at this time assumed that gases had no chemical properties, hence their attention was centered on the physical behavior of gases. A kinetic-molecular theory of gases began to develop. Notable in this direction were the experiments of Robert Boyle, the English physicist and chemist whose studies of the “spring of the air” (elasticity) led to the formation of what became known as Boyle's law, a generalization of the inverse relation between pressure and volume of a gas (see Gases).
V PHLOGISTON: THEORY AND EXPERIMENT
While natural philosophers were thus speculating on mathematical laws, early chemists in their laboratories were attempting to use chemical theories to explain the very real chemical reactions they were observing. The iatrochemists paid particular attention to sulfur and the theories of Paracelsus. In the second half of the 17th century, the German physician, economist, and chemist Johann Joachim Becher built a system of chemistry around this principle. He noted that when organic matter burned, a volatile material seemed to leave the burning substance. His disciple, Georg Ernst Stahl, made this the central point of a theory that survived in chemical circles for nearly a century.
Stahl assumed that when anything burned, its combustible part was given off to the air. This part he called phlogiston, from the Greek word for “flammable.” The rusting of metals was analogous to combustion and therefore also involved loss of phlogiston. Plants absorbed the phlogiston from the air and thus were rich in it. Heating the calx, or oxides, of metals with charcoal restored phlogiston to them. It followed from this that the calx was an element, and the metal a compound. This theory is almost exactly the reverse of the modern concept of oxidation-reduction (see Chemical Reaction), but it involves the cyclic transfer of a substance—even if in the wrong direction—and some observed phenomena could be explained by it. However, recent studies of chemical literature of the period show that the phlogiston explanation had only minor influence among chemists until it was attacked by the wealthy amateur French chemist Antoine Laurent Lavoisier in the last quarter of the eighteenth century.
A The 18th Century
At about the same time, another observation led to advances in the understanding of chemistry. As more and more chemicals were studied, chemists saw that certain substances combined more easily with, or had a greater affinity for, a given chemical than did others. Elaborate tables were drawn up showing relative affinities when different chemicals were brought together. Use of these tables made it possible to predict many chemical reactions before testing them in the laboratory.
All these advances led in the 18th century to the discovery of new metals and their compounds and reactions. Qualitative and quantitative analytical methods began to be developed, and the science of analytical chemistry was born. Nonetheless, as long as the part played by gases was believed to be only physical, the full scope of chemistry could not be recognized.
The chemical study of gases, generally called “airs,” became important after the British physiologist Stephen Hales developed the pneumatic trough to collect and measure the volume of gases released from various solids by heating in a closed system and collecting over water. The pneumatic trough became a valuable device for the collection and study of gases uncontaminated by ordinary air. The study of gases advanced rapidly and led to a new level of understanding of various different gases.
The initial understanding of the role of gases in chemistry occurred in Edinburgh in 1756, when British Chemist Joseph Black published his studies on the reactions of magnesium and calcium carbonates (see Carbonates). When these compounds were heated, they gave off a gas and left a residue of what Black called calcined magnesia, or lime (the oxides). The latter reacted with “alkali” (sodium carbonate) to regenerate the original salts. Thus, the gas carbon dioxide, which Black called fixed air, took part in chemical reactions (was “fixed,” as he said). The idea that a gas could not enter a chemical reaction was overthrown, and soon a number of new gases were recognized as being distinct substances.
The British physicist Henry Cavendish isolated “flammable air” (hydrogen) in the next decade. He also introduced the use of mercury instead of water as the confining liquid over which gases were collected, making it possible to collect water-soluble gases. This variant was used extensively by the British chemist and theologian Joseph Priestley, who collected and studied almost a dozen new gases. Priestley's most important discovery was oxygen, and he quickly realized that this gas was the component of ordinary air that was responsible for combustion and made animal respiration possible. However, he reasoned that combustible substances burned more energetically in this gas, and metals formed calxes more readily, since it was devoid of phlogiston. Hence, the gas accepted the phlogiston present in the combustible substance or the metal more readily than ordinary air, which was already partially filled with phlogiston. He named this new gas “dephlogisticated air” and defended that belief to the end of his life.
Meanwhile chemistry had been making rapid progress in France, particularly in the laboratory of Lavoisier. He was troubled by the fact that metals gained weight when heated in the air when presumably they were losing phlogiston.
In 1774 Priestley visited France and told Lavoisier about his discovery of dephlogisticated air. Lavoisier quickly saw the significance of this substance, and the way was opened for the chemical revolution that established modern chemistry. He used the name “oxygen,” meaning acid former.
B The Birth of Modern Chemistry
Lavoisier showed by a series of brilliant experiments that air contains 20 percent oxygen and that combustion is due to the combination of a combustible substance with oxygen. When carbon is burned, fixed air (carbon dioxide) is produced. Phlogiston therefore does not exist. The phlogiston theory was soon replaced by the view that oxygen from the air combines with the components of the combustible substance to form oxides of the component elements. Lavoisier used the laboratory balance to give quantitative support to his work. He defined elements as substances that could not be decomposed by chemical means and firmly established the law of the conservation of mass. He replaced the old system of chemical names (which was still based on alchemical usage) with the rational chemical nomenclature used today, and he helped to found the first chemical journal. After his death on the guillotine in 1794, his colleagues continued his work in establishing modern chemistry. A little later the Swedish chemist J?ns Jakob Berzelius proposed symbolizing atoms of the elements by the initial letters or pairs of letters from their names.
VI THE 19TH AND 20TH CENTURIES
By the beginning of the 19th century the precision of analytical chemistry had improved to such an extent that chemists were able to show that the simple compounds with which they worked contained fixed and unvarying amounts of their constituent elements. In certain cases, however, more than one compound could be formed between the same elements. At the same time the French chemist and physicist Joseph Gay-Lussac showed that the volume ratios of reacting gases were small whole numbers (which implies the interaction of discrete particles, later shown to be atoms). A major step in explaining these facts was the chemical atomic theory of the English scientist John Dalton in 1803.
Dalton assumed that when two elements combined, the resulting compound contained one atom of each. In his system, water could be given a formula corresponding to HO. He arbitrarily assigned to hydrogen the atomic weight of 1 and could then calculate the relative atomic weight of oxygen. Applying this principle to other compounds, he calculated the atomic weights of other elements and drew up a table of the relative atomic weights of all the then known elements. His theory contained many errors, but the idea was correct, and a precise quantitative value could then be assigned to the mass of each atom.
A Molecular Theory
The major weaknesses in Dalton's theory were that he did not account for the law of multiple proportions and made no distinction between atoms and molecules. Thus, he could not distinguish between the possible formulas for water HO and H2O2, nor could he explain why the density of water vapor, with its assumed formula HO, was less than that of oxygen, assumed to have the formula O. The solution to these problems was found in 1811 by the Italian physicist Amedeo Avogadro. He suggested that the numbers of particles in equal volumes of gases at the same temperature and pressure were equal and that a distinction existed between molecules and atoms. When oxygen combined with hydrogen, a double atom of oxygen (a molecule in our terms) was split, each oxygen atom then combining with two hydrogen atoms, giving the molecular formula of H2 O for water and O2 and H2 for molecules of oxygen and hydrogen.
Unfortunately, Avogadro's ideas were overlooked for nearly 50 years, and during this time great confusion prevailed among chemists in their calculations. It was not until 1860 that the Italian chemist Stanislao Cannizzaro reintroduced Avogadro's hypotheses. By this time chemists had found it more convenient to take the atomic weight of oxygen, 16, as the standard to which to relate the atomic weights of all the other elements instead of taking the value 1 for hydrogen, as Dalton had done. The molecular weight of oxygen, 32, was then used universally and, expressed in grams, was called the gram molecular weight of oxygen, or more simply, 1 mole of oxygen. Chemical calculations were standardized, and fixed formulas written.
The old problem of the nature of chemical affinity remained unsolved. For a time it appeared that the answer might lie in the newly discovered field of electrochemistry. The discovery in 1800 of the voltaic pile, the first true battery, gave chemists a new tool, which led to the discovery of such metals as sodium and potassium. It seemed to Berzelius that positive and negative electrostatic forces might hold elements together; at first his theories were generally accepted. As chemists prepared and studied more new compounds and reactions in which electrical forces did not seem to be involved (the nonpolar compounds), the problem of affinity was shelved for a time.
B New Fields of Chemistry
The most striking advances in chemistry in the 19th century were in the field of organic chemistry (see Chemistry, Organic). The structural theory, which gave a picture of how atoms were actually put together, was nonmathematical, but employed a logic of its own. It made possible the prediction and preparation of many new compounds, including a large number of important dyes, drugs, and explosives that gave rise to great chemical industries, especially in Germany.
At the same time, other branches of chemistry made their appearance. Stimulated by the advances in physics then being made, some chemists sought to apply mathematical methods to their science. Studies of reaction rates led to the development of kinetic theories that had value both for industry and for pure science. The recognition that heat was due to motion on the atomic scale, a kinetic phenomenon, led to the abandonment of the idea that heat was a specific substance (termed caloric) and initiated the study of chemical thermodynamics (see Thermodynamics). Continuation of electrochemical studies led the Swedish chemist Svante August Arrhenius to postulate the dissociation of salts in solution to form ions carrying electrical charges. Studies of the emission and absorption spectra of elements and compounds became important to both chemists and physicists (see Spectroscopy; Spectrum). In addition, fundamental research in colloid and photochemistry was begun. By the end of the 19th century, studies of this type were combined into the field known as physical chemistry (see Chemistry, Physical).
Inorganic chemistry also required organization. The number of new elements being discovered continued to grow, but no method of classification had been developed that could bring order to their reactions. The independent development of the periodic law by the Russian chemist Dmitry Ivanovich Mendeleyev in 1869 and the German chemist Julius Lothar Meyer in 1870 eliminated this confusion and indicated where new elements would be found and what their properties would be (see Elements, Chemical; Periodic Law).
At the end of the 19th century chemistry, like physics, seemed to have reached a stage in which no striking new fields remained to be developed. This view changed completely with the discovery of radioactivity. Chemical methods were used in isolating new elements such as radium, in the separation of the new class of substances known as isotopes, and in the synthesis and isolation of the new transuranium elements. The new picture of the actual structure of atoms obtained by physicists solved the old problem of chemical affinity and explained the relation between polar and nonpolar compounds. See Nuclear Chemistry.
The other major advance for chemistry in the 20th century was the foundation of biochemistry. This began with the simple analysis of body fluids; methods were then rapidly developed for determining the nature and function of the most complex cell constituents. By midcentury biochemists had unraveled the genetic code and explained the function of the gene, the basis of all life; the field had grown so vast that its study had become a new science, molecular biology. See also Genetics.
C Recent Research in Chemistry
Recent advances in biotechnology and materials science are helping to define the frontiers of chemical research. In biotechnology, sophisticated analytical instruments have made it possible to initiate an international effort to sequence the human genome. Success in this project will likely completely change the nature of such fields as molecular biology and medicine. Materials science, an interdisciplinary combination of physics, chemistry, and engineering, is guiding the design of advanced materials and devices. A recent example is the discovery of high-temperature superconductors, ceramic compounds that lose their resistance to the flow of electricity above 77K (-196 C/-321 F; see Superconductivity). Characterization of surfaces is being advanced by the invention of the scanning tunneling microscope, which can provide images of certain surfaces with atomic-scale resolution. See Microscope; Superconductivity.
Even in conventional fields of chemical research, new, more powerful analytical tools are providing unprecedented detail of chemicals and their reactions. For example, laser techniques are providing snapshots of gas-phase chemical reactions on the femtosecond (a millionth of a billionth of a second) time scale. From the soot produced by graphite electrodes has been isolated a new form of carbon, called buckminsterfullerene, that has the shape of a soccerball, and the chemical formula C60. This compound and its chemistry have been characterized with astonishing rapidity using the vast array of analytical techniques currently available. Certain alkali metal salts of this compound have even been found to be superconducting.
D The Chemical Industry
The growth of chemical industries and the training of professional chemists had an interestingly shared history. Until about 150 years ago chemists were not trained professionally. Chemistry was advanced by the work of those who were interested in the subject, but who made no systematic effort to train new workers in the field. Physicians and wealthy amateurs often hired assistants, only some of whom continued their masters' work.
Early in the 19th century, however, this haphazard system of chemical education changed. Many provincial universities were established in Germany, a country with a long tradition of research. A research center in chemistry was set up at Giessen by the German chemist Justus Liebig. This first teaching laboratory became so successful that it drew students from all over the world; other German universities soon followed.
A large group of young chemists was thus trained just at the time when chemical industries were beginning to exploit new discoveries. This exploitation had its start during the Industrial Revolution; the Leblanc process for the production of soda, for example—one of the first large-scale production processes—was developed in France in 1791 and was commercialized in England beginning in 1823. The laboratories of such growing industries were able to employ the newly trained chemistry students and also to use university professors as consultants. This interplay between the universities and the chemical industry benefited both of them, and the accompanying rapid growth of the organic chemical industry toward the end of the 19th century created the great German dye and pharmaceutical trusts that gave Germany scientific predominance in the field until World War I.
After the war, the German system was introduced into all the industrial nations of the world, and chemistry and chemical industries progressed even more rapidly. Among some of the more recent industrial developments, increasing use has been made of enzymatic reaction processes (see Enzyme), mainly because of the low costs and high yields that can be achieved. Industries are at present studying production methods using genetically altered microorganisms for industrial purposes (see Genetic Engineering).
E Chemistry and Society
Chemistry has had an enormous influence on human life. In earlier periods chemical techniques were used to isolate useful natural products and to find new ways to employ them. In the 19th century techniques were developed for synthesizing completely new substances that were either better than the natural ones or could completely replace them more cheaply. As the complexity of synthesized compounds increased, wholly new materials with novel uses began to appear. Plastics and new textiles were developed, and new drugs conquered whole classes of disease. At the same time, what had been entirely separate sciences began to be drawn together. Physicists, biologists, and geologists had developed their own techniques and ways of looking at the world, but now it became evident that each science, in its own way, was the study of matter and its changes. Chemistry lay at the base of each of them. The resulting formation of such interscientific disciplines as geochemistry or biochemistry has stimulated all of the parent sciences.
The progress of science in recent years has been spectacular, although the benefits of this progress have not been without some corresponding liabilities. The most obvious dangers come from radioactive materials, with their potential for producing cancers in exposed individuals and mutations in their children. It has also become apparent that the accumulation in plant and animal cells of pesticides once thought harmless or of by-products from manufacturing processes often have damaging effects. These dangerous materials have been manufactured in enormous amounts and dispersed widely, and it has become the task of chemistry to discover the means by which these substances can be rendered harmless. This is one of the greatest challenges science will have to meet. See also Environment.
Contributed By:
Henry M. Leicester
Reviewed By:
Arthur B. Ellis
Aaron Ihde
