For about two centuries after Boyle, scientists continued to make useful discoveries but made little progress in understanding the true nature of matter or chemical behavior. Perhaps the greatest source of confusion and defeat in these centuries was a theory of burning (combustion) called the phlogiston theory. It was originated by the German chemists Johann Joachim Becher and Georg Ernst Stahl in the late 1600s. According to this theory, phlogiston, an “essence” like yellowness or hardness in the theories of the ancient philosophers, escaped from substances during the burning process. By this time, chemists were learning to gain knowledge the modern way: by testing theories with experiments (seescience). But such tests failed to confirm the existence of phlogiston.
The first clue to a more useful theory came when an English chemist, Joseph Priestley, discovered in 1774 that a gas (now known as oxygen) was essential to the burning process. (Oxygen was also discovered by the Swedish chemist Carl Wilhelm Scheele at about the same time.) A few years earlier another English scientist, Henry Cavendish, had identified hydrogen as an element. The French chemist Antoine-Laurent Lavoisier used the discoveries of Priestley and Cavendish in a series of experiments from which he formulated the presently accepted theory of combustion. He also showed that burning, the rusting of metals, and the breathing of animals are all processes in which oxygen combines chemically with other substances. Lavoisier's most significant finding was that the products of a chemical reaction have the same total mass as the reactants, no matter how much the substances are changed. This means that, even when chemical changes take place, something essential stays the same. These contributions are often considered to mark the beginning of modern chemistry.
The span of time from about the beginning of the 1st century AD to about the 17th century is considered the period of alchemy. The alchemists believed that metals could be converted into gold with the aid of a marvelous mineral called the philosopher's stone, which they never succeeded in finding or making. They did discover new elements, and they invented basic laboratory equipment and techniques that are still used by chemists. However, the alchemists learned very little that was worthwhile concerning the fundamental nature of matter or of chemical behavior. They failed because their basic theories had almost nothing to do with what actually happens in chemical reactions.
In the 13th century such men as Roger Bacon, Albertus Magnus, and Raymond Lully began to realize how futile it was to search for the philosopher's stone. They suggested that alchemists might rather seek to help the world with useful new products and methods.
In the 16th century, another important leader in the new trend was Theophrastus Bombastus von Hohenheim, an aggressive, talented Swiss who used the Latin name Paracelsus. He insisted that the object of alchemy should be the cure of the sick. The elements, he said, were salt, sulfur, and mercury (long connected with the “elixir of life,” another nonexistent alchemical substance), and they would give health if present in the body in the proper proportions. On this basis he practiced medicine and attracted many followers. Thus began iatrochemistry, or chemistry applied to the study of medicine and the treatment of disease.
One of the first scientific chemists was Robert Boyle. In 1660 he helped found one of the first scientific organizations in Europe, the Royal Society of London. In a book called The Sceptical Chymist (1661) he rejected previous theories of the composition of matter and compiled the first list of the elements that are recognized today. He also discovered the relationship between the volume and the pressure of a gas.
Modern chemistry is only about two centuries old. The earlier history of chemistry may be divided into three periods: magic; alchemy; and “primitive modern,” a period of transition between alchemyand truly modern chemistry.
The Period of Magic
The period of magic extended from prehistoric times to about the beginning of the 1st century AD. Most people believed that natural processes were controlled by spirits, and they relied on magic to persuade the spirits to help while they conducted practical operations. Very little progress was made toward understanding how the universe is made, but much practical knowledge was gathered. Perhaps 9,000 years ago, people devised reliable techniques for making and sustaining fire. Gradually they learned to use fire to harden pottery, extract metals from ores, make alloys, and develop materials such as glass. Certain elements that occur naturally in a pure state, such as gold, copper, and sulfur, were recognized and valued for their properties. This was the period of the Sumerian, Babylonian, Egyptian, and Greek cultures.
About 400 BC the Greek philosopher Democritus theorized that all matter was made up of tiny, indivisible units he called atoms, but his idea was not based on scientific evidence. Other Greek philosophers, including Thales and Aristotle, also speculated on the nature of matter, though their theories, too, had little in common with modern chemical knowledge. They believed that earth, air, fire, and water (some imagined a fifth substance called “quintessence”) were the basic elements of all matter. They speculated on the possibility of removing such qualities as hardness, heat or cold, and color from common materials and combining them to make rarer or more valuable substances. They knew that iron could be drawn from a dirty, brown earthen rock and that bronze was made by combining copper and tin. Therefore it seemed possible that if yellowness, hardness, and other qualities could be properly combined, the product would be gold. Such speculations gave rise to alchemy.
