Six-minute summary: Henry Cavendish

Hello readers! It’s time for another six-minute summary of a historical figure. This week, we’ll explore the life of Henry Cavendish, an eighteenth-century scientist best known for discovering hydrogen and measuring the density of the Earth. However, Cavendish kept many of his findings private, and much of his pioneering research only reached public attention decades after he passed away. He was one of those classic, old-fashioned scientists who studied anything and everything, but he stood out from the crowd due to his meticulous attention to detail, unwavering focus, and curiously outdated fashion sense.

The Honourable Henry

Henry Cavendish was born in 1731. Not much is known about his childhood, and so his 1851 biography fills the “early life” void with an entire chapter on his ancestry. His family tree can be traced all the way back to the barons of William the conqueror, not just from one parent, but both. Henry Cavendish was so upper class that people addressed him as “The Honourable” Henry Cavendish even before he had done anything particularly honourable.

At the age of eighteen, Cavendish went to Cambridge, where he studied for four years. His time there was seemingly uneventful, but there are no surviving records of his academic achievements or social life. All we know is that he left without bothering to get a degree, which wasn’t especially odd for the time. After all, The Honourable Henry didn’t need a degree in order to pursue his interests. His father, Lord Charles Cavendish, was a member of the Royal Society with his own laboratory devoted to meteorology and thermometers. Lord Cavendish soon started taking The Honourable Henry along to Royal Society meetings and dinners, and Henry was officially elected to join them in 1760. After this, he never missed a meeting or a dinner; indeed, these events comprised the entirety of his social life.

Chemistry and gases

Henry Cavendish published his first paper with the Royal Society in 1766. He had made some crucial discoveries on combustion, gases, and the composition of the air – all at a time when chemists believed combustible materials to contain a mystical substance called phlogiston (more on that later). Within this restrictive belief system, Cavendish made some huge strides towards understanding what was really going on.

Firstly, he saw the importance of condensation that formed when materials were burnt within sealed glass containers. Other scientists had noticed this and overlooked it, but not Cavendish. He knew that nothing had been added or removed from the containers, and he had made careful measurements which proved that the air inside contained less oxygen after burning. Indeed, twice as much hydrogen than oxygen was missing from the air at the end of the experiment, and he suspected that they had both ended up in the condensation. As such, he judged that water was one part oxygen, two parts hydrogen (not that he used these names).

Cavendish also measured the composition of atmospheric air, finding it to be one part oxygen and four parts nitrogen (again, using different names). He took air samples for sixty days in a row, at specific times of day, at specific places in the town and in the countryside, and he ended up measuring hundreds of samples. He proved that atmospheric air was always the same, irrespective of time or place – something that hadn’t yet been tested.

Henry Cavendish’s work was of a much higher quality than many of his contemporaries. He was careful to trap the gases from his experiments without contaminating them or letting them escape. Better still, he devised new, robust ways to test their properties, including their density, combustibility, and solubility, and he recorded values for hundreds of different materials. Perhaps most importantly, he kept detailed and comprehensible notes that were of great use to later scientists.

A brief overview of phlogiston theory

At the time that Henry Cavendish was conducting experiments, scientists knew very little about burning and oxidation. It was widely believed that flammable substances contained phlogiston, which was released when they burned. The air absorbed phlogiston, but only up to a point. Once air became saturated with phlogiston, it could no longer support dephlogistication – which is how they explained candles going out in sealed containers.

This theory stunted scientific enquiry for several decades. Even when Joseph Priestly discovered oxygen, he believed it to be “dephlogisticated” air. It took years for phlogiston theory to be debunked, even though it was widely known that some metals gained mass when burnt, rather than losing it. Scientists were so reluctant to give up the theory that they rebranded phlogiston as a “principle” rather than a substance, and some even proposed that it had negative mass (cough-Robert Boyle-cough), despite this contradicting the original idea. Only in 1770 did Lavoisier prove that combustion requires oxygen. Therefore, although many of Henry Cavendish’s ideas were groundbreaking, they were recorded and interpreted within a flawed conceptual framework.

Density of the Earth

The most famous experiment by Henry Cavendish allowed him to determine the density of the Earth. In 1772, the Royal Society set up the “Committee of Attraction” to determine the Earth’s density, and Cavendish was involved in the initial experiment, which involved finding a uniformly shaped mountain in Scotland and measuring how much it deflected a plumb bob (a weight on a string). Henry Cavendish was not happy with the various assumptions in this approach, and knew he could do better. However, the experiment that he eventually used, which now bears his name, was devised by a scientist named John Mitchell. He was a friend of Cavendish, but he died before he could use it.

Henry Cavendish made a few modifications to Mitchell’s experimental designs after they came into his possession in 1797. However, the basic principle was the same: there were two, heavy, stationary spheres, and between them was a long, horizontal pole with two smaller spheres on either end. This pole was suspended by a wire at its centre, allowing it to rotate. The two smaller spheres were attracted to the larger spheres by gravity, but the twisting of the metal suspending wire made the pole oscillate back and forth. Cavendish measured these oscillations to find the gravitational attraction between the spheres. Isaac Newton’s universal law of gravitation had been published all the way back in 1687, but this was the first time that someone had measured the universal gravitational constant – and amazingly, the value that Henry Cavendish obtained was within 1% of the value we use today.

The accuracy of Cavendish’s measurement was down to his meticulous experimental procedure. He realised that the experiment was very sensitive to air currents, and so he set it up in a sealed room. He also shut out the light in order to avoid temperature variations throughout the day. After this, he sat and watched the spheres oscillate for hours on end, observing them through a telescope from afar. Nobody managed to improve his measurements for over a century.

Electricity

Henry Cavendish also made some interesting discoveries regarding electricity. He published a paper with the bizarre title “Attempts to imitate the effects of the Torpedo”, which is less bizarre once you find out that the Torpedo is a fish from the Mediterranean that can give mild electric shocks. Scientists were arguing whether fish electricity was the same as laboratory electricity, and so Cavendish decided to build an apparatus that mimicked the Torpedo fish, to convince people that they really were electric.

Lots of his ideas seem a little strange to us now (such as his proposition that electricity was a fluid that exerted pressure), but he made some important observations that were well before their time. He noticed that electricity had very different effects depending on its “intensity” and its “quantity” – but the concepts of voltage and current would not be proposed until decades later. Faraday even referenced Cavendish’s work as an inspiration in the mid-1800s.

Most of Cavendish’s electrical discoveries went unpublished, and were only unearthed by James Clerk Maxwell over a century later when he went through his notes. He had measured electric potential, invented an early unit of capacitance, found the relationship between voltage and current, and also found the inverse square law of electric force with distance. These last two were re-discovered by later scientists, and we now call them Ohm’s Law and Coulomb’s Law respectively.

Later life

Lord Charles Cavendish died in 1783, leaving behind a huge estate that went almost entirely to Henry. As such, Henry Cavendish lived a very comfortable life and died in 1810 as once of the richest men in Britain. He devoted his huge house entirely to his experiments: the upper floors and the garden were for astronomy, the drawing room was converted to a laboratory, and he measured the density of the Earth in an outhouse. He also had a townhouse in the city that was set up as his library, which he allowed other scientists to use. Beyond this, we know very little. When his biography was written in 1851, the biographer couldn’t find much to write about besides his science.

Neurodivergence?

Henry Cavendish’s social life and acquaintances were so limited that few records remain of his character. He was known for being shy and eccentric, but he was seemingly respected by his peers. According to some accounts, he would only talk to one person at a time – but they had to be known to him, and they had to be male. He would communicate to his female servants via notes, and he supposedly constructed an additional staircase to avoid his housekeeper, and went for walks at night in order to avoid his neighbours.

The other Royal Society scientists apparently found it difficult to talk to him. If they said something that interested him, he would give a quiet reply, but he was well-known for giving mere grunts in response, and for abandoning conversations in order to hide in empty corners of the room. His fashion sense only added to his eccentricity, as he always wore a suit that was 50 years out of style.

Some historians have suggested that Henry Cavendish was autistic. Of course, historical diagnoses should be taken with a pinch of salt, but it seems that Cavendish’s potential neurodivergence aided him greatly in some ways, and held him back in others. His meticulous attention to detail and his unshakeable focus helped him make some of the most rigorous scientific observations of the century, but his nervousness may well have held him back from discussing work with a broader audience of fellow scientists. He didn’t publish much throughout his lifetime – but then perhaps he didn’t feel the need.

In summary…

Henry Cavendish was a diligent and dedicated scientist with an obvious knack for experimental design and interpreting experimental results. His determination of the density of the Earth was incredible, but unfortunately his chemistry achievements were restricted by the shackles of phlogiston theory. Thanks for reading, and have a lovely week!