Science: A History in 100 Experiments

Computer artwork of James Watt’s improved version of Thomas Newcomen’s steam engine.

By experimenting with the scale model of a Newcomen engine and applying his understanding of Black’s discoveries, Watt realized that this kind of engine is not very efficient. On every stroke of the engine, the whole cylinder and piston combination has to be heated up to more than the boiling point of water, in order to allow it to fill with steam. Then, it has to be cooled sufficiently for the steam to condense, even though the steam itself (as he later appreciated) gives up latent heat as it condenses. The heat required to raise the temperature of the cylinder and piston was thrown away with every stroke of the engine.

Watt realized that it would be much more efficient to have an engine that used two cylinders, one of which was kept hot all the time and contained the moving piston, while the other, without a piston, was kept cold all the time. (He wrote in his journal that this idea came to him on a Sunday afternoon in May 1765, as he walked across the Glasgow Green.) In his early models, the cold cylinder, without a piston, was simply immersed in a tank of water. The two cylinders were connected to each other, but at first outside air was still used to push the piston down. Steam pushed the piston up as before, but when the piston reached the top of its stroke a valve opened automatically to let the steam flow into the cold chamber, where it condensed, reducing the pressure and allowing the piston to fall. At the bottom of the stroke, another atomatic valve opened to let fresh steam into the cylinder. Soon, this setup was improved by sealing off the piston’s cylinder from the atmosphere and using hot steam to push the piston down as well as to push it up. But the key concept was the ‘separate condenser’. Watt’s steam engine design was patented in1769.

Because Black had not published all of his discoveries, and Watt had a fairly lowly position in Glasgow, at first Watt did not know about Black’s work on latent heat, and independently made the same discovery. Specifically, in one series of experiments he noticed that when one part of boiling water is added to thirty parts of cold water, the rise in temperature of the cold water can hardly be measured. However, when the equivalent amount of steam at the temperature of boiling water is bubbled through the cold water, it can raise the temperature of the water to boiling point. This discovery by Watt led to discussions with Black, and Black’s understanding of heat helped Watt to make improvements to his steam engine design. Black even helped to fund the development of Watt’s idea into a practical machine. But it was in partnership with the venture capitalist Matthew Boulton in the 1770s that Watt developed the engines that drove the Industrial Revolution.

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Steam engines under construction at Boulton and Watt’s Soho Foundry, at Soho, near Birmingham, UK.

Watt went on to apply science in many other areas of practical importance, including developing a process for bleaching cloth, and a successful early method for copying handwritten letters, a forerunner of the photocopier. In all of this work he provided the archetypal example of an experimenter/inventor, blurring the line between ‘pure’ science and ‘practical’ science. As Humphry Davy wrote of him: ‘Those who consider James Watt only as a great practical mechanic form a very erroneous idea of his character; he was equally distinguished as a natural philosopher and a chemist, and his inventions demonstrate his profound knowledge of those sciences, and that peculiar characteristic of genius, the union of them for practical application.’

In the early 1770s, Joseph Priestley, who was a non-conformist minister, philosopher and scientist, carried out some experiments that hinted at the importance of plants in making air fit to breathe. In 1771, while a minister in Leeds, he put some mint in a pot in a closed container glass with a lit candle. The candle soon went out, but the mint thrived and continued to grow. Twenty-seven days later, without having ever opened the container, he re-lit the candle by focusing sunlight through the glass of the container using a curved mirror. This showed him that the mint had somehow revived the air in the closed container. The following year, he carried out similar experiments with mice. First, he kept a mouse in a similar enclosed container with no plants, and noted how long it took before the mouse collapsed. Then, he repeated the experiment with living plants in the container with the mouse. This time, the mouse survived. Priestley realized that this meant that living plants provided something to the air that animals need in order to live, and that candles need in order to burn. At this time, however, he had no idea what the ‘something’ was.

In 1774, Priestley left Leeds and was sponsored by Lord Shelburne, who provided him with a base on Shelburne’s estate in Calne, Wiltshire. Continuing his experiments there, he studied the gas released by what was then known as the red calx of mercury (now called mercuric oxide) when it was heated by focusing the rays of the Sun on it. He trapped the gas as it was given off, leaving mercury behind, and carried out a long series of experiments with it. He found that a lighted candle put into the gas flared up brightly, and that a glowing taper would re-light if plunged into a tube of this ‘pure air’, as he called it.

In 1775, he did another mouse experiment. He put a full grown mouse in a container filled with ordinary air, and found that it could survive for only 15 minutes. But when he put a similar mouse in the same container filled with ‘pure air’, it survived for half an hour, and then, when he took the seemingly dead mouse out of the container and warmed it by the fire, it revived. News of these experiments was quickly spread via the Royal Society. Priestley had discovered oxygen, although it would not be given that name until later. A Swedish chemist, Carl Scheele, made the discovery at about the same time, but his results were not published until 1777.

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Engraving of the laboratory of the English chemist, Joseph Priestley (1733–1804).

In 1779, a Dutch physician and chemist, Jan Ingenhousz, settled in England after travelling widely in Europe. By then, Priestley had moved on, and Ingenhousz took over his laboratory in Calne, under the same sponsorship. Ingenhousz was also interested in the way air could be ‘revived’ by plants, and had independently carried out experiments similar to those Priestley had carried out at the beginning of the 1770s. At Calne, he took this work a stage further by putting green plants under water in transparent containers. He observed that bubbles of gas were produced from the underside of the green leaves when they were exposed to sunlight, but that in the absence of sunlight this bubbling stopped.

It was a simple matter to catch the gas produced by the plants and test it. Ingenhousz found that a glowing taper plunged into the gas would relight, and this and other tests showed that it must be Priestley’s ‘pure air’ – what we now call oxygen. As a result of these experiments, Ingenhousz is credited with discovering photosynthesis, the chemical process by which plants use energy from sunlight and (among other things) carbon dioxide from the air to build their tissues, with oxygen released as a by-product. Animals use oxygen from the air to power their cells, releasing carbon dioxide as a waste product, so that there is a mutual interdependence between plants and animals. Although these details were worked out only later, the broad picture was clear to Ingenhousz in 1779.

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Photosynthesis in Canadian pondweed (Elodea canadensis). The bubbles around the plant contain oxygen, a by-product of photosynthesis. Photosynthesis is the process by which most plants convert sunlight into chemical energy.

Ingenhousz summed up his discoveries in a book, Experiments upon Vegetables – Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring it in the Shade and at Night. He was fascinated by the interdependence between plants and animals, and at the end of the book he wrote: ‘If these conjectures were well grounded, it would throw a great deal of new light upon the arrangement of the different parts of the globe and the harmony between all its parts would become more conspicuous.’ This comes close to the idea of Gaia, the Earth as a single living organism, two centuries ahead of its time.

The scientific sensation of the 1780s was the discovery of a ‘new’ planet in the Solar System. This transformed the view of the heavens that had held since ancient times, and began the process of opening up astronomers’ images – of, first, the Solar System, and then of the whole Universe.

The Ancients had observed five planets in the sky, named after Roman gods – Mercury, Venus, Mars, Jupiter and Saturn. By the 1780s, it was known that these planets orbit the Sun, with Mercury closest to the Sun and Saturn furthest out, and Earth was also known to be a planet, orbiting the Sun between Venus and Mars. Of course, the planet discovered in 1781, now know as Uranus, was not really new. It had been around for as long as the other planets, orbiting even further out than Saturn, and had even been observed many times, but it had been mistaken for a star or a comet. It is possible that one of the ‘stars’ identified by Hipparchos in his star catalogue in the second century BC was actually Uranus, although the planet is extremely difficult to spot with the naked eye. Telescopes made it easier to spot, and it is now certain that the planet was identified as a star by John Flamsteed in 1690. A French astronomer, Pierre Lemonnier, observed Uranus several times between 1750 and 1769, without realizing its true nature.

The reason for those missed opportunities, even after the advent of the telescope, is that Uranus is so far from the Sun that it moves very slowly across the sky as seen from Earth. The other planets, as well as being brighter and easier to see, move noticeably against the background stars, which gives them their name, from the Greek word for a ‘wanderer’. But this also highlights the importance of applying the ‘experimental’ method to observations as well as to experiments. It is no good looking at the night sky casually from time to time and speculating about what you see. You have to make methodical observations over a long period of time, keeping careful records and comparing observations from different times to work out what is going on.

That is exactly what William Herschel, assisted by his sister Caroline, was doing in the early 1780s. Herschel was a successful musician, living in Bath, who had developed a passion for astronomy and built his own telescopes, observing the skies from the garden of the house he shared with Caroline. He was actually carrying out a methodical search for double stars when, on 13 March 1781, he noticed an object that appeared in his telescope as a tiny disc, rather than a star-like point of light. (Stars do not appear as discs even in the best telescopes because they are so much further away than planets.) On 17 March, he looked for the object again, and found that it had moved against the background stars. The natural assumption was that he had found a comet, and he reported the discovery as such to the Royal Society. But when Herschel sent details of his discovery to the Astronomer Royal, Nevil Maskelyne, Maskelyne replied: ‘I don’t know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it.’

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William Herschel discovered Uranus in 1781 with this telescope.

This was a crucial point. Planets move in roughly circular orbits around the Sun, staying at more or less the same distance. Comets dive in from the outer parts of the Solar System, swing past the Sun and head back out into the depths of space. Other observations confirmed Maskelyne’s speculation. In particular, the Russian astronomer Anders Johan Lexell calculated the orbit of the object from the available observations and showed that it was indeed nearly circular. In 1783, Herschel wrote to the Royal Society that ‘by the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System’. By then, he had already been appointed ‘King’s Astronomer’ (not to be confused with Astronomer Royal) by George III, with an income of £200 per annum, which enabled him to become a full-time astronomer.

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Letter by William Herschel (1738–1822), dedicating an engraving of the planet Uranus to King George III of Britain.

In order to thank his patron, Herschel named the planet Georgium Sidus (George’s Star). But this did not go down too well outside the United Kingdom, and the astronomical community eventually settled on the name Uranus – with the stress on the first syllable. Ouranos was, in Greek mythology, the father of Cronus and grandfather of Zeus, who were Saturn and Jupiter in the Roman pantheon, fitting the place of the planet in the Solar System.

Although earlier experiments, such as those of Priestley and Ingenhousz, had shown the importance of some component of air in maintaining life, at the beginning of the 1780s the details of the process were still far from clear. Like many of his colleagues, the French chemist Antoine Lavoisier, a member of the French Academy of Sciences, speculated that the process resembled a slow form of combustion, with the life-giving component of air being converted into ‘fixed air’ (carbon dioxide) by, in effect, being burned in the body. But unlike those colleagues, Lavoisier, together with his fellow acadamician Pierre Laplace, carried out a proper scientific experiment, based on quantitative principles, to test the hypothesis.

Their experiment involved a guinea pig, which was placed in a container within another container, itself insulated from the outside world, with the gap between the two containers filled with snow at 0 ºC. Under these conditions, the animal was quiet and did not move about much. They waited for ten hours, and collected and measured the water that had melted from the snow as a result of the warmth of the guinea pig’s body. It came to 13 ounces (369 grams). Then, in a separate series of experiments Lavoisier and Laplace measured how much fixed air the animal breathed out in ten hours while it was resting. Finally, they compared their guinea-pig measurements with the amount of snow that could be melted by burning enough charcoal to make the same amount of fixed air. This was slightly less than 13 ounces, but the agreement was close enough to convince Lavoisier and a wider circle of scientists that animals keep warm by combining the substance we now call carbon, obtained from their food, with something from the air (oxygen) to make fixed air (carbon dioxide). This was a key step in seeing animals, including human beings, as systems obeying the same laws as burning candles or falling stones.

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Antoine Lavoisier (1743–1794) in his laboratory with his wife and his assistants. His wife (Marie-Anne Pierrette Paulze, 1758–1836) is taking notes at far right.

It was Lavoisier who gave oxygen its name, and who established that burning does indeed involve oxygen from the air combining with the burning substance. This replaced the old idea, still adhered to even by people such as Priestley, that a substance called ‘phlogiston’ is escaping from the substance as it burns. Lavoisier published his definitive demolition of the phlogiston model in the Mémoires of the French Academy in 1786, using the term ‘air’ where we would say ‘gas’:

1 There is true combustion, evolution of flame and light, only in so far as the combustible body is surrounded by and in contact with oxygen; combustion cannot take place in any other kind of air or in a vacuum, and burning bodies plunged into either of these are extinguished as if they had been plunged into water.

2 In every combustion there is an absorption of the air in which the combustion takes place; if this air is pure oxygen, it can be completely absorbed, if proper precautions are taken.

3 In every combustion there is an increase in weight in the body that is being burnt, and this increase is exactly equal to the weight of the air that has been absorbed.

4 In every combustion there is an evolution of heat and light.

Lavoisier also gave their modern names to many other substances, and produced the first list of 33 chemical elements, as well as introducing a system of symbols to represent the elements, although not all of them turned out to be elements as we know them today. The key point, though, is that he discarded the old idea of four mystical ‘elements’ (Earth, Air, Fire, and Water) and replaced it with the idea of an element as a substance that could not be broken down into any simpler substances using chemical processes, while more complex substances were made by combining elements. Indeed, Lavoiser’s definition still stands: ‘We must admit, as elements, all the substances into which we are capable, by any [chemical] means, to reduce bodies by decomposition.’ His naming system used logical rules based on this idea, so that, for example, ‘vitriol of Venus’ became ‘copper sulphate’.

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Nineteenth-century artwork of the ice calorimeter developed in the period 1782 to 1784 by the French scientists Antoine Lavoisier (1743–1794) and Pierre-Simon Laplace (1749–1827). The central space (centre right) would contain burning oil (upper right), or an animal such as a guinea pig; the surrounding chamber would contain ice; the outer, melted snow. The lid would be added and the amount of heat produced would be measured in terms of the volume of meltwater from the ice (lower left).

His book Traité Éleméntaire de Chimie (Elementary Treatise on Chemistry) was published in 1789, and laid the foundations of chemistry as a proper scientific subject. It is seen by chemists as their equivalent to Isaac Newton’s Principia. Lavoisier also spelled out clearly what we now call the law of conservation of mass, which states that matter is neither created nor destroyed in chemical reactions, but only converted from one form into another. In the same year, he also founded a journal, Annales de Chimie, which carried research reports about the new science.

As far as such things can be pinned down to a specific year or a specific event, the publication of Lavoisier’s book marks the moment when chemistry shed the last traces of alchemy and magic, and became a proper scientific discipline.

During the 1790s, a series of experiments led to two major discoveries: that electricity can flow from one place to another, and that electricity is important in operating the muscles of living animals. The second discovery came first, when the Italian physician Luigi Galvani was dissecting a frog. Galvani was also interested in the nature of electricity, and had in his laboratory a hand-cranked machine that could generate electric sparks by the friction of two surfaces rubbing together. This kind of ‘static’ electricity had been known about since the time of the Ancient Greeks. While Galvani was dissecting a pair of frog’s legs, a metal scalpel that had been in contact with the machine and had become electrified touched the sciatic nerve of one of the legs. The leg kicked as if it were still alive.

Galvani carried out many experiments to investigate the phenomenon. He found that legs from a dead frog would twitch if they were connected directly to the electric machine, or if they were laid out on a metal surface during a thunderstorm. But his most important discovery was a result of an observation, rather than a planned experiment.

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Luigi Galvani’s 1791 experiment on the legs of a frog. The upper diagram shows a silver rod (left) and a brass rod (right) being placed in contact with a foot and the spine of the frog. Bringing the two rods together resulted in the leg twitching as the muscles contracted. The lower diagram shows the metal rod connecting foils of two different metals, with the same result.

When preparing frogs’ legs for study, he would hang them up on brass hooks to dry out. When the hooks came into contact with an iron fence, the legs twitched. In case this was due to some influence from electricity in the air, Galvani took the legs and hooks indoors, away from any source of electricity (including his electrostatic generator) and brought the hooks into contact with iron again. Again, the legs twitched. He concluded that electricity must be manufactured in the body, and stored in the muscles of the frog. He called this ‘animal electricity’, and proposed that a fluid manufactured in the brain carries this electricity through the nerves of the body to its muscles. But he believed that this animal electricity was something different from the natural electricity of lightning, or the electricity produced artificially through friction.

Most of Galvani’s colleagues went along with this idea, which reinforced the idea of a special ‘life force’, or spirit, which distinguished living things from the non-living world. But one person in particular strongly disagreed. He was another Italian, a physicist called Alessandro Volta. Volta said that electricity was indeed the cause of the twitching of the legs of the dead frogs, but that it had not been stored in the muscles, and that there was no difference between animal electricity and natural electricity. Instead, he suggested that it was being generated from an outside source, an interaction between the two metals, brass and iron, that were in contact with one another.

Volta had already done a lot of work with electricity, including designing and building better friction machines to generate electric charge, and a device to measure electric charge. He first tested his new idea by putting different kinds of metal in contact with one another and touching the join with his tongue, which tingled as electricity flowed across the join. He realized that the saliva in his mouth was contributing to the effect, and in order to magnify the tiny current he felt with his tongue into something more dramatic he developed a new device, which he described in a letter to the Royal Society in 1800, two years after Galvani had died.

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A drawing made by Alessandro Volta (1745–1827) of the first electric battery, called the ‘voltaic pile’.