Henry Cavendish

Henry Cavendish, FRS (October 10, 1731 - February 24, 1810) was a British scientist noted for his discovery of hydrogen or what he called "inflammable air".^[1] He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and gave the element its name. Cavendish is also known for his measurement of the Earth's density and early research into electricity.

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Personal life

Henry Cavendish was born on October 10, 1731 in Nice, France, where his family was living at the time. His mother was Lady Anne Grey, daughter of the Duke of Kent and his father was Lord Charles Cavendish, son of the second Duke of Devonshire. The family traces its lineage across eight centuries to Norman times and was closely connected to many aristocratic families of Great Britain.

At age 11, Henry Cavendish was a pupil at Dr. Newcome's School in Hackney. At age 18 (in 1749) he entered the University of Cambridge in St Peter's College, now known as Peterhouse, but left four years later without graduating. His first paper, "Factitious Airs", appeared 13 years later, in 1766.

Cavendish was silent and solitary, viewed as somewhat eccentric, and formed no close personal relationships outside his family. By one account, Cavendish had a back staircase added to his house in order to avoid encountering his housekeeper because he was especially shy of women. The contemporary accounts of his personality have led some modern commentators, such as Oliver Sacks, to speculate that he had Asperger's syndrome, though he may have been merely painfully shy. His only social outlet was the Royal Society Club, whose members dined together before weekly meetings. Cavendish seldom missed these meetings, and was profoundly respected by his contemporaries. However his shyness made those who "sought his views... speak as if into vacancy. If their remarks were...worthy, they might receive a mumbled reply."^[2] He also enjoyed collecting fine furniture exemplified by his purchase of a set of "ten inlaid satinwood chairs with matching cabriole legged sofa" documented to have been acquired by Cavendish himself.^[3]

Because of his asocial and secretive behaviour, Cavendish often avoided publishing his work, and much of his findings were not even told to his fellow scientists. It wasn't until the late nineteenth century, long after his death, that James Clerk Maxwell looked through Cavendish's papers and found discoveries for which others had been given credit. Examples of what was included in Cavendish's discoveries or anticipations were Richter's Law of Reciprocal Proportions, Ohm's Law, Dalton's Law of Partial Pressures, principles of electrical conductivity and Charles's Law of Gases.

Cavendish died in 1810 and was buried in Derby Cathedral, along with many of his ancestors. The University of Cambridge's Cavendish Laboratory was endowed by several of Henry Cavendish's later relatives, especially William Cavendish, 7th Duke of Devonshire (Chancellor of the University from 1861 to 1891).

Gases and the atmosphere

  Cavendish is considered to be one of the so-called pneumatic chemists of the eighteenth and nineteenth centuries, along with, for example, Joseph Priestley, Joseph Black, and Daniel Rutherford. By combining metals with strong acids, Cavendish made hydrogen (H₂) gas, which he isolated and studied. Although others, such as Robert Boyle, had prepared hydrogen gas earlier, Cavendish is usually given the credit for recognizing its elemental nature.

Cavendish observed that hydrogen, which he called "inflammable air", reacts with oxygen, then known as "dephlogisticated air", to form water. James Watt and Antoine Lavoisier made a similar observation, resulting in a controversy as to who should receive credit for it.

Cavendish also accurately determined the composition of Earth's atmosphere. He found that 79.167% is "phlogisticated air", now known to be nitrogen and argon, and 20.8333% is "dephlogisticated air", now known to be 20.95% oxygen. Cavendish also found that 1/120 of the Earth's atmosphere is a third gas, which was identified as argon about 100 years later by William Ramsay and Lord Rayleigh.

Density of the Earth

Main article: Cavendish experiment

  In addition to his achievements in chemistry, Henry Cavendish is also known for the Cavendish experiment, the first to measure the force of gravity between masses in a laboratory and to produce an accurate value for the Earth's density. His work led others to accurate values for the gravitational constant (G) and the Earth's mass.

The equipment Cavendish used was designed and built by geologist John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797 – 1798,^[2] and published the results.^[4]

The experimental apparatus consisted of a torsion balance to measure the gravitational attraction between two 350-pound lead spheres and a pair of 2-inch 1.61-pound lead spheres.^[2] Using this equipment, Cavendish found that the Earth's average density is 5.48 times greater than that of water. Poynting later noted that the data should have led to a value of 5.448^[5], and indeed that is the average value of the twenty-nine determinations Cavendish included in his paper.^[6]

It is not unusual to find books that erroneously describe Cavendish's work as a measurement either of the gravitational constant (G) or the Earth's mass^[7][8], and this mistake has been pointed out by several authors.^[9][10] In reality, Cavendish's stated goal was to measure the Earth's density, and his result was later used to calculate G. The first time that this constant was used was in 1873, almost 100 years after the Cavendish experiment.^[11] Cavendish's results also can be used to calculate the Earth’s mass.

Henry Cavendish performed his experiment in an outbuilding in the garden of his Clapham Commons estate. For years afterward, his neighbors would point out the building and tell their children that it was where the world was weighed.^[3]

Electrical researches

Cavendish's electrical experiments did not become known until they were collected and published by James Clerk Maxwell a century later, in 1879, long after other scientists had been credited with the same results. Among Cavendish's discoveries were the following:^[12]

• The concept of electric potential, which he called the "degree of electrification"
• An early unit of capacitance, that of a sphere one inch in diameter
• The formula for the capacitance of a plate capacitor
• The concept of the dielectric constant of a material
• The relationship between electric potential and current, now called Ohm's Law. (1781)
• Laws for the division of current in parallel circuits, now attributed to Charles Wheatstone
• Inverse square law of variation of electric force with distance, now called Coulomb's Law

For further reading

• Dictionary of National Biography, Vol. 3, p.1261.
• Cavendish: The Experimental Life, C. Jungnickel and R. McCormmach, Bucknell University Press, 1999.

Notes and references

1. ^ ^{a b} Cavendish, Henry (1766). "Three Papers Containing Experiments on Factitious Air, by the Hon. Henry Cavendish". Philosophical Transactions 56: 141 – 184. Retrieved on 2007-11-06.
2. ^ ^{a b c} Bryson, B. (2003), "The Size of the Earth": A Short History of Nearly Everything, 59 – 62.
3. ^ ^{a b} McCormmach, R and Jungnickel, C (1996), Cavendish, American Philosophical Society, Philadelphia, ISBN 0871692201, p. 242, 337.
4. ^ Cavendish, Henry, (1798), 'Experiments to Determine the Density of the Earth', Philosophical Transactions of the Royal Society of London Vol. 88, 469 – 526.
5. ^ Poynting, J. H. (1894), "The Mean Density of the Earth" London: Charles Griffin and Company, page 45.
6. ^ Cavendish, Henry, "Experiments to Determine the Density of the Earth", reprinted in A Source Book in Geology, K. F. Mather and S. L. Mason, editors, New York: McGraw-Hill (1939), pages 103 – 107.
7. ^ Tipler, P. A. and Mosca, G. (2003), Physics for Scientists and Engineers: Extended Version, W. H. Freeman ISBN 0-7167-4389-2.
8. ^ Feynman, R. P. (1970), Feynman Lectures On Physics, Addison Wesley Longman , ISBN 0-201-02115-3
9. ^ Clotfelter, B. E. (1987), The Cavendish Experiment as Cavendish Knew It, American Journal of Physics 55 (3), 210-213.
10. ^ Falconer, I. (1999), Henry Cavendish: the man and the measurement , Measurement, Science & Technology 10 (6): 470-477.
11. ^ Cornu, A. and Baille, J. B. (1873), Mutual determination of the constant of attraction and the mean density of the earth, C. R. Acad. Sci., Paris Vol. 76, 954-958.
12. ^ Electricity. Encyclopedia Britannica (1911).

• The Electrical Researches of the Honourable Henry Cavendish edited by James Clerk Maxwell, Cambridge: University Press (1879).

• Experiments on Air by Henry Cavendish, Edinburgh: William F. Clay (1893) - Alembic Club reprint number 3.

• "The Mean Density of the Earth" by J. H. Poynting, London: Charles Griffin and Company (1894).

• The Laws of Gravitation: Memoirs by Newton, Bouguer and Cavendish, edited and translated by A. Stanley MacKenzie, New York: American Book Company (1900).

• A History of Chemistry by F. J. Moore, New York: McGraw-Hill (1918) - See especially pages 34 – 36.