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The development of the Phlogiston Theory
Antoine Lavoisier and the demise of the Phlogiston Theory

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The Phlogiston Theory

The Natural Philosophers of Cavendish's day adhered to the Phlogiston Theory. This theory that dated from 1667 stated that anything that could be combusted (burnt) contains an ethereal-like substance called phlogiston. When combustion was complete it meant that all the phlogiston had been released into the air. Phlogiston had no substance - as one proponent described it, it could not be 'put in a bottle'. However, the phlogiston theory seemed at odds with the fact that when a metal is heated in the air its calx (its oxide) is formed. This increase in weight did not seem to fit the theory but it was explained by assuming that phlogiston could have negative weight.



William Anderson's sketch of Henry Cavendish

Non-exhaustive List of Henry Cavendish's Achievements

Work on Air

  • 1781 He analysed samples of atmospheric air on 60 different occasions and from different places (even using a balloon to collect air samples from the upper atmosphere). He performed over 400 tests.

  • 1783 He concluded that the atmosphere had a constant composition of "phlogisticated air" (nitrogen) and "dephlogisticated air" (oxygen) in the ratio 5 : 1

  • 1785 He extracted nitrogen gas and oxygen gas from a sample of atmospheric air and realised that there was 1/120 parts remaining. He recognised the existence of another gas (or gases). In fact this represented the rare gases in the sample and argon would not be discovered for another 100 years.

Work on "fixed air" (carbon dioxide)

  • He looked at the chemical properties of "fixed air" including its solubility and specific gravity.

  • He investigated fermentation and recognised that the gas released is "fixed air". This gas was identified as the same gas that is given off when chalk or magnesia are heated.

Investigation into "calcareous earth" (calcium carbonate or chalk)

  • He produced calcium hydrogen carbonate (calcium bicarbonate) in solution

  • He realised the difference between hard and soft water

  • He found that water could be softened by adding calcium hydroxide

Work on Heat

  • One of the first to realise the conservation of mass and heat

  • He explained the mechanical equivalent of heat

  • He measured heats of fusion and evaporation of chemicals (latent heats)

  • He measured the freezing point of mercury, vitriolic acid and nitric acid

  • He explained why mercury in thermometers reduces in volume at lower temperatures

As an Intrument Maker

  • He made a new eudiometer (an instrument with a graduated glass tube used to ignite mixtures of gases with an electric spark). It can measure changes in gas volumes during reactions

  • Improved meteorological instruments using the information he gained from his latent heat studies.

Physics and Mathematics

  • He calculated the mean density of the Earth (his value of 5.448 g/cm3 is very close to the present value of 5.51 g/cm3)

  • He designed a torsion balance (called the Cavendish balance) to measure the gravitational constant of the Earth

  • He calculated the mass of the Earth (he called this 'weighing the world'). To do this he needed to calculate the gravitational constant (he was 1% off of the current value) and would then have used Newton's Law of Universal Gravitation. (Cavendish did not give a value for the mass of the Earth. Today's value is (5.9722 ± 0.0006) × 1024 kg).



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Henry Cavendish 1731 - 1810

Henry Cavendish was born into an English aristocratic family. His father was Lord Charles Cavendish, the third son of the Duke of Devonshire. In 1729 Charles Cavendish married Lady Anne Grey whose father was the Duke of Kent. In 1730 the winter was so cold that the River Thames was frozen over. When Lady Anne became ill her doctors advised that she spend the winter in warmer climes. Lord Charles and Lady Anne relocated to Nice in southern France. Not long after their arrival Lady Anne became pregnant and the Cavendish's first son, Henry, was born in Nice in October 1731. Lady Anne's health did not improve as had been hoped, so she consulted a specialist in Geneva, Switzerland before returning to England. It was not long after her return that Lady Anne found herself pregnant for a second time. Her health declined, however, and a few months after the birth of the Cavendish's second son, Frederick, she passed away.

Lord Charles never remarried and he was left to take care of the upbringing of his two sons. Henry was tutored at home until he was eleven years old when he entered the fashionable Dr Newcomb's Academy, a private boarding school in Hackney, East London. The school, which was founded in 1685, was non-denominational and offered a 'modern' curriculum. This would have included mathematics, natural science, French and the classical languages.

When Henry Cavendish left the Academy he entered St Peter's College, Cambridge (now known as Peterhouse) in 1749. St Peter's College was small with only twenty four registered students. Henry spent his time attending mathematics and natural philosophy lectures and reading scientific literature. He became particularly enthralled with Sir Isaac Newton's 'Principia'. Henry did not complete his degree but left Cambridge after only three years. Some biographers believe that this was because he did not agree with having to sit the obligatory religious tests in order to graduate. On leaving Cambridge, according to one biographer, Henry accompanied by his brother Frederick, spent some time in Pars. (It was usual for 'gentlemen' to tour the Continent after university and sometimes these tours lasted a few years. However, if Henry Cavendish did so, it would seem that his tour was limited to Paris).

On returning to England, Henry went to London to live with his father in Great Marlborough Street. Lord Charles provided a separate apartment for his son, as well as an allowance of £500 a year. Lord Charles was an experimental scientist and the house included a research laboratory. Although his father fully supported and encouraged Henry's scientific career, some of the family were not so pleased. It is reported that the Duke of Devonshire instructed his wife to "stay away from Henry Cavendish because he is not a gentleman - he works".

In 1758 Lord Charles introduced Henry to the Royal Society dinners and the meetings that followed them. Henry Cavendish was elected as a member of the Royal Society two years later in 1760. Between 1769 and 1773 he served on over twenty committees for the Royal Society. These included:

  • Astronomical committee that studied the transit of Venus (1769)

  • Committee studying the gravitational attraction of mountains (1774)

  • Committee organising the exploration of the North Pole (1773)

  • Committee for the publication of the Society's journal "Philosophical Transactions".

and, in 1769, Henry Cavendish was appointed to the Council of the Royal Academy (where he served for one year with his father before his father retired.)

A list of just a few of Henry Cavendish's achievements is shown in the box to the left. He was a prolific investigator, renowned for his precision, and his experiments covered most areas of science including astronomy, meteorology, mineralogy, optics and geology. In chemistry he is most famous for his discovery of "inflammable air" that we now know as hydrogen. Cavendish produced his "inflammable air" by adding dilute acid to metals, such as zinc and iron, and collecting the gas over water. He was awarded the Royal Society Copely Medal in 1766 for his paper "Three Papers Containing Experiments on Factitious Air". This paper included his observations on the reactions of "fixed air" (carbon dioxide) and atmospheric air.


Drawing of apparatus from Cavendish's
"Three Papers Containing Experiments on Factitious Air"


Lord Charles Cavendish died in 1783 when Henry Cavendish was forty years old. His inheritance made Cavendish one of the wealthiest men in England and the largest holder of bank stock. Cavendish left Great Marlborough Street and bought a house near Clapham Common where he built a laboratory. He also maintained a house at 11 Bedford Square where he kept his extensive library. Cavendish would use the house in Bedford Square on the nights when he attended the Royal Society dinners and meetings.

Henry Cavendish was asocial (he suffered from social phobia) and he avoided people as much as possible, even at the Royal Society. Studying accounts of his behaviour from the hindsight of the 21st century, it has been suggested that he might have suffered from Autism Spectrum Disorder. He certainly showed gynophobia (from the Latin meaning a horror of women). His female servants were instructed never to let themselves been seen by him, under threat of dismissal. He communicated with his housekeeper by leaving instructions on notepaper (his request for his dinner was always leg of lamb!). Cavendish even installed secret staircases in the house so that he could move around unseen.

The other Fellows of the Royal Society described Cavendish as 'very shy', 'odd', 'reclusive' and 'solitary'. When he attended the Thursday dinners and meetings, Cavendish would take his seat at the last moment to avoid engaging in conversation. When he did speak he was said to have a squeaky, hesitant voice and found talking so difficult that some listeners assumed that he had a speech impediment. He refused to sit for a portrait. The only drawing of Cavendish that exists was made by William Anderson who was invited to a Society dinner as an undercover guest specifically to make a covert sketch of him (see left).

Despite being one of the richest men in England, Cavendish always wore the same old, faded, velvet coat and three cornered hat. It was a style that had been popular in the previous century! His contemporaries have described his dress as 'a little neglected' and 'shabby'. He was reported to have been tall, thin and he slouched both when walking and when seated. Despite his fortune, Cavendish lived frugally (although there is evidence that he had a weakness for fine furniture).

An exception to Cavendish's solitary existence was his association with another member of the Royal Society, Charles Blagden. The two men met in 1775 and Blagden began to assist Cavendish with his experiments around 1782. Charles Blagden was thirty four and Cavendish fifty one at that time. Blagden even persuaded Cavendish to make journeys outside of London. In 1787 they made their third annual visit to the West Country. These were working visits and Cavendish made sure that specialized equipment was sent ahead and waiting for them on their arrival. They travelled from London to Plymouth and then on to Cornwall. Introduction letters from James Watt and Matthew Boulton gave the visitors access to tin mines. Josiah Wedgewood vouched for them to gain entry to clay pits. Blagden and Cavendish made a detour to Dartmoor to measure the heights of the hills. Later, in 1793, Cavendish made a journey to Derbyshire and Lincolnshire without his companion, stopping to investigate quarries and collieries en route. The reason for this trip was to meet James Watt who had recently measured the specific gravity of steam. When in Birmingham, Cavendish was able to watch the new Watt-Boulton steam engine being put through its paces.

Henry Cavendish devoted his life to science and his social anxiety did not prevent him from communicating with other researchers. In 1772, Joseph Priestley had travelled to London to meet Cavendish after reading his "Factitious Air" paper published six years earlier. In 1781 Priestley informed Cavendish, presumably by letter, that he had used an 'electrostatic machine' to ignite ordinary (atmospheric) air with "inflammable air". Cavendish repeated Priestley's experiment but he used "dephlogisticated air" (oxygen) in place of ordinary air. He discovered that the 'mist' formed as a product of the reaction was water.

Cavendish had performed hundreds of experiments but he never published a book and he wrote only twenty papers in his lifetime, although he did keep detailed laboratory notebooks. He neglected to publish his findings on igniting "inflammable air" with "dephlogisticated air" and so producing water. Instead he communicated his observations to Priestley and Blagden and one of these two gentlemen informed Antoine Lavoisier in Paris. Lavoisier published his 'Réflexions sur le phlogistique' (Reflections on Phlogiston) in 1783 and received the credit for the discovery of the formation of water. Lavoisier named the "inflammable air" hydrogen (meaning water former). Cavendish published one year later in 1784 but when his notebooks were 'rediscovered' in the mid-19th century they provided the proof that he was the first to have made water by igniting oxygen gas and hydrogen gas.

Henry Cavendish died in 1810. He died alone, as he no doubt preferred. His final instruction to his manservant was to leave him in his room and not to return "until a certain time". When the manservant did return he found his master dead. Cavendish's substantial estate was left to his heir, Lord George Cavendish. Henry Cavendish was interred in the family vault at All Saint's Church, Derby, close to the Cavendish's ancestral home, Chatsworth House.

In 1879, James Clerk Maxwell was given access to two sealed packets that had been handed down to the Duke of Devonshire. In these packets were Henry Cavendish's laboratory notebooks. Maxwell published Cavendish's work on research into electricity under the title "The Electrical Researches of the Honourable Henry Cavendish'. Maxwell discovered that there were five laws attributed to other scientists that should have been attributed to Henry Cavendish. These are:

  • Richter's Law of Reciprocal Proportions

  • Ohm's Law

  • Coulomb's Law

  • Charles's Law

  • Dalton's Law of Partial Pressures


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