Henry Cavendish (1731-1810) was an English scientist who made pioneering investigations in chemistry and used a torsion balance experiment, devised by John Michell, to make the first accurate measurements of the mean density of the Earth and the strength of the gravitational constant. He also carried out pioneering work on electricity, but much of his work was not published in his lifetime, and only became widely known when Cavendish’s papers were edited and published by James Clerk Maxwell in 1879.
Cavendish could afford not to publish his results, because he did not have to make a living out of science. Born on 10 October, 1731, at Nice, in France, Cavendish was the son of Lord Charles Cavendish, and grandson of the both 2nd Duke of Devonshire (on his father’s side) and the Duke of Kent (on his mother’s side). His father, himself a Fellow of the Royal Society, was administrator of the British Museum. Henry Cavendish studied at Cambridge University from 1749 to 1753, but left without taking a degree (not particularly unusual in those days), and studied in Paris for a year
before settling in London. He lived off his private fortune, and devoted his time to the study of science. Apart from his scientific contacts, he was reclusive, and published little, although he used some of his money to found a library, open to the public, located well away from his home. He was once described as “the richest of the wise, and the wisest of the rich.”
Among his unpublished discoveries, Cavendish anticipated Ohm’s Law and much of the work of Michael Faraday and Charles Coulomb. He also showed that gases could be weighed, and that air is a mixture of gases, not a pure substance.
Cavendish died on 28 February 1810, and left more than a million pounds in his will. The famous Cavendish Laboratory in Cambridge, named after Henry Cavendish, was founded in 1871 with funds provided by the 7th Duke of Devonshire, a relative of Cavendish and himself a talented mathematician.
One of the great English scientists of the second half of the eighteenth century , Cavendish, among other things, discovered hydrogen gas. But for a long time not many people knew just how clever he was, because as well as being almost unbelievably rich, so that he could do whatever he liked, Cavendish was also incredibly shy, and he didn’t bother to tell the world about most of his amazing discoveries. But he did write down accurate notes of all of his experiments, which were discovered after he died.
His father was only the fourth out of five brothers and six sisters, so he didn’t inherit a title himself; but he was certainly aristocratic, and he inherited a lot of money.
Henry’s father, Charles Cavendish, had married his mother, Anne de Grey, in 1729. Anne was only 22, and she was ill almost for the rest of her short life, with what seems to have been tuberculosis. Henry was born in Nice, in 1731, where his parents had gone to escape from the English winter, and his brother Frederick was born in England in 1733.
Before the end of that year, their mother was dead. Charles Cavendish never remarried, so Henry never really had a mother, which may partly explain why he grew up to be such a peculiar man.
In 1738, Charles Cavendish sold his country estate and moved to London with his two sons, in the year Henry had his seventh birthday. Both boys went to school in London, then on to Peterhouse (a Cambridge college, but never called Peterhouse College). After Henry had left Peterhouse in 1753, Frederick fell from an upstairs window and suffered a head injury which caused permanent brain damage. He was well enough to manage a fairly normal life, with the help of servants, but after the accident he could not do anything very intellectual.
But Henry was clever enough for at least two ordinary people. He went on the usual Grand Tour with his brother, then settled down at the family house in Great Marlborough Street to be a scientist. He wasn’t interested in anything else at all, and although he received an allowance from his father of £500 a year, he hardly spent any of it. He only ever owned one suit of clothes at a time, which he wore every day until it was worn out. Then he bought another in exactly the same style, even though this got more and more old-fashioned as time passed.
Later on, after his father died in 1783, when Henry was 52, and he had a huge fortune, Cavendish carried on just the same. He ate mutton every day, and one day when he was expecting some scientific friends for dinner (he only had scientific friends), his housekeeper asked him what to serve. “A leg of mutton,” he replied. She said this would not be enough. “Well then,” he said, “get two.”
One day, his bank manager called round. He was worried because Henry had £80,000 in his current account. This was a vast fortune when a fashionable gentleman could live comfortably on £500 a year, but Cavendish was so rich he had forgotten about it. The banker asked Cavendish if he would like to invest the money more profitably. Cavendish was so angry at having been bothered about the money that he told the bank manager to go away at once, or he would close the account.
Rather nervously, the manager asked if Cavendish might like to invest just half the money. To shut him up, Cavendish said the banker could do what he liked with the £40,000 as long as he went away at once. The honest banker put the money into safe investments, where it made a profit and made Henry Cavendish even richer.
When he died, in 1810, Cavendish was worth almost exactly a million pounds. This would be equivalent to about a billion pounds today, making him nearly as rich as Bill Gates. He left all the money to relatives, and one of their descendants, William Cavendish, the seventh Duke of Devonshire, used some of the fortune to establish the Cavendish Laboratory, in Cambridge, in the 1870s.
The only thing Henry Cavendish spent money on was houses, to give himself space for his scientific work, and laboratory equipment to put in the space. After his father died, he rented out the house in Great Marlborough Street, and bought one at Clapham Common, which was then a quiet, leafy area just outside the bustle of London.
Cavendish only ever went out on scientific business. He became a Fellow of the Royal Society in 1760. He hadn’t done any real science then, but in those days rich people who were interested in science were welcome as Fellows even if they hadn’t actually done much science. Cavendish often went to their meetings. But even there he was so shy that if he was late he would wait quietly outside the door until somebody else came along, so that he wouldn’t have to go into the room on his own. He also went to dinner with other Fellows, who had a dining club that met regularly.
Most of the time, Cavendish only communicated with his servants by writing notes to them, and several people who knew him have written how if he came across a woman he did not know he would cover his eyes with his hand and run away. But in the summer he would travel round Britain in a coach, visiting other scientists and studying geology.
The reason Cavendish was regarded as “the wisest of the rich” was thanks to his work in chemistry. This was because he did publish a lot of papers on this work, although he didn’t publish all of it. At the time, nobody knew about most of his other work, even though it was just as important. For example, in electricity we now know that Cavendish was the first person to discover what is known as Ohm’s Law, but he never told anybody, so Ohm had to discover it again later.
In the 1760s, Cavendish started experimenting with gases, carefully following Black’s example by measuring and weighing everything as he went along. He found that the gas given off when acids react with metals is different from ordinary air, and from Black’s fixed air. It burned very easily, and Cavendish called it “inflammable air;” we call it hydrogen. Indeed, the gas burned so vigorously that Cavendish soon decided that it must be pure phlogiston.
He also studied Black’s fixed air and the properties of Priestley’s fizzy mineral water. But in 1767, probably because he read Priestley’s book on electricity, he dropped his chemical experiments, and turned his attention to electricity. Hardly any of this work was published at the time, which was a great loss to science. Among other things, Cavendish proved that electricity obeys an inverse square law. This is now known as Coulomb’s Law, because Coulomb was the first person to publish it. Cavendish also measured the strength of the electric force very accurately.
Then, in the 1780s Cavendish went back to chemistry. He’d got interested in the way that air seems to be lost when things burn in it. For example, if a lighted candle is stood on a little island in a bowl of water, with a glass jar over the top, as he candle burns the level of water rises. This is because the volume of air is shrinking. About a fifth of the air disappears in this way before the candle goes out. We say that this is because one fifth of the air is oxygen, and the oxygen gets used up in burning.
Cavendish still tried to explain what was going on in terms of phlogiston, even though Priestley had already discovered oxygen and found that it makes up about a fifth of ordinary air. The explanation got horribly complicated and is exceedingly difficult to understand. What matters is that Cavendish carried out experiments in which oxygen (dephlogisticated air, to him) and hydrogen (pure phlogiston, he thought) were exploded together in a metal container, using an electric spark.
Apart from making a satisfying bang, the experiment at last started chemists, although not Cavendish, thinking along the right lines about oxygen, and what happens when things burn. Hydrogen and oxygen combine to make water. Cavendish found that his two gases always joined together in the same proportions to make water. He weighed everything carefully before and after each experiment, so he found that the weight of water produced was exactly the same as the weight of gas lost. Putting the numbers in, he found that 423 measures of “phlogiston” combine exactly with 208 measures of “dephlogisticated air” to make pure water with no gas left over.
This was a key moment in chemistry because it showed that water is a compound substance. It is somehow made by two other substances joining together, not any old how but joining together always in exactly the same proportions. Actually 2:1 exactly, we now know, for hydrogen and oxygen combining to make water.
This was the first step towards understanding how atoms combine to make molecules. Cavendish couldn’t take the step properly because he was stuck with the idea of phlogiston. But his discovery was immediately picked up and developed in France, by Antoine Lavoisier.
In 1785 Cavendish was able to remove both oxygen and nitrogen gases from air and was left with a tiny amount of unreactive gas. It was only in the 1890s that William Ramsay and Lord Rayleigh realised that their newly discovered inert gas, argon was the same as Cavendish’s leftover “air”. This highlights his skill at rigorous quantitative experiments. He used calibrated equipment, obtained reproducible results, repeated those experiments and averaged the results, and always tried to allow for sources of error.
While Lavoisier and others took his insight into the nature of air forward, Cavendish carried on experimenting, going to scientific meetings and dinners, and publishing some, but not all, of his discoveries. But he did keep records. Most of his electrical discoveries languished until they were published by Maxwell, in 1879, by which time other scientists had duplicated them and received the credit. The best source for appreciating these discoveries is The Scientific Papers of the Honourable Henry Cavendish, F.R.S. (http://www.amazon.co.uk/Scientific-Honourable-Cavendish-Cambridge-Collection/dp/1108018238). The 1911 edition of Encyclopedia Britannica lists some of Cavendish’s discoveries as the concept of electric potential, a unit of capacitance, the concept of dielectric constant, Ohm’s Law, Wheatstone’s laws for the division of current in parallel circuits, and Coulomb’s Law. We can only wonder how rapidly nineteenth-century science might have progressed if he had bothered to publish all this.
Since the one thing he is remembered for is the torsion balance experiment, I shall not go into details here. But Wikipedia is OK on this: https://en.wikipedia.org/wiki/Cavendish_experiment
Cavendish lived to be 78 and died quietly at home in 1810