Henry Cavendish Was Instrumental in Unveiling the Components of the Air That We Breathe

Historical profile Airs and graces Henry Cavendish was instrumental in unveiling the components of the air that we breathe. Mike Sutton looks back at his life

50 | Chemistry World | October 2010 www.chemistryworld.org mathematics at the University of or tin released the same quantity In short Cambridge (without taking a degree) of the gas, regardless of whether before joining London’s scientific  Henry Cavendish’s they were dissolved in spirit of salt community. He became a fellow of meticulous lab (hydrochloric acid) or dilute oil of the Royal Society in 1760 and was work helped to open vitriol (sulfuric acid). elected to its council in scientists’ eyes to the So Cavendish reached the 1765. existence and properties reasonable (but erroneous) Cavendish of hydrogen, carbon conclusion that inflammable air was took an active dioxide and nitrogen a constituent of zinc, iron and tin interest in  He proved that the dew and liberated by acids. He seems to mathematics, formed when hydrogen have suspected that the ‘air’ might astronomy, was exploded with actually be pure phlogiston, the fiery meteorology oxygen was water matter which early modern chemists and physics.  He also isolated argon, like Ernst Stahl (1660–1734) But his first which then remained believed to exist in all combustible scientific unidentified for over substances. However, he also publication, 100 years considered the possibility that it was which appeared  He had an obsessive a more complex substance in which in three parts eye for detail and often phlogiston played some part. in Philosophical considered his work Part two of Cavendish’s paper Transactions in 1766 unworthy of publication concerned fixed air (carbon (and earned him the Royal dioxide). Heating chalk or limestone Society’s Copley medal), to produce quicklime (needed for was a substantial contribution making mortar and cement) was to chemical knowledge. It known to release an ‘air’ which could concerned what Cavendish called cause death if inhaled. Black studied factitious airs – gaseous substances it in the 1750s while investigating the liberated from solids by heat or acids. medicinal properties of magnesia alba (basic magnesium carbonate). Airy fairy He showed that the fixed air At the time, many natural produced by heating limestone was If you were philosophers still regarded identical to the ‘air’ released when to indulge in a atmospheric air as a unitary element. mineral acids reacted with chalk and periodic table Chemists such as Stephen Hales other calcareous substances, and to day dream then and Joseph Black had previously the ‘air’ generated during alcoholic the words noble and collected and studied other ‘airs’, but fermentation. inert, linked to argon and its their chemical nature was not well Cavendish repeated Black’s neighbours, may conjure images of understood. Cavendish, however, experiments with greater precision, lordly idleness. Such an association developed ingenious new ways to collecting the fixed air over would be appropriate, if somewhat capture and store gases released mercury since it was soluble in ironic. The first person to detect during chemical reactions, enabling water. He calculated its density inert gases was indeed an aristocrat, him to measure precisely their as 1.57 times that of common air but one who obsessively pursued an volumes and weights. – reasonably close to the correct active career and was acclaimed as Part one of Cavendish’s 1766 figure of 1.65 – and measured with the greatest English scientist since paper concerned the inflammable moderate accuracy the quantity Sir Isaac Newton. air – hydrogen, as we now call it – of gas produced by the action of Henry Cavendish (1731–1810) was which was liberated by dissolving acids on mineral carbonates and grandson to two dukes – Devonshire metals in acids. Others, including bicarbonates. For example, he found on his father’s side, Kent on his Robert Boyle, had collected and that 1000 units of marble (calcium mother’s. As the younger son of a burned the gas, but Cavendish was carbonate) contained 407 or 408 younger son he was off the main line the first to study it quantitatively, Cavendish apparatus for units by weight of carbon dioxide; of inheritance, but still had a very estimating it to be 11 times less making and collecting the actual figure is 440. adequate personal income. Other dense than common air. He showed hydrogen Part three of Cavendish’s young men in his position might that fixed weights of zinc, iron massive work examined the ‘airs’ have gone into the church, the army released by fermentation, and by or parliament, become patrons of decaying animal and vegetable literature and the arts, or simply material. A Scots-Irish chemist and led lives of pleasurable indolence. surgeon called David MacBride Cavendish, however, devoted had identified carbon dioxide in himself to physics and chemistry. the vapours emitted by putrefying His father, Lord Charles organic matter. But it was Cavendish Cavendish, was an amateur scientist who showed that they also contained and a fellow of the Royal Society. another inflammable air, different He ensured that his son acquired a from the one produced by adding modern education by sending him to acids to metals. This heavier the forward-thinking Dr Newcome’s inflammable air appeared to consist Hackney Academy, rather than to mainly of marsh gas (methane). the public school Eton. Cavendish Cavendish also compiled a then spent 3 years studying lengthy manuscript which could www.chemistryworld.org Chemistry World | October 2010 | 51 Historical profile

potentially have become part four heating manganese dioxide with of his paper series. In it, he oil of vitriol. But after obtaining the recorded experiments on other same gas from other substances inflammable airs produced by without using acid – including heating organic solids such as saltpetre (potassium nitrate) and wood shavings and hartshorn mercuric oxide – he renamed it (red deer horn). Dissatisfied with fire air, because it accelerated PHILOSOPHICAL TRANSACTIONS PHILOSOPHICAL inconclusive results, Cavendish combustion. Unfortunately for characteristically abandoned this Scheele, publication of his discovery line of work, leaving it unfinished was delayed until 1777 and by then and unpublished. In fact, after Priestley had already independently producing one more chemical made the same discovery. paper on the mineral content of Priestley – a Unitarian minister, a pump water, Cavendish was to lecturer at the Warrington Academy, turn his back on chemistry for and a fellow of the Royal Society several years to focus instead – had scientific interests as broad on physics. as Cavendish’s. He began studying fixed air in 1770 while living close Physical pursuits to a brewery where it was readily Cavendish published little obtainable, and he later published during his physics period, a pamphlet on the manufacture despite performing countless of artificial mineral water which experiments and covering launched the fizzy drink industry. copious pages with notes. In 1772 Cavendish and Priestley Biographers attribute discussed repeating research done this to his near-obsessive by Hales, which involved adding perfectionism. aqua fortis (nitric acid) to various Cavendish certainly set minerals and metals. When Priestley himself high standards – his did the experiment he identified model was Newton’s book another gas which he called nitrous Principia Mathematica. His air (nitric oxide). In contact with extensive manuscript writings atmospheric air, the nitrous air indicate that he hoped to deal formed a reddish gas (nitrogen with heat and electricity as dioxide), which dissolved in water to authoritatively as Newton leave a gaseous residue that no longer had handled gravitational supported life or fire. He seized on attraction and inertial this as a quantitative method for motion. Unfortunately, the puzzles estimating the ‘goodness’ (oxygen Cavendish grappled with were too social contact outside his circle of content in our terms) of atmospheric complex to be unravelled using the scientific acquaintances. air. Then, in 1774, Priestley produced resources then available. what we now call oxygen by heating It took another century of effort by Breath of fresh air Cavendish’s 1784 mercury’s red oxide. some outstanding experimenters and In 1781 Cavendish returned to paper in Philosophical Cavendish and Priestley, like theorists to gain the comprehensive chemistry with renewed energy. Transactions on the most of their contemporaries, still understanding of thermodynamics During the previous decade, Carl synthesis of water accepted the phlogiston theory and electromagnetism that Scheele in Sweden and Joseph of combustion. It seemed to them Cavendish sought. Some of his Priestley in England had discovered that Cavendish’s inflammable air discoveries, if published, would have another new gas, oxygen. It was (hydrogen) must be (or must contain) been valuable additions to scientific given various names, reflecting phlogiston, while Scheele’s fire air knowledge. But rather than present different theoretical interpretations was starved of it. This seemingly the world with work that was of its origins and behaviour, but the explained why they combined incomplete, he preferred to continue one which eventually stuck was explosively when stimulated by a experimenting in private. coined by a Frenchman, Antoine flame or electric spark. But then Such reticence was characteristic Lavoisier. Cavendish made a Lavoisier proposed that combustion of Cavendish. One colleague significant contribution to this was not the loss of phlogiston but noted that he had ‘a most reserved unfolding drama. the gain of something (oxygen), disposition and particularly shy Seventeenth century natural supporting his claim by meticulously habits’, while another recorded philosophers had shown that an weighing the ingredients and that he spoke ‘with great difficulty enclosed body of atmospheric products of combustion reactions. and hesitation, and very seldom’. air could sustain animal life (or However, it was not until Priestley Often ill at ease with his fellow flames) for only a limited time but it visited Paris in 1774, and told men of science, he was even more remained unclear whether breath ‘He always tried Lavoisier about his own discovery uncomfortable with his domestic and fire contaminated the air, or to avoid social of dephlogisticated air, that the servants – particularly the women. consumed some vital portion of French savant realised exactly what He instructed them to remain out it. Scheele – an apothecary with contact outside was gained during the process of of his sight at all times, and ordered a genius for chemical research – his circle of combustion. At first he called the his dinner by leaving a note on the isolated this vital component in the new gas eminently respirable air, but hall table. Unsurprisingly, he never early 1770s. He first called it vitriolic scientific later named it oxygen, combining married, and always tried to avoid air, probably because he made it by acquaintances’ two Greek words meaning 52 | Chemistry World | October 2010 www.chemistryworld.org reached Lavoisier who repeated the experiment. Before Cavendish’s ‘Experiments on air’ papers had HAMISH KIDD HAMISH appeared in the Philosophical Transactions for 1784 and 1785, Lavoisier had incorporated it (without acknowledgement) into one of his own papers. English scientists were quick to protest and Lavoisier duly made amends. Another priority dispute began when the engineer and scientist James Watt accused Cavendish of plagiarising his research on the composition of water. This misunderstanding was apparently provoked by a malicious correspondent. After seeing Cavendish’s paper Watt acknowledged that their theories were different and withdrew the accusations. Cavendish was initially unconvinced by Lavoisier’s account of combustion, pointing out that to identify oxidation so closely with acidity was a mistake, since some acids contain no oxygen. For some years, Cavendish continued to call oxygen dephlogisticated air, but gradually accepted the new nomenclature as it came into general use. Meanwhile, his final contribution to atmospheric chemistry was ignored, and remained so for many years. By repeatedly sparking atmospheric nitrogen with excess oxygen in a closed container over water, Cavendish converted it into a acid-begetter to recognise the Cavendish statue over the soluble oxide. (In his language, the fact that many oxides were acidic entrance to the original dephlogisticated air deprived the when dissolved in water. Similarly, Cavendish laboratories in phlogisticated air of its phlogiston, Lavoisier called Cavendish’s Cambridge, UK converting it to acid.) He then inflammable air hydrogen – the absorbed the surplus oxygen with water-begetter. liver of sulfur (potassium sulfide). But an irreducible gaseous residue Feeling gassy remained, occupying approximately In 1781 another paper of Priestley’s 1/120th of the original volume. He encouraged Cavendish once again published this observation in 1785, to study gases. Priestley and his but was unable to explain it. It was assistant John Warltire had exploded not until 1894 that Lord Rayleigh mixtures of atmospheric air and and William Ramsay identified the hydrogen with electric sparks and residue as argon – the first of the noticed dew forming inside the noble gases to be discovered. As vessel. Cavendish, with his usual usual, the noble Cavendish was well thoroughness, proved conclusively ahead of his time. that this dew was pure water, produced by combining inflammable Mike Sutton is a visiting fellow in air with the dephlogisticated air ‘His final the department of humanities of (oxygen) which made up one fifth Northumbria University, UK of atmospheric air. (He called the contribution other four-fifths of the atmosphere to chemistry Further readng phlogisticated air – air so saturated C Jungnickel and R McCormmach, Cavendish: – discovering the experimental life, 2nd edition, Bucknell, with fiery matter that it could absorb Lewisburg, US, 1999 no more. We call it nitrogen.) argon – was J R Partington, A history of chemistry, volume 3, Cavendish told several Macmillan, London, UK, 1962 investigators about his synthesis ignored for L Tomory, Let it burn: distinguishing inflammable airs 1766–1790, Ambix, 2009, 56, 253 HAMISH KIDD HAMISH of water, and news of it soon many years’ www.chemistryworld.org Chemistry World | October 2010 | 53