The Greek philosopher Empedocles, a native of Sicily, introduces a theory which will be accepted in Europe until the 17th century. He states that all matter is made up, in differing proportions, of four elemental substances - earth, air, fire and water. Not until the arrival of a 'sceptical chemist' (the title of a book by Robert Boyle in 1661) is there a serious threat to this Greek theory of the elements.
Soon an equivalently simple notion is put forward to account for the make-up of living creatures, in the theory of the four humours.
Democritus and the atom: c.420 BC
He states that all matter is composed of eternal, indivisible, indestructible and infinitely small substances which cling together in different combinations to form the objects perceptible to us. The Greek word for indivisible is atomos. This theory gives birth to the atom.
Aristotle's variable atoms: 4th century BC
Aristotle, practical as ever in his determination to get things worked out in detail, proposes a new theory to explain how the four elements of Empedocles and the atoms of Democritus produce the wide range of substances apprehended by our senses.
He suggests that there are two pairs of alternatives - hot and cold, moist and dry - which provide the exact nature of matter. In broad terms the four possible combinations are the four elements: earth (cold and dry), air (hot and moist), fire (hot and dry), water (cold and moist). But it is the infinitely variable balance between these qualities which creates the different atoms of stone or wood, bone or flesh.
Greek science in Alexandria: from the 3rd century BC
This limitation is removed when the centre of the Greek world transfers, in the 3rd century BC, to Alexandria. In this bustling commercial centre, linked with long Egyptian traditions of skilled work in precious metals, people are interested in making practical use of Greek scientific theory. If Aristotle says that the difference in material substances is a matter of balance, then that balance might be changed. Copper might become gold.
Among the practical scientists of Alexandria are men who can be seen as the first alchemists and the first experimental chemists. Their trade, as workers in precious metals, involves melting gold and silver, mixing alloys, changing the colour of metals by mysterious process.
These are the activities of chemistry. The everyday items of a chemical laboratory - stills, furnaces, flasks - are all in use in Alexandria.
There are strong mystical influences in Egypt, some of them deriving from Babylonian astrology, and this tradition too encourages experiment. Astrologers believe in many hierarchies, among the planets in the heavens but also among metals in the earth. Lead is the lowest of the metals, gold the highest. Left to itself, out of sight in the earth, lead may slowly be transformed up the scale to achieve ultimate perfection as gold.
If this process could be accelerated, in the back of a jeweller's shop, there would be certain immediate advantages. In the early centuries, the experiments of chemistry and alchemy go hand in hand.
Alchemy in Asia: 8th - 10th century AD
There are two important centres of alchemical experiment in medieval Asia. One is Baghdad under the caliphate, where from the 8th century there is enthusiastic translation and study of Greek scientific texts. Arab alchemists, in their pursuit of synthesized gold, make practical advances in techniques of distillation. And they identify several chemical substances.
The other great centre is China, where alchemical experiments have a slightly different purpose. The quarry is still gold, but as an elixir of eternal life. This is the pursuit of the Daoists (one of whom describes, with gentle irony, an Experiment which goes wrong in the 9th century). It is Daoists who make the most startling chemical discovery of the period - gunpowder.
Gunpowder: 10th century
At this early stage in China the military use of gunpowder is limited to grenades and bombs lobbed at the enemy from catapults. Its real destructive force will only emerge when the explosion is confined, in the development of artillery.
Science's siesta: 8th - 15th century AD
In the profoundly Christian centuries of the European Middle Ages the prevailing mood is not conducive to scientific enquiry. God knows best, and so He should - since He created everything. Where practical knowledge is required, there are ancient authorities whose conclusions are accepted without question - Ptolemy in the field of astronomy, Galen on matters anatomical.
A few untypical scholars show an interest in scientific research. The 13th-century Franciscan friar Roger Bacon is the most often quoted example, but his studies include alchemy and astrology as well as optics and astronomy. The practical scepticism required for science must await the Renaissance.
Van Helmont: AD 1648
A book is published in Amsterdam in 1648 which can be seen as a definitive turning point between alchemy and chemistry. Entitled Ortus Medicinae (Origin of Medicine), it is the collected papers of Jan Baptista van Helmont, an aristocrat who has lived quietly on his estate near Brussels conducting scientific experiments.
Van Helmont is inclined to mysticism. He believes in alchemy and in the philosopher's stone which, if found, could turn base metals into gold. But he also conducts experiments on entirely scientific principles. Some, like his famous five-year project with a willow tree, lead him to the wrong conclusion. But the method is valid.
Van Helmont weighs out 200 lbs of dried earth, places it in an earthenware container and plants a willow tree weighing 5 lbs. For five years he waters the plant daily. At the end of the experiment the willow tree weighs 169lbs and the earth, when dried, not much less than 200 lbs. Van Helmont concludes, reasonably that the wood, bark and leaves of the tree must be composed of water, which he therefore considers to be the chief constituent of all matter.
He is half right - any willow tree is about 50% water. What van Helmont is unaware of is that the tree has also absorbed carbon and oxygen, as carbon dioxide or CO2, from the air.
Ironically, Van Helmont himself becomes the first scientist to postulate the existence of carbon dioxide. He burns 62 lbs of charcoal and finds that he is left with only 1 lb of ash. What has happened to the rest? Van Helmont is convinced, ahead of his time, of the indestructibility of matter. Indeed he is able to demonstrate that metal dissolved in acid can be recovered without loss of weight.
So he now reasons that the missing 61 lbs have escaped in the form of an airy substance to which he gives the name gas sylvestre (wood gas).
The identity of this wood gas is not discovered until a century later (by Joseph Black), but van Helmont is the first to have suggested the existence of gaseous substances other than air. It is he who coins the word 'gas' - deriving it from Chaos (sounding similar in Flemish), which is used in Greek mythology to mean the original emptiness before creation.
The principles of experiment enter chemistry in the work of van Helmont, and are developed by another aristocrat fascinated by the puzzles of science - Robert Boyle.
Robert Boyle: AD 1661-1666
But Boyle also uses the air pump to make significant discoveries of his own - most notably that reduction in pressure reduces the boiling temperature of a liquid (water boils at 100° at normal air pressure, but at only 46°C if the pressure is reduced to one tenth).
Boyle's best-known experiment involves a U-shaped glass tube open at one end. Air is trapped in the closed end by a column of mercury. Boyle can show that if the weight of mercury is doubled, the volume of air is halved. The conclusion is the principle known still in Britain and the USA as Boyle's Law - that pressure and volume are inversely proportional for a fixed mass of gas at a constant temperature.
Boyle's most famous work has a title perfectly expressing a correct scientific attitude. The Sceptical Chymist appears in 1661. Boyle is properly sceptical about contemporary theories on the nature of matter, which still derive mainly from the Greek theory of four elements.
His own notions are much closer to the truth. Indeed it is he who introduces the concept of the element in its modern sense, suggesting that such entities are 'primitive and simple, or perfectly unmingled bodies'. Elements, as he imagines them, are 'corpuscles' of different sorts and sizes which arrange themselves into compounds - the chemical substances familiar to our senses. Compounds, he argues, can be broken down into their constituent elements. Boyle's ideas in this field are further developed in his Origin of Forms and Qualities (1666).
Chemistry is Boyle's prime interest, but he also makes intelligent contributions in the field of pure physics.
In an important work of 1663, Experiments and Considerations Touching Colours, Boyle argues that colours have no intrinsic identity but are modifications in light reflected from different surfaces. (This is demonstrated within a few years by Newton in his work on the spectrum.)
As a man of his time, Boyle is as much interested in theology as science. It comes as a shock to read his requirements for the annual Boyle lecture which he founds in his will. Instead of discussing science, the lecturers are to prove the truth of Christianity against 'notorious infidels, viz., atheists, theists, pagans, Jews and Mahommedans'. The rules specifically forbid any mention of disagreement among Christian sects.
The phlogiston theory: 18th century AD
Two natural processes, burning and rusting, particularly intrigue the chemists of the 17th and 18th centuries. A concept is put forward in 1667 in Germany in a book by Johann Joachim Becher. explaining such changes as the release of a particular substance, present in all materials which are capable of changes of this kind. The theory was developed by George Ernst Stahl, who in a 1702 edition of Becher's work gave the mystery substance the name phlogiston - from the Greek phlogizein, to set alight.
Stahl is correct in his link between burning and rusting, for each depends on oxidization (a chemical reaction with oxygen). But experimental evidence immediately provides a stumbling block for the phlogiston theory.
If phlogiston is a substance released both in burning and rusting, then the resulting ash and rust should weigh either the same as or less than the weight lost by the original object (there is much debate as to the weight or weightlessness of phlogiston). But experiments reveal that oxygen-rich rust is heavier than unrusted iron, while ash is much lighter than the burnt organic material. Yet this difficulty is not enough to prevent most scientists believing in the existence of phlogiston, until Lavoisier and the discovery of oxygen finally disprove the case.
Demons in the ore: AD 1742-1751
From the mid-18th century there is rapid acceleration in the discovery of new elements, as chemists improve their analytical methods in the laboratory. These substances are not at first recognized as elements (a concept only firmly established in the 19th century), but in each case it is evident that a previously unidentified material has been isolated.
Two of the earliest in this series of discoveries take place in Sweden. Both involve the analysis of familiar metallic ores, and both acquire their lasting names from the superstitions of German miners.
Miners in the Harz mountains have often been frustrated by a substance which appears to be copper ore but which, when heated, yields none of the expected metal. Even worse, it emits noxious fumes. The miners blame this on the influence of a spirit, the mischievous kobold, and the name becomes attached to this kind of ore.
The only use found for the residue of such ore after roasting is in the making of glass, to which it adds a beautiful blue colour. In about 1735 Georg Brandt is able to show in his Swedish laboratory that the blue derives from a previously unknown metal. The mischievous spirit has been identified, and Brandt gives its name to the new substance - as cobalt.
A similar demon is blamed by miners in Saxony for another ore which yields a brittle substance instead of copper. In German a Nickel is a dwarfish troublemaker, and the miners call the disappointing ore Kupfernickel (copper scamp). The impurity in ore of this type is analyzed in Sweden in 1751 by Axel Cronstedt. He identifies its components as arsenic and a previously unknown hard white metal, quite distinct from copper. Following the example of Brandt, he honours the offending demon in the naming of the new substance and calls it nickel.
Several other new metallic elements are isolated in the following decades. But the main focus of research moves now to the gases.
Joseph Black and fixed air: AD 1754-1756
Joseph Black presents his doctoral thesis to the university of Edinburgh in 1754 and publishes it in expanded form two years later as Experiments upon Magnesia Alba, Quicklime, and Some Other Alcaline Substances. The experiments which he describes are a classically complete series of compound transformations of calcium, carbon and oxygen - though it is not as yet possible to express his results in these terms.
Black has observed that if he heats chalk (calcium carbonate), he gets quicklime (calcium oxide) and a gas, the presence of which he can identify by its weight. Unwilling as yet to speculate on its identity, he calls it fixed air - because it exists in solid form until released.
As a next stage, Black demonstrates that he can reverse the process. Mixing water with the quicklime, he gets a substance (slaked lime) whch will take up the fixed air again - leaving him with his original amount of chalk and the water.
In other experiments Black is able to show that this same unknown gas, his fixed air, is produced as a result of burning charcoal, of fermentation and of breathing. He demonstrates this last point to his students by breathing through a tube into a jar of limewater (a clear solution of slaked lime). The liquid turns cloudy as grains of chalk form in it.
Black's fixed air is the gas sylvestre of which the existence has been postulated by van Helmont a century earlier. Its composition as carbon dioxide is not discovered until the 1780s, when Lavoisier achieves it by burning carbon in oxygen.
Black's proof that such a gas exists prompts an energetic search for others. Hydrogen is identified by Cavendish in 1766, and oxygen almost simultaneously by Scheele and Priestley in the 1770s. Meanwhile Black has observed another important scientific principle, latent heat.
Cavendish and hydrogen: AD 1766
In 1766 Henry Cavendish presents his first paper to the Royal Society. Under the title Factitious Airs he describes his experiments with two gases. One is the 'fixed air' identified by Joseph Black. The other is a gas which Cavendish calls 'inflammable air', soon to be given the name hydrogen by Lavoisier.
Hydrogen has been observed as a phenomenon for at least two centuries. The 16th-century alchemist and charlatan Paracelsus finds that the dissolving of a metal in acid releases a form of air which will burn. But Cavendish is the first to identify it as specific substance. He believes that he has found the inflammable essence, phlogiston.
The study of gases in the laboratory is by now a standard chemical process thanks to the pneumatic trough developed in the early part of the 18th century by Stephen Hales. An upturned vessel, full of water, stands in a shallow trough of water. Gas is collected in the top of the vessel, displacing water and being sealed in by it. With this device Cavendish is able to calculate the specific gravity of hydrogen.
He finds that it is one fourteenth that of common air (it is the lightest substance known). Within less than two decades of his observation, a dramatic use is found for this very light new gas - in ballooning.
Priestley and oxygen: AD 1774
Joseph Priestley, a nonconformist minister, is employed as librarian from about 1773 in an English nobleman's house, Bowood in Wiltshire. He is provided with a laboratory to carry out his chemical researches. And he has recently acquired a large 12-inch lens, with which he can focus intense heat on chemical substances.
In August 1774 he directs his lens at some mercury oxide. He discovers that it gives off a colourless gas in which a candle burns with an unusually brilliant light. Experimenting further with this gas, he records a few months later that 'two mice and myself have had the privilege of breathing it'. The mice were presumably offered the privilege first.
Priestley has isolated oxygen. He foresees a medical use for it ('it may be peculiarly salutary for the lungs in certain cases'), but he does not fully appreciate its chemical significance - largely because he believes in the phlogiston theory. He calls the new gas 'dephlogisticated air', on the assumption that the phlogiston has been removed from it.
In October 1774, visiting Paris with his noble patron, he describes his discovery to a gathering of French scientists. Among them is Lavoisier, who develops Priestley's experiments in his own laboratory and realizes that he has the evidence to disprove the phlogiston theory.
Priestley meanwhile isolates a great many other gases. Though he is the first to publish his discovery of oxygen, he has in fact been preceded in the identification of both oxygen and nitrogen by the Swedish chemist Carl Wilhelm Scheele.
Scheele separates air in 1773 into two gases which he calls 'fire air' (oxygen) and 'foul air (nitrogen). His findings only become known with the publication of his book Air and Fire in 1777, but it is established that the experiments date from four years earlier. Like Priestley, Scheele is handicapped by his belief in the phlogiston theory. When isolating hydrogen, he concludes - as has Cavendish - that it is pure phlogiston.
Cavendish and water: AD 1784
During the last three decades of the 18th century, with more and more chemical substances becoming identified, there is great interest in which of them may be elements - in Boyle's sense of being pure substances unmixed with anything else. Of the four ancient Greek elements, earth is clearly no longer a candidate. Air is separated in 1773 by Scheele into oxygen and nitrogen. Water receives its dismissal from the club at Cavendish's hands in a paper entitled Experiments in Air (1784).
Cavendish mixes hydrogen and oxygen, in the proportion 2:1, in a glass globe through which he passes an electric spark. The resulting chemical reaction leaves him with water, which stands revealed as a compound (H2O).
Lavoisier: AD 1777-1794
Although Antoine Laurent Lavoisier has no single glamorous discovery to add lustre to his name (such as postulating the first gas, or identifying oxygen), he is regarded as the father of modern chemistry. The reason is that during the last two decades of the 18th century he interprets the findings of his colleagues with more scientific clarity than they have mustered, and creates the rational framework within which chemistry can develop.
He gives evidence of this in his response to Priestley's discovery of 'dephlogisticated air'. He undertakes a series of experiments which reveal the involvement of this new gas in the processes where phlogiston has been assumed to play a key role.
He is able to show that Priestley's gas is involved in chemical reactions in the processes of burning and rusting, and that it is transformed in both burning and breathing into the 'fixed air' discovered by Joseph Black. His researches with phosphorus and sulphur cause him to believe that the new gas is invariably a component of acids. He therefore gives it in 1777 the name oxygen (from the Greek for 'acid maker'). On a similar principle Lavoisier coins the word hydrogen ('water maker') for the very light gas isolated by Cavendish.
With these two names chemistry takes a clear and decisive step into the modern era. It is an advance which Lavoisier soon consolidates.
With three other French colleagues Lavoisier publishes in 1787 Méthode de nomenclature chimique (Method of Chemical Nomenclature). Their scheme, soon universally accepted, sweeps away the muddled naming of substances which has descended from alchemy and replaces it with a logical system of classification. This is an achievement of French rationalism comparable to the metric system, in the planning of which Lavoisier is also involved.
In 1789 Lavoisier follows this book on chemical methodology with the related fruits of his own researches - Traité élémentaire de chimie (Elementary Treatise of Chemistry). In this he attempts a list of the known elements.
Lavoisier names more than thirty elements, which he defines - in the tradition begun by Boyle a century earlier - as substances which can be broken down no further by any known method of analysis. The majority are metals, but there are by now three gases which Lavoisier identifies as elements - oxygen, hydrogen and nitrogen (which he calls azote, 'without life').
Lavoisier is immensely active in public affairs, in addition to his scientific work. Unfortunately his tasks have included, under the ancien régime, membership of the ferme générale or tax authority.
By the time of the Terror, in 1794, it makes little difference that Lavoisier has been on the liberal and reformist side on contemporary issues. An order is given for the arrest of all the former members of the ferme générale. In May 1794, in a trial lasting only part of a day, twenty-eight of them including Lavoisier appear before a revolutionary tribunal. Condemned to death, they are guillotined that same afternoon.
A colleague of Lavoisier, who has worked with him on the commission to introduce the metric system, comments: 'It took only a moment to cut off that head; a century may not be enough to produce another like it.'
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