Some refer to Cavendish as 'the man who weighed the earth', but it would be more accurate to describe him as a man who spent his life as a careful investigator in chemistry, heat and electricity.
He was a reclusive man, described by McCormmack & Jungnickel as (quote) 'a scion of one of the most powerful aristocratic families, a scientific fanatic, and a neurotic of the first order' [ref1.p.1.]. He found it difficult to talk to anyone beyond a small circle of scientists, and – people say now – may have suffered from Aspergers. Devoting himself to a life of experimentation in his father's house, he published little, although Clerk Maxwell was astonished to find later such a large quantity of original work amongst his papers, including his demonstration of hydrogen as being an element.[Ref2.,p.56]
He had been born in Nice, because his mother needed a warmer climate than England had, but she died when he was only four. Both he and his brother were brought up by their father, Lord Charles Cavendish, a man from the nobility who himself had a strong interest in the sciences. Both of Lord Cavendish's sons went to Cambridge, but Henry, a serious student of mathematics and Newtonian philosophy – amongst many not-so-serious sons of nobility – chose not to bother with the final exams [p. 138.] Thus, from his early 20s he became a lifelong experimenter. His first manuscripts are dated from when he was 34 [p.192], on such things as arsenic, acids [p.203]; carbon dioxide (what he called “fixed air”); “factitious air” (Copley Medal, when he was 36)[p.209]; with later manuscripts on aurorae, Indian astronomy and the Hindu civil year.
But as we mentioned at the start, he is most well-known for the famous 'Cavendish experiment', which verified Newton's law of Universal Gravitation.
This law always fascinated me as a child. How could a coffee cup be attracted to a teapot? A chair to another chair – especially when Newton had dismissed any test as impractical, and had calculated that two large spheres only a millimeter apart would take about a month to come together?
His acquaintance from the Royal Society, as well as from student times, the Rev. John Michell [Ref.2], had completed an apparatus for this attraction, shortly before his death. The apparatus then passed to Cavendish, who, at 67 was already highly respected by colleagues in the Royal Society for his painstaking experimentation.
So Cavendish largely rebuilt the Michell apparatus – two massive lead balls, about 350 lbs each, near to two much smaller balls hanging on a rod suspended on a fine wire. Each little ball was of course, as Newton had said, attracted to its nearest large ball, causing the rod to twist ever so slightly against the springiness of the thin suspension wire.
His results were presented to the Royal Society in 1798, more than 100 years after Newton had postulated the law, and his 57 pp paper, describing all the possible errors, was published in Philosophical Transactions. He deduced the average density of the earth as about 5 times that of water, which is correct, but most importantly, his careful experimentation yielded an accurate value for Newton's universal constant, “big G”.
Henry Cavendish never married, and his reclusiveness was very well-known. It is said that if a female servant entered the room he would ask her to leave, or he would himself scurry away. He died in 1810, by now one of the wealthiest men in England, because he never spent anything, and because Earls and Dukes in the Cavendish family always seemed to leave him money and estates.
The Cavendish Lab, which can now boast at least 30 Nobel prize winners, was built in Cambridge about 60 years after his death.
References:
1. “Cavendish; The Experimental Life”; by Christa Jungnickel and Russell McCormmach, Bucknell, 1999.
2. “Speculative Truth; Henry Cavendish, Natural Philosophy, and the Rise of Theoretical Science”; by Russell McCormmach; Oxford Univ.Press, 2004.
3. “Remarkable Physicists, from Galileo to Yukawa”; by Ioan James; Cambridge Univ.Press, 2004.