Measuring pressure is tricky, just ask Stephen Hales’ horse!

Robert Boyle was fascinated by the world and he spent his life trying to explain why things are as they are. He was a chemist, physicist and inventor, and his work went on to be a major influence on the founder of modern chemistry, Isaac Newton. He was particularly interested in ‘air’. We’re all used to breathing, we’ve seen things on fire and noticed that if we exhale when it’s cold some sort of vapour emerges from our mouths. What’s going on? We’re surrounded by something that we can’t see, but what is it?

Yale University Press’ Little Histories collection is a family of books that take a closer look at some of the most significant events, ideas, discoveries and people throughout history. To celebrate the launch this week of the paperback edition of William Bynum’s A Little History of Science, we’re proud to present an exclusive extract from this acclaimed title. This piece, taken from the chapter ‘Airs and Gases’, introduces some of the first investigations into the nature of ‘air’ and shows how these experiments went on to influence our modern understanding of the subject.LHSceincefeatured1“‘Air’ is a very old word. The word ‘gas’ is much newer, only a few hundred years old, and the shift from air to gases was crucial. For the ancient Greeks, air was one of the four fundamental elements, just one ‘thing’. But Robert Boyle’s experiments in the seventeenth century had challenged this view, and scientists had come to realise that the air that surrounds us, and that we all breathe, is made up of more than one substance. From then on it was much easier to understand what was happening in many chemical experiments. Lots of experiments produced something that bubbled up, or went up in a puff and then disappeared into the air. Sometimes the experiment seemed to change the air: chemists often produced ammonia, which made their eyes water, or hydrogen sulphide, which stank of rotten eggs. But without being able to collect the gases in some way, it was hard to know what was going on. Isaac Newton had showed that measurement was important, but it was hard to measure a gas if it was just loose in the atmosphere.

So chemists had to find ways to collect pure gases. The most common way of doing this was to conduct the chemical experiment in a small closed space, like a sealed box. This enclosed space was then connected by a tube to an upside-down container completely filled with water. If the gas didn’t dissolve in the water – and some gases do – it could bubble up to the top and push the water down. Stephen Hales (1677 – 1761), an ingenious clergyman, devised a very effective ‘water bath’ for collecting gases. Hales spent most of his long life as the vicar of Teddington, then a country village, now swallowed up into London. A modest and retiring man, he was also extremely curious and a constant experimenter. Some of his experiments were pretty horrible: he measured the blood pressure in horses, sheep and dogs by directly sticking a hollow tube into an artery. This was attached to a long glass tube, and he simply measured how high the blood rose, which equalled the blood pressure. For a horse, the glass tube has to be nine feet tall (2.7 metres) to prevent blood spurting out the top.

Hales also studied the movement of sap in plants and measured the growth of different parts of plants. He painted tiny specks of ink at regular intervals on their stems and leaves, and then recorded the distances between the specks before and after the plant had grown. He showed that not all the parts grew at the same rate. Hales then used his apparatus for collecting gases to see how plants react in different conditions. He saw that they were using ‘air’, as the atmosphere was called. (in 1727 his book Vegetable Staticks laid the foundations for the later discovery of photosynthesis, which is how plants use sunlight as a source of energy, and are able to change carbon dioxide and water into sugars and starches, and ‘breath’ out oxygen. It is one of the most fundamental processes on our planet. But we are getting ahead of ourselves, and at that stage no one knew about oxygen.


Stephen Hales had led the way with his water bath, and his demonstration that plants, as well as animals, need air. This ‘air’ was understood to be a gas that was released when something butned. A Scottish doctor and chemist, Joseph Black (1728-99), collected this ‘air’ (which he called ‘fixed air’) and showed that while plants could live in it and use it, animals would die if they were placed in a container with just fixed air to breathe. They needed something else. Balck’s ‘fixed air’ is now called carbon dioxide (CO2), and we know it’s an essential part of the life cycles of plants and animals. (It is also a ‘greenhouse gas’, a main cause of the ‘greenhouse effect’, which is leading to global warming.)

A reclusive aristocrat, Henry Cavendish (1731 – 1810), spent his days in his private laboratory in his London house, experimenting and measuring. He discovered more about fixed air, and collected another air, one that was very light, and exploded when sparked in the presence of ordinary air. He called it ‘inflammable air’. We now call it hydrogen, and it turned out that the explosion produced a clear liquid that was nothing other than water! Cavendish also worked with other gases, such as nitrogen.”

This collection of scientists all made pioneering contributions to our understanding of gases and all are celebrated in A Little History of Science by William Bynum. Pick up a copy of the book to explore the rest of the chapter and discover the remarkable stories of science. | More Remarkable People | Buy A Little History of Science

A Little History of Science

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