A little bit of engineering (562404), страница 8

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Chemistry, History of

Chemistry, History of, 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.

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.

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.

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.

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 Baghdād 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.

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.

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.

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.

Boyle’s Law

Boyle’s law, developed by English scientist Robert Boyle, states that the pressure of a gas times its volume is equal to a constant number, for a gas at a constant temperature. This relationship means that pressure increases as volume decreases, and vice versa. In this graph, the product of pressure and volume anywhere along one of the lines of constant temperate should be equal.

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).

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.

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.

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