A Level GCSE Secondary Secondary Science

Chemical Anniversaries: 1766 – Three Papers by Henry Cavendish

The January 1766 edition of Philosophical Transactions, the journal of the Royal Society, included a forty-page, illustrated paper by Henry Cavendish. It was Cavendish’s first published work and was in three parts. The first dealt with inflammable air (hydrogen) and the second and third parts with fixed air (carbon dioxide). Despite the passing of three hundred years and changes in the language, both English and chemical, many of the experiments Cavendish describes would be familiar to students of chemistry in schools today.

A Strange Man

Henry Cavendish was born in 1731. He was a member of the family of the Dukes of Devonshire who owned vast estates in Derbyshire and elsewhere. During the eighteenth century the family made huge sums of money from mining copper and other metals in the Pennine hills. Henry thus had a privileged upbringing and was educated at the University of Cambridge. He did not however complete a degree and grew into a strange man.

Cavendish was extremely shy and could not bear to speak to women. He never married and female servants had to keep out of his sight. He lived alone and communicated with his housekeeper by notes. Other scientists were his only social contacts, mainly at the regular meetings of the Royal Society in London. With a fortune inherited from an uncle, Henry settled in a house in London where he could do his research and build up a remarkable library with no worries about earning a living.

Throughout his life, Cavendish carried out a wide range of experiments in physics and chemistry. He made important discoveries, many of which he did not bother to publish so that later scientists earned the credit. When he died in 1810 he had barely dented the fortune he had inherited. It passed to his relatives and eventually was used to fund the Cavendish Laboratory at Cambridge University.

Chemistry in 1766

Physics became a mathematical science in the seventeenth century thanks to the insights and experiments of Galileo, Newton and others. The same could not be said of chemistry. While the alchemists’ goal of converting lead to gold was a fading dream in the eighteenth century there were few accepted chemical laws or concepts. While Robert Boyle in the 1660s had stated that an element was a substance that could not be broken down into anything simpler, the list of known elements in the 1760s was still relatively short. There was still debate about whether water and air were elements or not. The most accepted explanation of burning was that flammable materials contained a mysterious substance called phlogiston which was given off when the substances burned.

Three papers on Air.

Cavendish’s paper in Philosophical Transactions can be read today and if you know the modern names for the terms he used then his experiments are simple to follow and to repeat. For example, Cavendish refers to “factitious air” by which he means a gas combined with another substance in a solid or liquid. An example, discovered by Joseph Black in the 1750s, is fixed air (carbon dioxide) which is combined with lime (calcium oxide) in limestone (calcium carbonate). All gases are called airs and are said to be “elastic” i.e. compressible, while those combined in solids or liquids are inelastic i.e. non-compressible.

In the first paper, Cavendish explores the properties of inflammable air (hydrogen). Gases that burned had been known for some time and in 1671 Robert Boyle obtained flammable air by reacting iron with acid. Cavendish first generates the gas by reacting zinc, iron and tin with both vitriolic acid (dilute sulfuric acid) and spirit of salt (hydrochloric acid). He collects the gas over water in graduated glass vessels as we still do today. The three metals he uses are the only ones available that will react with acids. He measures the volumes of gas given off from measured amounts of metals. Like many generations of school pupils, he holds a flame to the gas and observes the explosive combustion. Concerned for his own safety he devises a method of igniting the gas from a safe distance. Having carried out many experiments he decides that the flammable air is always the same substance.

In further experiments he measures the flammability of the gas by mixing it with measured quantities of “ordinary” air and finds that the loudness of the “pop” and speed of burning varies. He also compares the weight or density of the gas and shows it is about a tenth of ordinary air. Just a few years later, Jacques Charles uses this property in hydrogen balloons. Cavendish also finds that the metals react with concentrated sulfuric acid and nitric acid in different ways, producing airs which are not flammable.

Cavendish’s experiments are methodical and precise and described in excellent detail so that they could be repeated today. Nevertheless, he still clings to the phlogiston theory for his explanation of what he observes. For Cavendish, flammable air is pure phlogiston, released from metals by the reaction with acids and producing flame and heat when mixed with ordinary air.

In part 2 of the paper, Cavendish repeats and extends Black’s work on fixed air, showing how it is produced when various earthy substances react with acids. He measures its solubility in water and other liquids and other properties
In part 3 he compares the fixed air produced by fermentation and the decay of organic material with the gas he studied in part 2 and concludes that the gases are the one and the same material.

After 1766

Cavendish continued to carry out his chemical experiments as well as making important discoveries in physics. Probably his most famous chemical work was published in his 1784 Experiments on Air paper. He proved that water was not an element but a compound of flammable air (hydrogen) and dephlogisticated air (oxygen).

Cavendish found patterns in chemical reactions for example, metals reacting with acids produce inflammable air. He showed that the products of these reactions were predictable even if the reactants were varied – inflammable air always had the same properties. He showed the value of making measurements and of describing his methods, results and conclusions in simple to understand language. Despite using old names and sticking with the phlogiston hypothesis Cavendish behaves like a modern chemist.

 

Tasks

  1. Write out word (and symbol equations) for the reactions of zinc, iron and tin with vitriolic acid and spirit of salt.
  2. Why did Cavendish only react zinc, iron and tin with acids?
  3. Why is Cavendish’s conclusion that the same flammable gas was produced in each reaction of a metal with an acid important?
  4. Cavendish made lots of measurements of volumes of gases and masses of reactants and products. Why are accurate measurements important?
  5. Why was his inheritance important for Cavendish?
  6. In what ways do you think Cavendish’s personality influenced his career in science?
  7. Cavendish only wrote up some of his work for publication. Why is publication important in science?
  8. Describe the reaction of hydrogen and oxygen as Cavendish might have done, using the terms he used.
  9. Imagine you are one of Henry Cavendish’s rare visitors to his London home, dining on a supper of mutton. Describe the visit including a tour of his laboratory.

 

Bibliography

The Oxford Dictionary of Scientists pub. OUP
Three Papers, Containing Experiments on Factitious Air, by the Hon. Henry Cavendish, F. R. S. Phil. Trans. January 1, 1766 56 141-184; doi:10.1098/rstl.1766.0019 (go to http://rstl.royalsocietypublishing.org/search?fulltext=Cavendish&submit=yes&andorexactfulltext=and&x=25&y=9)

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