The Clapham Society Local History Series — 31

Eccentric Scientist who weighed the Earth
by Derrick Johnson

This article first appeared in the South London Press on 13 October 2017
(Entitled: ‘Cavendish was in his element in a laboratory’)

One of the most distinguished, and eccentric, former inhabitants of South London was Henry Cavendish who is remembered in the name of Cavendish Road off the South Side of Clapham Common.

Henry was born in 1731 in Nice where his family was living at the time. His mother, who died when he was only a year old, was Lady Anne Grey, fourth daughter of the First Duke of Kent, and his father Lord Charles Cavendish, third son of the Second Duke of Devonshire. At the age of 11 he attended Hackney Academy a private school in East London. Although later educated at Cambridge University at what became Peterhouse College he left without taking a degree – a common practice at the time. However, like his father he was passionately interested in science – he became a member of the Royal Society of London and established his own laboratory. On his father’s death in 1783 he moved this from their London home to a large mansion in Clapham, the site of which is at the junction of the present Cavendish Road and Clapham Common South Side. Henry used the upper rooms of the house as an observatory, the drawing room as a laboratory and had a forge in an adjoining room. The house became known as Cavendish House. It was demolished in 1905.

Henry published no books and few papers but achieved much. He noticed that the action of certain acids on certain metals produced a highly inflammable gas which was later called hydrogen. He measured its properties and recognised it as an element. He noted that water was produced by burning it in an atmosphere of oxygen and estimated that it consisted of two parts hydrogen and one part oxygen.

In 1785 he carried out an experiment on the composition of air obtaining impressively accurate results. He removed both the nitrogen and oxygen (phlogisticated and dephlogisticated air in the terms of the times) but there remained a volume of gas amounting to 1/120 of the original volume of nitrogen. Over 100 years later the British physicists William Ramsay and Lord Rayleigh realised that their newly discovered inert gas, argon, was responsible for Henry’s residue – he had not made an error.

In addition to his work on gases Henry carried out research in mechanics, optics and magnetism but his most famous experiment was to determine the density of the earth, and hence its weight, in what became known as the Cavendish Experiment. The apparatus he used was a torsion balance consisting of two small lead spheres and two much larger stationary ones and he measured the gravitational attraction between them from the period of oscillation. He was noted for the precision of his measurements and for taking extraneous factors into account. Measuring an astronomically small attraction (1/50 millionth of the weight of the balls) the result that he obtained for the density of the earth is within 1% of the currently accepted figure.

Henry also carried out electrical experiments but the bulk of his work was not published until a century later. Among his discoveries was the concept of capacitance, dielectric constant, electric potential and what later became known as Ohm’s Law.

Henry was a shy man, uncomfortable in society and particularly the company of women. He said little, developed no known personal attachments outside his family and was generally regarded as eccentric. His only social outlet was the Royal Society Club whose members dined together before their weekly meetings. He seldom missed one but his shyness made those who sought his views speak as if into vacancy. If their remarks were worthy they might receive a mumbled reply but more often than not they would hear a peeved squeak (he appears to have had a high pitched voice) and turn to find an actual vacancy and the sight of Cavendish fleeing to find a more peaceful corner. Henry only communicated with his housekeeper by notes left in the hall and his female servants were not allowed to see him. It is said that he had a back staircase added to the house so that he could avoid them. He always dressed in an old fashioned suit as in the illustration.

Henry inherited the bulk of his father’s substantial estate and when he died in 1810 was one of the wealthiest men in Britain. He was often to be seen walking down the middle of the road after dark in his unusual clothing – spare a thought for him as you wait at the traffic lights at Cavendish Road!