The Elements

Metadata

• Author: Philip Ball

Highlights

1624 the French chemist Étienne de Clave was arrested for heresy. De Clave’s inadmissible ideas did not concern the interpretation of holy scripture. Nor were they of a political nature. They did not even challenge the place of man in the universe, as Galileo was doing so boldly. — location: 204

He believed that all substances were composed of two elements – water and earth – and ‘mixts’ of these two with three other fundamental substances or ‘principles’: mercury, sulphur, and salt. It was not a new idea: the great French pharmacist Jean Béguin, who published Tyrocinium chymicum (The Chemical Beginner), one of the first chemistry textbooks, in 1610, maintained until his death a decade later that all matter had essentially those same five basic ingredients. — location: 206

it contradicted the system of elements propounded by the ancient Greeks and endorsed by Aristotle, their most influential philosopher — location: 211

Aristotle took this scheme from his teacher Plato, who in turn owed it to Empedocles, a philosopher who lived during Athens’s Golden Age of Periclean — location: 211

democracy in the fifth century BC. According to Empedocles there were four elements: earth, air, fire, and water. — location: 212

The word of Aristotle became imbued with God’s authority, and to question it was tantamount to blasphemy. Not until the late seventeenth century did the discoveries of Galileo, Newton, and Descartes restore the Western world’s ability to think for itself about how the universe was arranged. — location: 215

Free of such constraints, the ancient Greeks themselves discussed the elements with far more — location: 223

The Aristotelian quartet was preceded by, and in fact coexisted with, several other elemental schemes. Indeed, in the sixteenth century the Swiss scholar Conrad Gesner showed that no fewer than eight systems of elements had been proposed between the times of Thales (the beginning of the sixth century BC) and Empedocles. — location: 224

Sini ke atas dah compiled ke Elemen dasar di awal peradaban_1

What are things made from? The Periodic Table is one of the pinnacles of scientific achievement, but it does not quite do justice to that question. — location: 230

Set aside the fact that the atomic building blocks are actually more subtly varied than the table implies (as we shall see later). Forget for a moment that these atoms are not after all fundamental and immutable, but are themselves composites of other entities. — location: 231

And make it a matter for discussion elsewhere that the atoms of the elements are more often than not joined into the unions called molecules, whose properties cannot be easily intuited from the nature of the elements themselves. — location: 234

Lead in petrol shows up in the snow fields of Antarctica; mercury poisons fish in South America. Radon from the earth poses health hazards in regions built on granite, and natural arsenic contaminates wells in Bangladesh. Calcium supplements combat bone-wasting diseases; iron alleviates anaemia — location: 240

There are elements that we crave, and those we do our best to avoid. — location: 242

four of them are endlessly permuted in the molecules of the body: carbon, nitrogen, oxygen, and hydrogen. Phosphorus is indispensable, not only in bone but in the DNA molecules that orchestrate life in all its forms. Sulphur is an important component of proteins, helping to hold them in their complex shapes. But beyond these key players is a host of others that life cannot do without. Many are metals: iron reddens our blood and helps it to transport oxygen to our cells, magnesium enables chlorophyll to capture the energy of sunlight at the foot of the food pyramid, sodium and potassium carry the electrical impulses of our nerves. — location: 243

Toxic’ arsenic and ‘sterilizing’ bromine are among them, showing that there is no easy division of elements into ‘good’ and ‘bad’.) — location: 249

The uneven distribution of elements across the face of the earth has shaped history – stimulating trade and encouraging exploration and cultural exchange, but also promoting exploitation, war, and imperialism. — location: 250

Southern Africa has paid dearly for its gold and the elemental carbon of its diamonds. Many rare but technologically important elements, such as tantalum and uranium, continue to be mined from poor regions of the world under conditions (and for reasons) that some consider pernicious and hazardous. — location: 252

only with the development of new ultrasensitive techniques of chemical analysis have we become alerted to the complexity with which they are blended in the world, — location: 255

And so today’s bottles of mineral water list their proportions of sodium, potassium, chlorine, and much else, banishing the notion that all we are drinking is H2O. — location: 257

Copper salts can be toxic, but copper bracelets are rumoured to cure arthritis. We take selenium supplements to boost fertility, while selenium contamination of natural waters devastates Californian ecosystems. Which of us can say whether 0.01 milligrams of potassium in our bottled water is too little or too much? — location: 262

Thales of Miletus (c.620–c.555 BC), one of the first known enquirers into the constitution of the physical world, posited only one fundamental substance: water. There is ample justification for this view in myth; the Hebrew god was not the only deity to bring forth the world from a primal ocean. — location: 275

Anaximander (c.611–547 BC), Thales’ successor, avoided the issue with his contention that things are ultimately made of apeiron, the ‘indefinite’ and unknowable first substance. Anaximenes (d. c.500 BC) decided that air, not water, was primary. For Heraclitus (d. 460 BC), fire was the stuff of creation. — location: 279

But the Milesian school of philosophers that Thales founded produced little consensus about the prote hyle or ‘first matter’ that constituted everything. — location: 278

Why not simply conclude that rock is rock, wood is wood? — location: 283

Why not accept them at face value, rather than as manifestations of something else? — location: 284

these ancient savants were searching for unity: to reduce the multifarious world to a simpler and less puzzling scheme. — location: 285

but there is also a practical reason to invoke fundamental elements: things change. Water freezes or boils away. Wood burns, transforming a heavy log to insubstantial ashes. Metals melt; food is ingested and most of it is somehow spirited away inside the stomach. — location: 287

To this end, Anaximander believed that change came about through the agency of contending opposite qualities: hot and cold, and dry and moist. When Empedocles (c.490–c.430 BC) postulated the four elements that gained ascendancy in Western natural philosophy, he too argued that their transformations involved conflict. — location: 292

Empedocles does not exactly fit the mould of a sober and dignified Greek philosopher. Legend paints him as a magician and miracle worker who could bring the dead back to life. Reputedly he died by leaping into the volcanic maw of Mount Etna, convinced he was an immortal god. — location: 295

Small wonder, perhaps, that his earth, air, fire, and water were wrought into different blends – the materials of the natural world – through the agency of the colourful principles Love and Strife. Love causes mixing; Strife, separation. Their conflict is an eternal waxing and waning: at one time, Love dominates and things mix, but then Strife arises to pull them apart. This applies, said Empedocles, not just to the elements but to the lives of people and cultures. — location: 296

Aristotle shared Anaximander’s view that the qualities heat, cold, wetness, and dryness are the keys to transformation, and also to our experience of the elements. It is because water is wet and cold that we can experience it. Each of the elements, in Aristotle’s ontology, is awarded two of these qualities, so that one of them can be converted to another by inverting one of the qualities. Wet, cold water becomes dry, cold earth by turning wetness to dryness (Fig. 1 — location: 304

philosophers as belonging to a kind of gentleman’s club whose members are constantly borrowing one another’s ideas, — location: 308

heaping lavish praise or harsh criticism on their colleagues, while all the while remaining ‘armchair’ scientists who decline, by and large, to dirty their hands through experiment. — location: 309

Leucippus of Miletus (fifth century BC) is generally credited with introducing the concept of atoms, but we know little more about him than that. He maintained that these tiny particles are all made of the same primal substance, but have different shapes in different materials. His disciple Democritus (c.460–370 BC) called these particles atomos, meaning uncuttable or indivisible. — location: 311

Democritus reconciled this fledgling atomic theory with the classical elements by positing that the atoms of each element have shapes that account for their properties. — location: 314

Fire atoms are immiscible with others, but the atoms of the other three elements get entangled to form dense, tangible matter. — location: 315

Democritus supposed that atoms move about in a void. Other philosophers ridiculed this idea of ‘nothingness’, maintaining that the elements must fill all of space. Anaxagoras (c.500–428 BC), who taught both Pericles and Euripides in Athens, claimed that there was no limit to the smallness of particles, so that matter was infinitely divisible. This meant that tiny grains would fill up all the nooks between larger grains, like sand between stones. Aristotle asserted – and who can blame him? – that air would fill any void between atoms. (This becomes a problem only if you consider that air is itself made of atoms.) — location: 320

Plato had it all figured out neatly. He was not an atomist in the mould of Democritus, but he did conceive of atom-like fundamental particles of the four Empedoclean elements. — location: 325

His geometrical inclinations led him to propose that these particles had regular, mathematical shapes: the polyhedra called regular Platonic solids. Earth was a cube, air an octahedron, fire a tetrahedron and water an icosahedron. — location: 326

The flat faces of each of these shapes can be made from two kinds of triangle. These triangles are, according to Plato, the true ‘fundamental particles’ of nature, and they pervade all space. The elements are converted by rearranging the triangles into new geometric forms. — location: 327

There is a fifth Platonic regular solid too: the dodecahedron, which has pentagonal (five-sided) faces. This polyhedron cannot be made from the triangles of the other four, which is why Plato assigned it to the heavens. There is thus a fifth classical element, which Aristotle called the aether. But it is inaccessible to earthly beings, and so plays no part in the constitution of mundane matter. — location: 330

This fourness of fundamental principles reaches further, embracing the four points of the compass (Chinese tradition acknowledges five elements, and five ‘directions’) and the four ‘humours’ of classical medicine. According to the Greek physician Galen (AD c.130–201), our health depends on the balance of these four essences: red blood, white phlegm, and black and yellow bile. — location: 344

What this really means is that the classical elements are familiar representatives of the different physical states that matter can adopt. Earth represents not just soil or rock, but all solids. Water is the archetype of all liquids; air, of all gases and vapours. Fire is a strange one, for it is indeed a unique and striking phenomenon. Fire is actually a dancing plasma of molecules and molecular fragments, excited into a glowing state by heat. It is not a substance as such, but a variable combination of substances in a particular and unusual state caused by a chemical reaction. In experiential terms, fire is a perfect — location: 363

symbol of that other, intangible aspect of reality: light. — location: 368

With Aristotle’s endorsement, the Empedoclean elements thrived until the seventeenth century. — location: 376

The Greek philosopher Epicurus (341–270 BC) established an atomistic tradition that was celebrated in 56 BC by the Roman poet Lucretius in his tract De rerum natura (On the Nature of — location: 377

Things). — location: 379

This atomistic poem was condemned by religious zealots in the Middle Ages, and barely escaped complete destruction. But it surfaced in the seventeenth century as a major influence on the French scientist Pierre Gassendi (1592–1655), whose vision of a mechanical world of atoms in motion represented one of the many emerging challenges to the Aristotelian orthodoxy. — location: 379

It may seem strange from today’s perspective that several of the substances recognized today as elements – the metals gold, silver, iron, copper, lead, tin, and mercury – were not classed as such in antiquity, even though they could be prepared in an impressively pure state. — location: 385

Metals, with the exception of fluid mercury, were considered simply forms of Aristotelian ‘earth’. — location: 388

Gold and copper are the oldest known metals, since they occur in their pure, elemental forms in nature. — location: 393

There is evidence of the mining and use of gold in the region of Armenia and Anatolia from before 5000 BC; copper use is similarly ancient in Asia. Copper mostly occurs not as the metal, however, but as a mineral ore: a chemical compound of copper and other elements, such as copper carbonate (the minerals malachite and azurite). These copper ores were used as pigments and colouring agents for glazes, — location: 394

is likely that copper smelting, which dates from around 4300 BC, arose from a happy accident during the glazing of stone ornaments called faience in the Middle East. — location: 397

Lead was smelted from one of its ores (galena) since around 3500 BC, but was not common until 1,000 years later. Tin seems to originate in Persia around 1800–1600 BC, and iron in Anatolia around 1400 BC. — location: 399

This sequence of discovery of the metals reflects the degree of difficulty in separating the pure metal from its ore: iron clings tightly to oxygen in the common mineral ore haematite (ochre), and intense heat and charcoal are needed to prise them apart. — location: 401

TABLE 1 The seven ‘classical’ metals and their correspondences — location: 408

Attempts to transmute other metals to gold may have been made as long ago as the Bronze Age. But after the eighth century AD they were no — location: 411

longer haphazard; they had a theoretical underpinning in the sulphur-mercury theory of the Arabic alchemist Jabir ibn Hayyan. Jabir is more the name of a school of thought than of a person. Many more writings are attributed to him than he could possibly have written, and there is some doubt about whether he existed at all. The Jabirian tradition works curious things with the Aristotelian elements. It accepts them implicitly but then, so far as metals are concerned, adds another layer between these fundamental substances and reality. — location: 412

According to Jabir, the ‘fundamental qualities’ of metals are the Aristotelian hot, cold, dry, and moist. But the ‘immediate qualities’ are two ‘principles’: sulphur and mercury. All metals are deemed to be mixtures of sulphur and mercury. — location: 416

mixtures of this sulphur and mercury yield not gold but the Holy Grail of alchemy, the Philosopher’s Stone, the smallest quantity of which can transform base metals to gold. — location: 419

It marks the beginning of a tendency to pay lip service to Aristotle while getting on with more practical concerns about what things are made of. — location: 425

The next step away from the traditions of antiquity involved the addition of a third ‘principle’ to Jabir’s sulphur and mercury: salt. — location: 426

The three-principle theory is generally attributed to the Swiss alchemist Paracelsus (1493–1541), although it is probably older. Paracelsus asserted that sulphur, salt, and mercury ‘form everything that lies in the four elements’. — location: 428

By the end of the seventeenth century, things had moved on again. There was no longer any perceived obligation to square one’s views with Aristotle, and the ‘principles’ were widely regarded as elements in their own right. Jean Béguin listed a popular scheme of five elements: mercury, sulphur, salt, phlegm, and earth. — location: 431

He claimed that none of them was pure – each contained a little of the others. — location: 433

the early chemists of the seventeenth — location: 442

century began trying to understand matter by practical means. — location: 442

In their endless quest for the Philosopher’s Stone, alchemists burnt, distilled, melted, and condensed all manner of substances and stumbled across many technologically important new compounds, such as phosphorus and nitric acid. But in the 1600s there appeared a transitional group of natural philosophers whose primary objective was no longer to conduct the Great Work of alchemical transformation but to study and understand matter at a more mundane level. — location: 443

These ‘chymists’ were neither alchemists nor chemists; or, rather, they were a bit of both. One of them was Robert Boyle (1627–91). — location: 446

The Eton-educated son of an Irish aristocrat, Boyle became part of the innermost circle of British science in the mid-seventeenth century. — location: 448

He was on good if not intimate terms with Isaac Newton (hardly anyone was intimate with Newton), and was involved in the founding of the Royal Society in 1661. — location: 449

Traditionally portrayed as a broadside against alchemy in general, Boyle’s classic book The Sceptical Chymist (1661) in fact aims to distinguish the learned and respectable alchemical ‘adepts’ (such as Boyle himself) from the ‘vulgar laborants’ who sought after gold by means of blind recipe following. — location: 451

assault on all the main schools of thought about — location: 454

the elements. These, he said, are simply incompatible with the experimental facts. — location: 454

The conventional four-element theory claimed that all four of Aristotle’s elements are present in all substances. But Boyle observes that some materials cannot be reduced to the classical elementary components, however they are manipulated by ‘Vulcan’, the heat of a furnace: — location: 455

In other words, elements are to be found not by theorizing but by experiment: ‘I must proceed to tell you that though the assertors of the four elements value reason so highly . . . no man had ever yet made any sensible trial to discover their number.’ — location: 460

But he then proceeds to question whether anything of this sort truly exists – that is, whether there are elements at all. Certainly, Boyle holds back from offering any replacement for the elemental schemes he demolishes, although he shows some sympathy for the idea, advocated by the Flemish scientist Johann Baptista van Helmont, that everything is made of water. — location: 465

By the end of the seventeenth century, then, scientists were not really any closer to enumerating the elements than were the Greek philosophers. Yet a hundred years later the British chemist John Dalton (1766–1844) wrote a textbook that outlined a recognizably modern atomic theory and gave a list of elements that, while still very incomplete and sometimes plain wrong, is in content and in spirit a clear precursor to today’s tabulation of the hundred — location: 468

and more elements. Why had our understanding of the elements changed so fast? — location: 471

Boyle’s demand for experimental analysis as the arbiter of elemental status is a central component of this change. Another reason for the revolution was the relinquishment of old preconceptions about what elements should be like. — location: 472

classical scholars, an element had to correspond to (or at least be recognizable in) stuff that you found around you. — location: 473

Many of the substances today designated as elements are ones almost all of us will never see or hold; in antiquity, that would seem an absurd complication. — location: 474

short, there is nothing obvious about the elements. Until the twentieth century, scientists had no idea why there should be so many, nor indeed why there should not be thousands more. The elements cannot be deduced by casual inspection of the world, but only by the most exacting scrutiny using all the complicated tools of modern science. — location: 478

It is often said that Antoine Laurent Lavoisier did for chemistry what Isaac Newton did for physics and Charles Darwin for biology. He transformed it from a collection of disparate facts into a science with unified principles. — location: 484

And Lavoisier? His was the fate of the Enlightenment’s brave new world: slaughtered during Robespierre’s Reign of Terror. — location: 489

Lavoisier (1743–94), like Condorcet, was misfortunate that the leading thinkers in France were likely, sooner or later, to become embroiled in politics. Whereas in England science was still the pursuit of ‘gentlemen’ with money and leisure to spare, France had its state-approved Academy of Sciences whose members commonly filled public offices and became highly visible figures in political life (Fig. 2 — location: 492

Lavoisier was a tax collector before he became a famous scientist, — location: 496

But his chemical expertise also secured him the prominent position of director on Louis XVI’s Gunpowder Administration, and as treasurer and effective secretary of the Academy of Sciences he vigorously opposed its dissolution by the anti-elitist Jacobin administration in 1793. — location: 496

Lavoisier was a sitting target for the Revolutionary witch-hunters, who — location: 498

were determined to purge the nation of anyone whose loyalty to the Republic they found reason to doubt. — location: 499

That is why, in 1794, Lavoisier was forced to bow his head to the blade that had just removed his father-in-law’s. — location: 499

Two centuries later, the debate still rages about whether Lavoisier was or was not the true discoverer of one of chemistry’s most important elements: oxygen. — location: 500

It has become the subject of a play written by two of the world’s leading chemists, the Nobel laureate Roald Hoffmann and the co-inventor of the contraceptive pill, Carl Djerassi. — location: 501

In Oxygen, the Nobel Committee of 2001 has decided to award ‘retro-Nobel’ prizes for great discoveries made before the prize was inaugurated in 1901. They decide that the first chemistry prize must go to oxygen’s discoverer, because, says one of the characters, ‘the Chemical Revolution came from oxygen’. — location: 503

Lavoisier gave the — location: 505

element its name, but he was certainly not the first to make it, nor to recognize it as a distinct and important substance. — location: 505

His experiments on water led him to conclude in 1783 that it ‘is not a simple substance at all, not properly called an element, as had always been thought’. — location: 514

he announced that ‘atmospheric air is composed of two elastic fluids of different and opposite qualities’, which he called ‘mephitic air’ and ‘highly respirable air’. Neither water nor air, in other words, is an element. — location: 515

He named the constituents of water hydrogen (‘water-former’) and oxygen, which combine in a two-to-one ratio reflected in the familiar chemical formula H2O. — location: 517

Air is a more complex substance. The fraction that is ‘highly respirable air’, Lavoisier realized, is an element in itself: oxygen. — location: 518

The name comes from the Greek for ‘acidformer’, as Lavoisier wrongly believed that — location: 519

oxygen was a component of all acids. — location: 520

For the ‘fluid’ that Lavoisier called mephitic air he proposed the name azot or azotic gas, a Greek term indicating that it is inimical to life. Lavoisier found that, when he isolated this component, it had the ‘quality of killing such animals as are forced to breathe it’. Reasonably enough, he concluded that it was noxious. In fact it is not poisonous but simply useless: separated from oxygen, it cannot sustain life. Lavoisier noted that this gas ‘is proved to form a part of the nitric acid, which gives a good reason to have called it nitrigen’. He preferred his azot, however, and so did the other French chemists – which is why nitrogen is known to this day as azote in France. — location: 520

Oxygen and nitrogen are elements, but most of these other gases are compounds formed by the reaction and joining together of two or more different elements. In oxygen gas, each atom of oxygen is bound to another atom of oxygen. In carbon monoxide, an oxygen atom is linked to an atom of carbon. — location: 535

In practice, Lavoisier found it necessary to use a chemical reaction to perform the separation: he allowed the oxygen to combine with other substances through combustion, leaving behind almost pure nitrogen. But modern techniques can perform the physical separation of these elements. — location: 546

The second half of the eighteenth century was the age of ‘pneumatick chemistry’, when the properties of gases, typically called ‘airs’, were the focus of the discipline. — location: 551

for example, the ‘fixed air’ studied by Scottish chemist Joseph Black (1728–99). In the 1750s, Black found that a gas was produced when carbonate salts were heated or treated with acid. The air, he reasoned, was ‘fixed’ in the solids until liberated. Unlike common air, fixed air turned lime water (a solution of calcium hydroxide) cloudy. We now recognize that this is due to the formation of insoluble calcium carbonate – basically chalk. — location: 556

Black’s student Daniel Rutherford (1749–1819) called this gas ‘mephitic air’ instead: mephitis is a noxious emission in legend, thought to emanate from the earth and cause pestilence. — location: 561

apt name, for animals died in an atmosphere of this new gas. — location: 562

Rutherford’s ‘air’ is not, however, the same as Lavoisier’s mephitic air, which is nitrogen. Yet Rutherford is himself credited with discovering nitrogen, for he found that it is an unreactive component of common air. — location: 563

Only about a fifth of common air is ‘good’, supporting life, Rutherford reported in 1772. — location: 564

The legacy of Aristotle’s elements was still strong, and the pneumatick chemists preferred to regard each gas as ‘common air’ altered in some manner – for example, in states of greater or lesser impurity. Even Lavoisier found this a hard habit to shake off. — location: 570

It invoked chemistry’s most notorious pseudoelement: phlogiston. — location: 574

We can trace this hypothetical substance back to Jabir ibn Hayyan’s sulphur, a supposed component of all metals. Real sulphur, the yellow solid mined from the earth, was a combustible substance, a component of gunpowder and the brimstone that bubbles beneath the fires of hell. — location: 575

Becher’s definition of terra pinguis: ‘Metals contain an inflammable principle which by the action of fire goes off into the air.’ — location: 581

was assumed that metals too give out phlogiston during calcination. — location: 587

Charcoal was deemed to be rich in phlogiston (why else would it burn so well in ovens and furnaces?), and so it was capable of restoring this substance to the calx, regenerating the metal. — location: 588

There is just one problem. It is true that wood, losing mass as it burns, seems to be giving out some substance into the air. But calcined metals gain weight. — location: 589

Then a French pharmacist named Pierre Bayen pointed out to Lavoisier that ‘calx of mercury’, which we would now call mercuric oxide, can be converted to mercury simply by heating, without the need for ‘phlogiston-rich’ charcoal. Moreover, the gas released in this process was not Black’s fixed air, but something quite different. What was this gas? That started to become clear to Lavoisier when Joseph Priestley came to dinner. — location: 598

In August 1774 Priestley conducted the same experiment as Bayen, heating mercuric oxide and collecting the gaseous product. He found that a candle flame placed in this gas burned even more brightly than in common air, — location: 603

Priestley never swayed from his firm conviction in the phlogiston theory as long as he lived, and he called his new gas ‘dephlogisticated air’. — location: 606

In 1775 Priestley discovered it had an even more miraculous property. Mice placed in a glass vessel full of ‘dephlogisticated air’ survived for much longer than mice in an identical vessel containing common air. — location: 607

when Priestley himself inhaled it, he reported that ‘my breath felt peculiarly light and easy for some time — location: 609

And in 1771–2 a Swedish apothecary named Carl Wilhelm Scheele, one of the finest experimental chemists of his age, performed the same experiment as Mayow and isolated a gas that enhanced burning. — location: 614

March 1775 Lavoisier announced that his own experiments with mercuric oxide revealed all calxes to be a combination of metals with such — location: 621

gas.   Seeing this report, Priestley realized that Lavoisier had not quite appreciated the ‘superior’ qualities of his ‘dephlogisticated air’ – it was not merely common air. He sent the Frenchman a sample of the gas to verify that this was so. As a result, Lavoisier presented a paper to the French Academy in April in which he identified the principle of combustion – Priestley’s gas – as an especially ‘pure air’. In keeping with his notorious arrogance, he made no mention of the contributions of Priestley and Bayen. — location: 622

Lavoisier may have been cavalier with his treatment of priority issues, but he went far beyond replicating the results of others. To Priestley, oxygen was always going to be regarded as a form of common air modified by the removal of phlogiston; Scheele too saw things very much in these terms. Lavoisier came to understand that this ‘pure air’ was actually a substance in its own right. In that case, air itself was not elemental but a mixture. It is Lavoisier who made oxygen an element. — location: 630

The chronology of events suggests that oxygen arose purely from attempts to explain combustion. — location: 633

But Lavoisier was equally keen to make this new element the explanatory principle of acidity, itself still a profound mystery to chemists. In this he was less successful. Many non-metallic elements, such as sulphur, carbon, and phosphorus, combine with oxygen to produce gases that dissolve in water to make acids, and that is why Lavoisier named the new element as he did (in German oxygen is still known as Sauerstoff, ‘acid stuff’). But not all acids contain oxygen; and those that do, do not derive their acidity from it. — location: 634

single element can exhibit very different characteristics depending on what it is combined with. Chlorine is a corrosive, poisonous gas; combined with sodium in table salt, it is completely harmless. Carbon, oxygen, and nitrogen are the stuff of life, but carbon monoxide and cyanide (a combination of carbon and nitrogen) are deadly. This was a hard notion for chemists to accept. Lavoisier himself came under attack for claiming that water was composed of oxygen and hydrogen: for water puts out fires (it is ‘the most powerful antiphlogistic we possess’, according to one critic), whereas hydrogen is hideously flammable. — location: 639

Lavoisier’s belief reveals that he still held a somewhat traditional view of elements. They were generally regarded as being rather like colours or spices, having intrinsic properties that remain evident in a mixture. But this is not so. — location: 638

The discovery of oxygen did not just make phlogiston redundant; the two were fundamentally incompatible. Oxygen is the very opposite of phlogiston. It is consumed during — location: 644

burning, not expelled. Burning ends when the air is devoid of oxygen, not when it is saturated with phlogiston. Indeed, it is this mirror-image quality that made phlogiston seem to work so well. Science needed an element like this to explain combustion – but it simply looked at the problem the wrong way round. Phlogiston was oxygen’s shadow. — location: 645

Like many of his contemporaries, he regarded heat as a physical substance, rather like the ancient elemental fire. He called it caloric, and it sounded suspiciously like phlogiston in another guise. Caloric was what made substances gaseous; oxygen gas was replete with it. When oxygen reacted with metals to form calxes, caloric was released (heat was given out), and in consequence the oxygen became dense and heavy. — location: 654

These ideas are evident in an essay of Lavoisier’s from 1773, in which he identifies the three different physical states of matter: solid, liquid, and gas. Here he makes the crucial distinction between the physical and chemical nature of substances, which confused the ancients and led to their minimal elemental schemes. ‘The same body’, says Lavoisier, ‘can pass successively through each of these states, and in order to make this phenomenon occur it is necessary only to combine it with a greater or lesser quantity of the matter of fire.’ — location: 657

Despite these throwbacks, Lavoisier transformed the way chemists thought about elements. At the beginning of the eighteenth century it was common to imagine just five of them. In 1789 Lavoisier consolidated his oxygen theory by publishing a textbook, Traité élémentaire de chimie (An Elementary Treatise on Chemistry), that defined an element as any substance that could not be split into simpler components by chemical reactions. And he listed no fewer than thirty-three of them. — location: 666

It would require nineteenth-century physics to show that some of these were — location: 669

fictitious (light, caloric). Several others were in fact compounds that chemists had not yet found how to decompose to their elements. But the message was clear: there is no ‘simple’ scheme of elements. There are lots of them out there, and it was up to chemists to find them. — location: 669

Now they see things very differently. The chemical composition of the air is not a precondition for life but the result of it. Around two billion years ago, primitive living organisms transformed the atmosphere from one largely devoid of oxygen to one with plenty of it. — location: 680

There is no known geological process that can maintain a high level of oxygen in our planet’s atmosphere. Eventually the gas will react with rocks and become locked away in the ground. Only biological processes can strip oxygen out of its combinations with other elements and return it to the skies. If all life on Earth were to end, the oxygen level would gradually dwindle to insignificance. For this reason, an oxygen-rich atmosphere is a beacon that proclaims the presence of life beneath it. — location: 682

To us this sounds fortuitous, but to photosynthetic cells it was the biggest outbreak of global pollution the world has ever seen. To them oxygen was sheer poison. It is perceived as a friendly element, but it is actually one of the most corrosive and destructive. — location: 696

After all, it takes only a single spark to persuade an entire forest to react with oxygen. The consequence in 1998–9 was a haze of smoke that covered Indonesia and altered the local climate. There is geological evidence for global wildfires in the distant past that make this one seem like a bonfire. — location: 699

for there was until recently no way to protect gleaming iron and steel from the avid combining power of oxygen. It turns old paintings brown as it transforms the varnish; exposed to air, most metals develop a rind of oxide within seconds. — location: 702

Nature, however, makes do. If the air is full of poison, it will learn to live on poison. We breathe oxygen not because it is inherently good for us but because we have evolved ways of making it less bad for us. Enzymes mop up the deadly compounds formed as oxygen is used to burn sugar in the energy factories of our cells. These compounds include hydrogen peroxide, used as an industrial and a domestic bleach, and the even more destructive superoxide free radical. Such substances damage the delicate biomolecules of our cells, including DNA. Cells have molecular mechanisms that strive to repair the damage, but its inevitable accumulation is an important factor in the ageing process. — location: 703

Thus there is nothing optimal or ideal about living on an oxygen-rich planet; it is simply the way things turned out. — location: 708

Oxygen is, after all, an extremely abundant element: the third most — location: 709

abundant in the universe, and the most abundant (47 per cent of the total) in the Earth’s crust. — location: 710

On the other hand, the living world (the biosphere) has contrived to maintain the proportion of oxygen in the atmosphere at more or less the perfect level for aerobic (oxygen-breathing) organisms like us. If there was less than 17 per cent oxygen in the air, we would be asphyxiated. If there was more than 25 per cent, all organic matter would be highly flammable: it would combust at the slightest provocation, and wildfires would be uncontrollable. A concentration of 35 per cent oxygen would have been enough to destroy most life on Earth in global fires in the past. — location: 710

(NASA switched to using normal air rather than pure oxygen in their spacecrafts for this reason, after the tragic and fatal conflagration during the first Apollo tests in 1967.) — location: 714

So the current proportion of 21 per cent achieves a good compromise. — location: 715

hypothesis that the biological and geological systems of the Earth conspire to adjust the atmosphere and environment so that they are well suited to sustain life – the so-called Gaia hypothesis. Oxygen levels have fluctuated since the air became oxygen rich, but not by much. In addition, today’s proportion of atmospheric oxygen is large enough to support the formation of the ozone layer in the stratosphere, which protects life from the worst of the sun’s harmful ultraviolet rays. Ozone is a UV-absorbing form of pure oxygen in which the atoms are joined not in pairs, as in oxygen gas, but in triplets. — location: 717

How is atmospheric oxygen kept at such a steady level? It is created, as we have seen, during photosynthesis, when organisms strip oxygen from water molecules. Photosynthetic organisms — location: 721

include all plants and many species of bacteria. Oxygen is consumed by animals and other aerobic organisms. It is tempting to regard the steady level of oxygen as a balance between these sources and sinks in the biosphere. But there is more to it than that. The oceans act as a kind of buffer against large variations in atmospheric oxygen, since the decomposition of marine organic matter (which removes oxygen from the air) slows down if oxygen levels fall. — location: 722

These so-called biogeochemical cycles are linked: changes in the cycling of oxygen, carbon, nitrogen, and phosphorus are interdependent. The meshed cogs create a more or less constant environment on our planet. — location: 727

Changes to the turning speed of — location: 728

one of the cogs – for example, owing to industrial and agricultural practices that pump carbon-rich gases into the atmosphere – can upset the other cogs in ways that are hard to predict. This is why there is so much uncertainty about the likely course of global climate change caused by human activities. — location: 729

Because the biogeochemical cogs are always turning, the chemistry of the Earth is not at equilibrium. When a chemical process reaches equilibrium, all change ceases. The chemical constancy of our planet’s environment is due not to inactivity but to perpetual change. It is the difference between a person staying on the same spot by standing still or by walking a treadmill. — location: 731

This disequilibrium of the Earth’s environment involves inorganic processes in sea and rock, but it is ultimately sustained by the biosphere – by life. The cogs are kept in motion mostly by the energy of the sunlight captured by photosynthetic organisms. If life ceased, the planet would gradually settle towards a static equilibrium that would be very different from today’s environment. — location: 734

We can see this by looking at the atmospheres of our neighbouring planets. Venus and Mars are of a similar size to Earth, and they were formed from a roughly similar mixture of elements. But their skies now contain only tiny amounts of oxygen – less than 1 per cent – and only small quantities of nitrogen. Their atmospheres are both about 95 per cent carbon dioxide, even though that of Mars is very tenuous while that of Venus is very thick. — location: 737

On Venus this dense blanket of the greenhouse gas raises surface temperatures to around 750 °C; on Mars the thin sheet keeps things at a frigid –50 °C or so. In either case, the absence of oxygen and the proximity of the mixture of atmospheric gases to an equilibrium mixture proclaims from afar that there is no life to be found on these worlds. — location: 740