Humphry Davy

How to use this course

Humphry Davy turned the electric battery into a tool for taking matter apart. Within three years of his first major paper on electrochemistry he had isolated potassium, sodium, calcium, strontium, barium and magnesium, metals that no chemical reagent could free from their oxides. He then showed that “oxymuriatic acid” was an element, chlorine, and with that overturned Lavoisier’s rule that every acid contains oxygen. He ended his active career with two engineering problems: the explosions that killed coal miners, and the corrosion of the copper hulls of the Royal Navy. He was also a public lecturer of extraordinary power, a friend of poets, and the man who hired Michael Faraday.

Each technical lesson opens with a paragraph for the general reader, then states the key results in modern form, reconstructs Davy’s experiments and reasoning, and ends with exercises whose solutions are folded. Davy worked before electrochemistry had quantitative laws. Faraday supplied them in 1833–34, and the lessons use those laws freely to explain what Davy saw.

Timeline

Penzance, 1778–1798

Humphry Davy was born in Penzance, at the western tip of Cornwall, on 17 December 1778. His father Robert was a woodcarver; his mother was Grace Millett. Humphry was the eldest of five children. John Tonkin, his godfather and guardian, paid for much of his schooling, first at the grammar school in Penzance and from 1793 at Truro Grammar School.

He was a restless, imaginative boy who fished, wrote verse and told stories. His father died in 1794, leaving debts, and Davy was apprenticed to John Bingham Borlase, a surgeon and apothecary in Penzance. The dispensary gave him chemicals and apparatus, and he began experimenting at home with whatever he could find. He set himself a broad course of reading that ran from theology and metaphysics to mathematics and languages.

Reading Lavoisier

In 1797 he learned French and worked through Lavoisier’s Traité élémentaire de chimie. He was nineteen, self-taught, and immediately critical. With his Quaker friend Robert Dunkin he performed experiments on heat, including the rubbing together of two pieces of ice that is discussed in Lesson 04, and he wrote an essay arguing that Lavoisier’s “caloric” did not exist.

Two contacts opened the way out of Penzance. Gregory Watt, son of James Watt, lodged with the Davy family for his health in 1797 and talked chemistry with him. Davies Giddy (later Davies Gilbert, himself a future President of the Royal Society) saw his talent and introduced him to Thomas Beddoes, a physician in Bristol who was founding an institution to treat disease with gases.

Bristol and the Pneumatic Institution, 1798–1801

Davy arrived at Beddoes’s Pneumatic Institution in Clifton, Bristol, on 2 October 1798, aged nineteen, to supervise the preparation and testing of gases. The idea behind the institution was that the newly discovered “airs” of Priestley, Black and Lavoisier might cure consumption and other diseases. James Watt designed apparatus for producing and inhaling them.

Beddoes printed Davy’s essays on heat and light in 1799 in a volume of contributions from the west of England. Davy later regretted their speculative tone. His real work was experimental, and it was dangerous. He inhaled nitrous oxide, hydrogen, carbon dioxide, “carburetted hydrogen” and other gases, and once nearly killed himself with water gas. The results went into his book of 1800, Researches, Chemical and Philosophical; Chiefly Concerning Nitrous Oxide, a careful study of the gas’s preparation, composition and physiological effects (Lesson 04).

Poets and gas

Bristol gave Davy a literary circle. Samuel Taylor Coleridge and Robert Southey breathed nitrous oxide under his supervision and recorded their sensations. Coleridge asked him to see the second edition of the Lyrical Ballads through the press. Davy wrote poetry all his life; more than 160 manuscript poems survive, though few were published. Coleridge later remarked that had Davy not been the first chemist of his age he would have been its first poet.

In 1800 news arrived of Alessandro Volta’s pile, the first source of continuous electric current. Davy built piles at once and began publishing on the chemical effects of galvanism in William Nicholson’s journal. This brought him to the notice of the new Royal Institution in London, and on 9 March 1801 he left Bristol for good.

The Royal Institution, 1801–1806

The Royal Institution had been founded in 1799 by Benjamin Thompson, Count Rumford, and a group of landowners led by Sir Joseph Banks, to diffuse useful knowledge and apply science to everyday life. In early 1801 a committee that included Banks, Rumford and Henry Cavendish interviewed Davy, and he was appointed assistant lecturer in chemistry, director of the laboratory and assistant editor of the journals, at £100 a year. His first lecture, on galvanism, was on 25 April 1801. By June 1802 he was the full lecturer.

His lectures became a London sensation. Fashionable audiences crowded Albemarle Street, and the traffic of carriages is said to have led to the street being made one-way. He combined dramatic demonstrations with a vivid, almost poetic style, and he was handsome and young. The managers also gave him practical work: lectures on agricultural chemistry, analyses of soils, and a study of the chemistry of tanning.

Meanwhile the Institution built him batteries. By 1802 he had the most powerful battery then existing, and his early observations of the bright discharge between carbon points led, by 1808, to the large-scale carbon arc. He received the Copley Medal in 1805, and by 1804 he was already a Fellow of the Royal Society. The stage was set for the Bakerian Lecture of 1806, which made his name throughout Europe.

Heat, light and laughing gas

For the general reader. As a teenager Davy tried to show that heat is a kind of motion, not an invisible fluid, by melting ice through friction. The experiment was weaker than he thought, but the conclusion was right. In Bristol he then made himself the subject of experiments with nitrous oxide, and he noticed that the gas dulled pain, nearly half a century before anyone used it in surgery.

Caloric and the ice experiment

Lavoisier listed caloric among the chemical elements: a weightless fluid whose flow is heat. Rumford had argued in 1798 from the boring of cannon that friction produces heat without limit, which a conserved fluid could not do. Davy’s version was more pointed. Two pieces of ice rubbed together below the freezing point melt. Liquid water has a larger heat capacity than ice and absorbs latent heat on melting, so on the caloric theory the water should contain more caloric than the ice it came from. Where could that caloric have come from, if not from motion?

The weakness of the experiment is quantitative. Rubbing by hand delivers little power, and ice in a room, or even in a cold one, receives heat from its surroundings by conduction and radiation. Twentieth-century critics argued that the melting Davy saw could be explained by heat leaking in, so the experiment could not decide the question. The kinetic theory was established by James Joule’s measurements in the 1840s.

Nitrous oxide

Priestley had discovered nitrous oxide in 1772. Davy prepared it pure by gently heating ammonium nitrate and studied its decomposition with care, distinguishing it from the other oxides of nitrogen by their proportions of nitrogen and oxygen:

Overheating gives other products, including nitric oxide and nitrogen, which is why he insisted on controlled temperatures. He then breathed the gas repeatedly, recording euphoria, laughter and, on one occasion, relief from the pain of an erupting wisdom tooth. In the 1800 book he suggested that, since the gas seemed able to destroy physical pain, it might be useful in surgical operations. Nobody acted on this until the American dentist Horace Wells tried it in 1844.

The chemical agencies of electricity

For the general reader. When water is connected to a battery, acid appears at one wire and alkali at the other. In 1806 many chemists believed the current was creating these substances out of water. Davy showed, by patient purification, that the acid and alkali came from impurities, and that pure water gives only hydrogen and oxygen. He then proposed that chemical attraction and electrical attraction are the same force.

The problem of the acid and alkali

In 1800 William Nicholson and Anthony Carlisle decomposed water with a voltaic pile, and others soon noticed acid near the positive wire and alkali near the negative one. Some, including the Italian experimenter Pacchiani, claimed muriatic acid was being created from water. Davy electrolysed water in cups of agate and then of gold, connected by moistened asbestos, using distilled water redistilled in silver. He traced the alkali to the glass and agate of the vessels and the acid to salt from the hands and to nitrogen of the air, which gave nitric acid. In vessels of gold, under hydrogen, with the purest water, only hydrogen and oxygen appeared, in the ratio of two volumes to one.

He also showed that salt solutions are decomposed with the acid going to the positive pole and the base to the negative, and that these substances could be transported through intervening solutions. Jöns Jacob Berzelius and Wilhelm Hisinger had reported the same polarity of decomposition for salts in 1803. Berzelius praised Davy’s lecture as one of the best memoirs ever to enrich chemical theory, and the Institut de France awarded Davy its prize for galvanism in 1807 although Britain and France were at war.

The hypothesis was qualitative but enormously fertile. It implied that every compound might be decomposed by a strong enough battery, which is precisely the idea Davy tested on the alkalis in 1807. Faraday, his assistant, later made electrolysis quantitative.

Potassium and sodium

For the general reader. Potash and soda had been known since antiquity, and Lavoisier suspected they were compounds, but no chemical method could take them apart. Davy reasoned that if electricity could break any chemical bond, a large enough battery would decompose them. On 6 October 1807 it did, and tiny globules of a shining metal burst into flame as they formed. His cousin and assistant Edmund Davy recalled him dancing around the laboratory with joy.

The experiment

Davy first electrolysed saturated aqueous solutions of potash. He obtained only hydrogen and oxygen: the water was decomposed and the potash was untouched. He then used potash that was only slightly moistened, enough to make it conduct, placed on a platinum disc connected to the negative side of his battery, with a platinum wire from the positive side touching its upper surface. The potash fused at the contacts. Gas was given off at the upper surface, and at the lower surface small globules with a high metallic lustre appeared, some burning explosively with a bright flame, others tarnishing and becoming coated with a white film. Soda gave a similar metal. He named them potassium and sodium and announced them in the Bakerian Lecture of November 1807.

Why water must be (almost) absent

In solution two cathode reactions compete. Their standard potentials are

Water is reduced at a far less negative potential, so in aqueous solution the current makes hydrogen. Any potassium formed would in any case react with the water at once: \(2\mathrm K+2\mathrm{H_2O}\to2\mathrm{KOH}+\mathrm H_2\). Davy’s trick was to supply just enough moisture for conduction while keeping most of the cathode in contact with fused hydroxide. Industrial sodium was later made by the same principle, electrolysing molten NaOH (the Castner process) and later molten NaCl (the Downs cell).

Consequences

The new metals were lighter than water, soft enough to cut with a knife, and so reactive that Davy stored them under naphtha. Some chemists doubted that such substances could be metals, or argued that they were compounds of the alkali with hydrogen. Gay-Lussac and Thénard soon made potassium in quantity by reducing potash with red-hot iron, which removed the battery as a bottleneck and gave chemists a powerful new reducing agent. Potassium is how boron, silicon and several other elements were first obtained (Lesson 07 and the Berzelius module).

Alkaline earths, boron and the amalgam method

For the general reader. After the alkalis, Davy turned to the “earths” lime, magnesia, baryta and strontia. They resisted direct electrolysis because they do not melt or dissolve easily. Following a method of Berzelius and Magnus Pontin in Stockholm, he collected the metals in a pool of mercury, then boiled the mercury away. Four new metals were announced in a single paper in June 1808.

The mercury cathode

Berzelius and Pontin found in 1808 that when a salt solution is electrolysed with a mercury cathode, the alkali or alkaline-earth metal dissolves in the mercury as an amalgam instead of reacting with water. Davy adapted the idea. He mixed the moistened earth with mercuric oxide on a platinum plate, made a hollow in the mixture to hold mercury, and dipped the negative wire into it. The metal formed in the mercury, and distilling off the mercury in a closed tube left a small residue of the metal. On 30 June 1808 he reported barium, strontium, calcium and magnesium to the Royal Society. He first called magnesium “magnium”, to avoid confusion with manganese.

The separation step depends on volatility. Mercury boils at 357 °C, while calcium, strontium, barium and magnesium boil at roughly 1100–1900 °C, so mercury distils off and the alkaline-earth metal stays behind. Davy never obtained these metals in bulk or very pure. Calcium and magnesium in quantity came decades later.

Boron and the earths that would not yield

In 1808–09 Davy heated boric acid with potassium and obtained a dark, combustible substance, boron. Gay-Lussac and Thénard reported the same reduction at almost the same time. With alumina, silica and zirconia he failed to obtain the metals, though he was convinced they existed and proposed names for them. For the metal of alumina he suggested “alumium” and then “aluminum”; the spelling “aluminium” was introduced by others to match the endings of sodium and potassium.

Chlorine, iodine and the nature of acids

For the general reader. Lavoisier named oxygen the “acid-maker” and taught that all acids contain it. Muriatic acid (hydrochloric acid) was the awkward exception, since nobody could find oxygen in it. Chemists assumed a hidden element, “murium”, and called the green gas made from the acid “oxymuriatic acid”. In 1810 Davy argued that the green gas contains no oxygen at all. It was an element, which he named chlorine. Acids, he later proposed, owe their character to hydrogen.

The argument

Carl Wilhelm Scheele had made the gas in 1774 from muriatic acid and manganese dioxide. On the Lavoisian view it was muriatic acid combined with extra oxygen, so heating it with charcoal ought to produce carbon dioxide. In 1809 Joseph Louis Gay-Lussac and Louis Jacques Thénard tried this and found no carbon dioxide; they noted that the gas might be an element but did not commit themselves. Davy repeated and extended these experiments:

• Charcoal heated to whiteness in the dry gas by the battery produced no carbon dioxide or carbon monoxide.
• Metals burned in the gas gave chlorides, with no water and no oxygen released.
• Ammonia and dry muriatic acid gas combined to give sal ammoniac without water.
• Hydrogen and the green gas combined in equal volumes to give muriatic acid gas and nothing else.

The simplest interpretation was that the gas was an element and muriatic acid a compound of it with hydrogen. Davy announced this on 15 November 1810, naming the element from the Greek for pale green. Many chemists, including Berzelius, held on to the oxymuriatic theory for years before accepting it.

Acids and hydrogen

In December 1813, in Paris during his Continental tour, Davy received a sample of the violet substance Bernard Courtois had found in seaweed ash in 1811, through André-Marie Ampère. He quickly recognized it as an element analogous to chlorine and wrote to the Royal Society on 10 December, four days after Gay-Lussac’s announcement that it was an element. A dispute over priority followed, though both men credited Courtois with the discovery. Hydriodic acid, like muriatic acid, contained hydrogen and no oxygen. By 1815 Davy proposed that acidity is linked to hydrogen that can be replaced by a metal, the idea that Justus von Liebig made precise in 1838 and that underlies the later definitions of Arrhenius and of Brønsted and Lowry.

The safety lamp and the physics of flame

For the general reader. Coal mines release methane, which miners called firedamp. Mixed with air and touched by a candle, it explodes. In 1812 an explosion at Felling Colliery near Gateshead killed 92 men and boys. Asked in 1815 to find a remedy, Davy studied how flames travel and found that a flame cannot pass through a narrow tube or a fine metal mesh. A lamp enclosed in wire gauze lets methane burn inside harmlessly while the flame cannot reach the gas outside.

Davy’s experiments

Davy obtained firedamp from the mines and established that it needs a high temperature to ignite, that mixtures burn only within a range of proportions with air, and that explosions would not pass through metal tubes narrower than about one-eighth of an inch. Shortening the tubes worked if their diameter was reduced, and a sheet of fine wire gauze acted like a great many short, narrow tubes. His letter describing the results was read in Newcastle on 3 November 1815, and he presented the gauze lamp to the Royal Society on 9 November. The lamp was first tried underground at Hebburn Colliery on 9 January 1816. Davy did not patent it.

Why the gauze stops the flame

A flame front sustains itself because heat released in the reaction zone raises the next layer of gas to ignition. Inside a narrow channel, the walls draw heat out of the reaction zone. If the channel is narrow enough, the heat lost to the walls outweighs the heat released and the flame dies. Combustion theory expresses this as a critical Péclet number:

where \(S_L\) is the laminar burning velocity, \(\alpha\) the thermal diffusivity of the unburned gas and \(d_q\) the quenching distance. For a stoichiometric methane–air mixture, \(S_L\approx0.37\ \mathrm{m\,s^{-1}}\) and \(\alpha\approx2.2\times10^{-5}\ \mathrm{m^2\,s^{-1}}\), giving \(d_q\) of a few millimetres; measured quenching diameters for methane–air in drilled holes are about 3.5 mm. A gauze with openings well below this is a flame arrester. Davy’s lamps used iron gauze with several hundred openings per square inch.

A detector, and a hazard

Inside the gauze, methane burns as a pale blue cap above the oil flame, and its height tells the miner how much firedamp is present; in air lacking oxygen the flame goes out. But the lamp could fail. A gauze heated red-hot by burning gas inside could ignite the mixture outside, a strong draught could blow the flame through the mesh, and a single broken wire defeated it. The lamp also allowed seams to be worked that had been closed as too dangerous. Explosions continued, and a parliamentary committee in 1835 noted that reliance on the lamp had encouraged working in fiery mines. George Stephenson developed a different safety lamp at Killingworth at about the same time, and a priority dispute followed. Davy received silver plate worth £2,000 from the coal owners and the Royal Society’s Rumford Medal.

Protecting copper: an early electrochemical cure

For the general reader. Warships of the 1820s were sheathed in copper below the waterline to keep off weed and shipworm, but the copper corroded quickly in seawater. Davy applied his theory that chemical change is electrical. If copper could be made slightly negative, it should stop dissolving. Small blocks of zinc or iron attached to the hull did exactly that. The cure worked chemically and failed practically, for a reason no one had foreseen.

Corrosion as a cell

Copper in seawater corrodes because two half-reactions run on the same surface: copper is oxidized, and dissolved oxygen is reduced.

Davy reasoned that copper corrodes because it is electrically positive with respect to the oxygen and acid in seawater. Connecting it to a more reactive metal would render it negative and stop the attack. In 1824 he reported to the Royal Society that a small piece of zinc or iron in contact with a much larger area of copper sheet preserved the copper in seawater.

Trials and failure

The Navy Board had asked Davy in 1823 to find a remedy. Experiments with Faraday’s help, then trials on ships, confirmed that the protectors prevented corrosion. But the protected copper rapidly became covered with weed and barnacles, which slowed the ships. Copper ions dissolving from unprotected sheathing poison marine organisms; by stopping the corrosion Davy had also stopped the antifouling. By the end of 1825 the Admiralty ordered the protectors removed from sea-going ships. The principle survived: sacrificial zinc and aluminium anodes protect ships’ hulls, pipelines and offshore structures today.

Fame, Faraday and the Royal Society, 1812–1827

In 1812 Davy was knighted, married the wealthy and sociable widow Jane Apreece, and gave up his regular lecturing at the Royal Institution. In the same year he published the first part of Elements of Chemical Philosophy; no further parts appeared. His Elements of Agricultural Chemistry, based on lectures to the Board of Agriculture, followed in 1813. The marriage brought wealth and society but little happiness, and the couple had no children.

Faraday

In 1812 a young bookbinder’s apprentice, Michael Faraday, attended Davy’s last lectures and sent him a bound volume of his notes. When an explosion of nitrogen trichloride injured Davy’s eye later that year, he took Faraday on, and on 1 March 1813 Faraday became chemical assistant at the Royal Institution. In October 1813 the Davys left for the Continent, with Faraday as assistant and, to his discomfort, at times as servant. Napoleon granted them passports despite the war. In Paris Davy worked on iodine; in Florence he used the Grand Duke of Tuscany’s great burning lens to burn diamond and show it is pure carbon; in Italy he met Volta. They returned to London in April 1815. The relationship later soured. In 1821 Davy sided with William Hyde Wollaston in the dispute over Faraday’s electromagnetic rotations, and he is reported to have opposed Faraday’s election to the Royal Society in 1824. It was nevertheless Davy who had opened the door for the greatest experimental physicist of the century.

President of the Royal Society

Davy was created a baronet on 20 October 1818, the first such honour for a man of science in Britain. He spent 1818–19 at Naples trying chemical methods for unrolling the carbonized papyri of Herculaneum, without much success. On the death of Sir Joseph Banks he was elected President of the Royal Society on 30 November 1820. He hoped to reform the Society and tie it more closely to government and to working scientists, but his manner was vain and abrasive, and factional quarrels marked his presidency. He was also a founding fellow of the Zoological Society of London. Ill health forced him to resign in 1827. He was awarded one of the Royal Society’s Royal Medals in the same year.

Last journeys, death and legacy, 1826–1829

Davy suffered a stroke in 1826 and never fully recovered. In 1827 he left for the Continent, travelling in Italy, Austria and Slovenia, fishing and writing. Salmonia, or Days of Fly Fishing (1828) is a set of dialogues on angling and natural history. His last book, Consolations in Travel, or The Last Days of a Philosopher, published in 1830, mixes dialogues on science, history and religion with visions of the future of humanity. On 20 February 1829 he had a second stroke in Rome. His brother John Davy, an army physician and chemist, came to look after him. Davy died at an hotel in Geneva on 29 May 1829, aged 50. He was buried in the Plainpalais cemetery in Geneva on 1 June, and a memorial tablet was later placed in Westminster Abbey.

Legacy

• Electrochemistry: the electrolytic isolation of the alkali and alkaline-earth metals, and the idea that chemical affinity is electrical, which Berzelius developed into a system and Faraday made quantitative.
• Chemical elements: potassium, sodium, calcium, strontium, barium, magnesium and boron obtained, chlorine and iodine recognized as elements, and Lavoisier’s oxygen theory of acids replaced by a hydrogen theory.
• Applied science: the safety lamp and the principle of the flame arrester; cathodic protection; the carbon arc, ancestor of arc lighting; early agricultural chemistry; the first suggestion of anaesthesia by nitrous oxide.
• Science and the public: his lectures made the Royal Institution a centre of public science, a tradition Faraday continued with the Christmas Lectures. His brother John published a memoir in 1836 and edited his collected works in 1839–40.
• Honours: the Royal Society has awarded the Davy Medal for chemistry since 1877, and the lunar crater Davy is named after him.

Sources and further reading

Biography

1. Humphry Davy, Wikipedia (with extensive references).
2. Science History Institute, Humphry Davy.
3. J. Davy, Memoirs of the Life of Sir Humphry Davy, 2 vols., London, 1836.
4. D. M. Knight, Humphry Davy: Science and Power, Blackwell, 1992; Cambridge University Press, 1996.
5. J. Z. Fullmer, Young Humphry Davy: The Making of an Experimental Chemist, American Philosophical Society, 2000.
6. R. Holmes, The Age of Wonder, HarperPress, 2008, chapters on Davy.
7. The Davy Notebooks Project, Lancaster University: transcriptions of Davy’s notebooks.

Science

1. H. Davy, Researches, Chemical and Philosophical; Chiefly Concerning Nitrous Oxide, Bristol, 1800.
2. H. Davy, The Bakerian Lecture: On some chemical agencies of electricity, Phil. Trans. R. Soc. 97 (1807) 1–56.
3. H. Davy, The Bakerian Lecture: On some new phenomena of chemical changes produced by electricity, particularly the decomposition of the fixed alkalies, Phil. Trans. R. Soc. 98 (1808) 1–44.
4. H. Davy, Electro-chemical researches on the decomposition of the earths, Phil. Trans. R. Soc. 98 (1808) 333–370.
5. H. Davy, Researches on the oxymuriatic acid, its nature and combinations; and on the elements of the muriatic acid, Phil. Trans. R. Soc. 100 (1810).
6. H. Davy, On the Safety Lamp for Coal Miners, London, 1818.
7. H. Davy, On the corrosion of copper sheeting by sea water, and on methods of preventing this effect, Phil. Trans. R. Soc. 114 (1824).
8. Bakerian Lecture, Wikipedia: list of lectures and titles.
9. Davy lamp, Wikipedia, on the lamp, its rivals and its record in the mines.
10. Potassium, Chemistry World element podcast.
11. I. Glassman, R. A. Yetter, N. G. Glumac, Combustion, 5th ed., Academic Press, 2014, on flame quenching.