Airs and Gases

A Little History Of Science: Airs and Gases

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 had to be nine feet tall (2.7 metres) to prevent the blood spurting out the top.

Hales also studied the movement of sap in plants and measured the growth of the 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 still 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 ‘breathe’ 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.) Remember the word pneuma, from Chapter 6? ‘Pneumatic’ just means ‘relating to air’, and pneumatic chemistry – the chemistry of airs – was one of the most important areas of science in the eighteenth century. (Did you notice that ‘airs’ was plural there?) Pneumatic chemistry was where it was at from the 1730s onward. It was not just that the older notion of ‘air’ was giving way to the much more dynamic idea of it actually being made up of several kinds of gases. Scientists were also discovering that most substances can exist as – or be transformed into – a gas, given the right conditions.

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 burned. 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. Black’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.

No one was as successful in pneumatic chemistry research as Joseph Priestley (1733–1804). Priestley was remarkable. A clergyman, he wrote books on religion, education, politics and the history of electricity. He became a Unitarian, a member of a Protestant group that believed that Jesus was only a very great teacher, not the Son of God. Priestley was also a materialist, teaching that all the things of nature could be explained by the reactions of matter: there was no need for a ‘spirit’ or ‘soul’. During the early days of the French Revolution, which he supported, his house in Birmingham was burned down by people who feared that liberal religious and social views like his might bring revolution across the Channel. He fled to the United States, where he lived the last ten years of his life.

Priestley was also a very busy chemist. He used fixed air to make soda water, so remember him the next time you have a fizzy drink.

Priestley identified several new gases, and, like all pneumatic chemists, he wondered what happens when things burn. He knew air played a part in burning, and he also knew that there was a kind of ‘air’ (a gas) that made things burn even more vigorously than the ‘ordinary’ air that surrounds us. He made this ‘air’ by heating a substance that we know as mercury oxide, and collecting the gas in a water bath. He showed that animals could live in it, as plants could in fixed air. Priestley’s new ‘air’ was something special: indeed, it seemed to be the principle that was involved in many chemical reactions, as well as in breathing and burning. He thought it could all be accounted for by a substance called ‘phlogiston’, and that all things that can burn contain phlogiston, which is released in the burning process. When the air around becomes saturated with phlogiston, they can no longer burn.

Many chemists used this idea of phlogiston to explain what happens when things burn, and why some ‘airs’ would make things in a closed container burn for a time, and then seem to make them go out. Burn a lump of lead, and the product (what is left behind) will be heavier than the original lump. This suggested that phlogiston, which scientists thought was contained in the lead and was released through burning, must have a negative weight – that is, it makes whatever contains it lighter than things that don’t contain it.

When most things burn, the products are gases that are difficult to collect and weigh. Burn a wooden twig, for example, and the product that is esay to see – the ash – is much lighter than the original twig; to get the total weight of the product, the gases given off would have to be collected, weighed and added on. In Priestley’s scheme, phlogiston took the place of what we call oxygen, except that it had almost exactly the opposite properties!

For Priestley, when things burned, they lost phlogiston, and became lighter; but we would say they combine with oxygen, and we now know that things get heavier when this happens. When the candle went out in a closed container, or if a mouse or bird died after a while of being sealed inside a closed container with ordinary air, Priestley said it was because the air was saturated with phlogiston; we now know that it’s because the oxygen has been used up. It reminds us that it is possible to do very careful experiments, and take careful measurements, but explain the results in very different ways.

The man who named oxygen is still known as the ‘father’ of modern chemistry. Antoine-Laurent Lavoisier (1743–94) met a violent death during the French Revolution. He was arrested, tried and guillotined, not because he was a chemist, but because he was a ‘tax farmer’. In pre-Revolutionary France, rich men could pay a fee to the State to become tax collectors, and then keep what they could collect. The system was rotten, but there is no evidence that Lavoisier abused it. In fact, he spent a lot of his time before the Revolution doing important scientific and technical research for the State, investigating a number of important questions in manufacturing and agriculture. But he was an aristocrat, and the Revolutionary leaders hated him and his class, and he paid the price.

Like Priestley, Cavendish and the other pneumatic chemists, Lavoisier was an enthusiastic experimentalist, and was helped by his wife. In fact Madame Lavoisier was an important figure in science. Marie-Anne Pierrette Paultze (1758–1836) married Lavoisier when she was only fourteen years old (he was twenty- eight), and they worked together in the laboratory, performing experiments, taking readings and recording the results. In addition, Madame Lavoisier was a charming hostess. She and her husband entertained learned men and women who discussed the latest developments in science and technology. Theirs was a happy marriage of real partners.

As a schoolboy, Lavoisier loved science. His sharp mind and scientific ambition were evident from an early age. Like most students who studied chemistry then, he grew up with the phlogiston idea, but he exposed a number of logical and experimental flaws in it. Lavoisier was determined to have the best apparatus available. He and his wife devised new laboratory equipment, always with the aim of improving accuracy in chemical experiments. He used very accurate scales to weigh the substances in his experiments. Several different kinds of experiments convinced him that when things burn, the total weight of all their products increases. This involved collecting and weighing the gases that combustion produced.

Lavoisier also continued to investigate what happens when we (and other animals) breathe. These experiments assured him that the substance involved in both combustion and respiration was a single, real element, and not some kind of substance like phlogiston. This element also seemed to be necessary for acids to form.

The chemical reactions of acids and alkalis (the latter are some- times called ‘bases’) had long fascinated chemists. Remember Robert Boyle’s invention of litmus paper? Lavoisier continued this line of work. Indeed, he believed that oxygen (which means ‘acid former’) is so important in acids that they always contain that element. We now know that this is not true (hydrochloric acid, one of the most powerful acids, contains hydrogen and chlorine, but no oxygen). Yet much of what Lavoisier said about oxygen is still part of our knowledge today. We now know it is the element needed for things to burn, or for us to breathe, and that those two seemingly different processes have much in common. Humans use oxygen to ‘burn’, or process, sugars and other things we eat, to give our bodies energy to carry out our daily functions.

Lavoisier and his wife continued with their chemical experiments during the 1780s, and in 1789, just on the eve of the French Revolution, Lavoisier published his most famous book. Its English title is Elements of Chemistry, and it is just that. It is the first modern textbook of the subject, full of information on experiments and equipment, and containing his reflections on the nature of the chemical element. We now call an element some substance that cannot be broken down any further by chemical experiments.

A compound is a combination of elements which, given the right experiment, can be broken down. So water is a compound, made up of two elements, hydrogen and oxygen. This distinction was at the heart of Lavoisier’s important book. His list of elements, or ‘simple substances’, did not contain all the elements chemists now recognise, as many had not yet been discovered. It did include surprising things such as light and heat. But Lavoisier laid down the basic framework for understanding the difference between an element and a compound.

Just as important was his belief that the language of chemistry must be precise. With several colleagues, Lavoisier reformed the language of his subject, demonstrating that to do good science, you needed to be precise in the words you use. (Linnaeus would have agreed.) Chemists needed to be able to refer to the compounds and elements they were experimenting with, so that any other chemist, anywhere in the world, would know they were dealing with exactly the same things. He wrote, ‘We think only through the medium of words.’ After Lavoisier, chemists increasingly shared a common language.