Science: A History in 100 Experiments

But perhaps his most spectacular realization was that fossils are the remains of once-living creatures. In the middle of the seventeenth century, it was widely accepted that these peculiar stones, resembling living things, are just pieces of rock that have been distorted by some unknown process to look like living things. But Hooke, with the evidence of his microscopic studies in front of him, said that fossils are not simply contorted pieces of rock. The details matched the patterns of living things too precisely for that to be true. He said that the fossils we now call ammonites must be ‘the shells of certain Shellfishes, which, either by some Deluge, Inundation, earthquake, or some such other means, came to be thrown to that place, and there to be filled with some kind of mud or clay, or petrifying water’. And he realized that because such remains are now found far from the sea, there must have been major changes to the Earth in the past. The reference to a ‘Deluge’ seems to have been a sop to those who believed in the literal truth of the story of the Biblical Flood. Hooke made his own views clear in a lecture at Gresham College in London, where he said that ‘parts which have been sea are now land’, and ‘mountains have been turned into plains, and plains into mountains, and the like.’ A profound inference to draw from looking at tiny objects through a microscope.

Isaac Newton is widely regarded as the greatest scientific thinker that ever lived, and the emphasis in this appreciation is usually on his skills as a theorist, propounding the laws of motion and, most famously of all, the law of gravity. But like his contemporaries, Newton was also a ‘hands-on’ scientist – a practical man who did his own experiments, often using equipment he had designed and built himself. This approach transformed science in the 1660s, largely because of the influence of the Royal Society, founded in the early years of that decade (it received its Royal Charter in 1663). The motto of the Society was (and is) Nullius in Verba, which can be loosely translated as ‘take nobody’s word for it’. From the outset, they did not simply accept hearsay reports of scientific discoveries, but carried out experiments and demonstrations themselves to test such claims. (In the early days, Hooke was the man who did the experiments.) Newton came to the attention of the Society in 1671 because of his practical skills – he had designed and built a new kind of telescope, useful for astronomical investigations, which focused light using a curved mirror rather than a lens. The telescope was shown to the Royal Society by Isaac Barrow, a Cambridge mathematician who had been one of the first people to recognize Newton’s ability. Newton himself was by then Lucasian Professor of Mathematics in Cambridge, but lived a quiet life and largely kept his many discoveries to himself.

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Isaac Newton portrayed in front of his own drawing (colour added) showing the splitting of white light from the sun into the spectrum. The illustration also shows that a second prism refracts the original colours, in this case red, without further change once split, or dispersed, the second prism refracts the original colours, in this case red, without further change.

The Society was sufficiently impressed to elect Newton as a Fellow on 11 January 1672, and to ask him what else he had been working on. His reply took the form of a long letter (what we would now call a scientific paper) in which he explained his ideas about light and the experiments on which those ideas were based. Newton’s key insight was that ‘pure’ white light is actually a mixture of all the colours of the rainbow. To the ancients, white light represented a pure entity, in the same way that spheres were thought to be perfect. To suggest that white light was a mixture of colours would have seemed to them as ludicrous as the idea that planets did not move in perfect circles in their orbits.

Of course, it was well known that when sunlight passed through triangular prisms or other pieces of glass it produced rainbow patterns of colour. But before Newton people assumed that the white light passing through the glass had become adulterated as it picked up imperfections from the glass, and this had changed its nature. Newton’s genius was to devise a simple experiment which proved that this was wrong.

In the first stage of the experiment, he worked in a darkened room with heavy curtains to shut out the sunlight. A small hole in the curtain let through a single beam of light, which fell upon a triangular prism. After passing through the prism, the light shone upon a white wall on the other side of the room, where it was spread out into a rainbow pattern of colours. It was Newton who identified seven different colours in this pattern – red, orange, yellow, green, blue, indigo, and violet.

This could still be explained as a change brought about by the passage of the light through the glass. But in the second part of the experiment Newton put a second prism, reversed compared with the first prism, between the first prism and the white wall. The first prism had spread white light out into seven colours; the second prism did not make the colours even brighter (more ‘impure’) but rather combined the seven colours back into a single spot of white light. He had turned a ‘rainbow’ back into white light. The explanation was that white light is really a mixture of all the colours of the rainbow, and that light is bent as it passes through the prisms (or, indeed, inside raindrops). Some colours are bent more than others, so they get spread out or squeezed back together depending on how the prism is oriented. It is as easy to bend them back into a single beam as it is to bend the different colours out of a single beam. This was the first step towards an understanding of spectroscopy (see here (#litres_trial_promo)), which would one day make it possible to determine the composition of the stars, using reflecting telescopes, some of them developed from Newton’s design.

This was just one of Newton’s insights into the nature of light, which were eventually gathered in a great book, Opticks, published in 1704, the year after he had become President of the Royal Society. In that book, he summed up his own understanding of the scientific method: ‘Analysis consists in making Experiments and Observations, and in drawing general Conclusions from them by Induction, and admitting of no Objections against the Conclusions but such as are taken from Experiment, or other certain Truths.’

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Replica of Newton’s reflecting telescope.

Newton’s remark about experiments ‘and Observations’ is important. Sometimes Nature does the ‘experiment’ for us, and the role of the scientist is ‘only’ to observe what is going on and work out why it has happened. But it often takes a very clever person to work this out. Ole Rømer’s discovery of the finite speed of light is a case in point.

In the seventeenth century, there was a great deal of interest in using studies of the eclipses of the moons of Jupiter (discovered by Galileo) as ‘clocks’ to determine longitude. These eclipses occur at regular intervals, as the moons orbit Jupiter in the same way that the Earth orbits the Sun. The moment when a particular moon disappeared behind Jupiter could be observed from different places on Earth, and the time at which it occurred could be compared with the local time measured from noon. This told the observers how far east or west they were from some chosen reference point. This technique was pioneered by the Italian Giovanni Cassini, who moved to the Paris Observatory in 1671. He sent the Frenchman Jean Picard to Denmark to use observations of Jupiter’s moons to establish the exact longitude of Tycho Brahe’s old observatory, so that Tycho’s records could be tied in to observations from Paris. Picard was helped by a young assistant, the Dane, Ole Rømer, who then went to Paris to work with Cassini (he was also for a time the tutor of the French Crown Prince, the Dauphin).

Over the next few years, Rømer continued to monitor the eclipses of Jupiter’s moons, and noticed that these did not always occur exactly when they were expected. He made a particular study of the moon Io, and noticed that the time between one eclipse and the next got shorter when the Earth was moving towards its closest to Jupiter (which happens when it is on the same side of the Sun as Jupiter), and longer when it was moving further away. Cassini himself thought for a time that this might be because light travels at a finite speed. When the Earth is moving towards Jupiter, the time between successive eclipses is shorter because in the time between eclipses the Earth has moved closer to Jupiter, so light from the second eclipse does not have so far to travel and gets here quicker. Similarly, when the Earth is moving away from Jupiter, the light from the second eclipse has further to travel, because the Earth has moved on in its orbit, and takes longer to reach us.

Curiously, Cassini abandoned the idea. But Rømer took it up, and made detailed observations and calculations to develop it further. In August 1676, Cassini, then still enamoured of the idea, announced to the French Academy of Sciences that the official tables of the eclipses of Io, used in calculating longitude, would have to be revised ‘due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.’ Cassini also predicted that the emergence of Io from eclipse on 16 November 1676 would be seen about ten minutes later than would have been calculated by the previous method. In fact, the emergence that occurred on 9 November was observed and matched the new calculations, prompting Rømer to make a more detailed presentation to the Academy.

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Danish astronomer Ole Rømer (1644–1710) with the tools of his trade.

Unfortunately, most of Rømer’s papers were lost in a fire in 1728, and the only account we have of that presentation is a rather garbled news story, which was translated into English and published in 1677 in the Philosophical Transactions of the Royal Society. But a later document survived and spells out his calculation and the dramatic result that he announced. With the best estimate available to him for the size of the Earth’s orbit, Rømer calculated that the speed of light must be (in modern units) 225,000 kilometres per second. If we make the same calculation, using Rømer’s observations, and plug in the modern value for the diameter of the Earth’s orbit, we get a speed of 298,000 kilometres per second. This is remarkably close to the best modern measurement of the speed of light: 299,792 kilometres per second.

Although not everyone was convinced at the time, the discovery made Rømer’s reputation. He visited England, where he was warmly received and discussed his observations and their implications with such people as Isaac Newton, Edmond Halley and the Astronomer Royal John Flamsteed. They were convinced. In his book, Opticks, Newton mentioned that light takes ‘seven or eight minutes’ to travel from the Sun to Earth. Rømer returned to Denmark in 1681 to become Astronomer Royal and Director of the Royal Observatory in Copenhagen, the spiritual heir to Tycho.

There is a particular kind of experiment that is important in everyday life, although it is not usually called an ‘experiment’, perhaps for fear of worrying the people taking part. This is the ‘medical trial’ – a kind of controlled experiment. One of the best examples of a medical trial is also one of the earliest, dating from the 1740s, when James Lind, a surgeon in the Royal Navy, carried out experiments to find a cure for scurvy. The story of scurvy also shows the value of careful experiments and observations, even when it takes a long time for these observations to be explained.

Scurvy is an illness that starts out with a general feeling of being under the weather and lethargy, then develops with spots on the skin, softening and bleeding of the gums, loss of teeth, open wounds on the skin, and eventually death. We now know that it is caused by a lack of vitamin C, but in the eighteenth century nobody knew anything about vitamins. What they did know was that scurvy was prevalent among sailors and soldiers on a restricted diet of dried meats and grains. The problem was highlighted by the fate of the first British circumnavigation of the globe, by a fleet led by Sir George Anson between 1740 and 1744. Out of an initial complement of about 2,000 men, more than half died from scurvy on the voyage. For a growing naval power such as Britain, finding a treatment or preventative for scurvy was a pressing problem.

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The mouth of a person suffering from scurvy, showing swollen and bleeding gums.

Lind was not the first person to suggest that citrus fruits might be used to treat scurvy, but he was the first person to carry out a scientific experiment to test the idea. His ideas about the cause of scurvy were completely wrong – he thought it was caused by a ‘putrefaction’ of the body, which he hoped to treat with acids. On a voyage in 1747, he tested this idea by adding different acids as supplements to the diet of different groups of men afflicted with scurvy. All the sailors ate the same foods, but every day one group had a quart of cider each, one group had 25 drops of elixir of vitriol (sulphuric acid) added to their diet, one group took six spoonfuls of vinegar each, one group had to drink half a pint of seawater, the members of another group each ate two oranges and one lemon a day, and the last group drank barley water and ate a spicy paste. The seawater drinkers were a ‘control’, because they were not given any medicine. Hence the term ‘controlled experiment’. By the time the experiment ended (because the ship had run out of fruit) the health of the sailors given oranges and lemons had improved dramatically; among the other groups, only the cider drinkers showed a slight improvement.

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James Lind (1716–1794).

The Royal Navy took note of the discovery, and some captains began to implement a policy of providing ships with a syrup made from oranges, and also with sauerkraut, which had also proved an effective antiscorbutic (from the Latin term for scurvy, scorbutus). Lind left the service soon after this voyage, and although he wrote a book, A Treatise of the Scurvy, which was published in 1753, it was largely ignored. But a second ‘experiment’ was carried out by James Cook on his first circumnavigation of the globe, starting in 1768. His ship carried three tons of sauerkraut. This tasted vile, but Cook persuaded his crew to eat it by a ‘method I never once knew to fail with seamen’. He had the food served at first only to officers, who ate it with the appearance of delight. The men soon petitioned for it to be added to their rations, and the result was that there were hardly any incidences of scurvy.

Although shore-based doctors continued to ignore the evidence, experience had shown the Navy what worked, even if they did not know why it worked. In 1794, lemon juice was issued to sailors on board the Suffolk on a 23-week voyage to India during which nobody died from scurvy. A year later, lemon juice began to be issued to every ship. The juice proved quite palatable, since it was drunk mixed with the sailors’ ‘grog’, a ration of rum diluted with water. This was later replaced by lime juice, which proved even more effective, and, by the middle of the nineteenth century, American sailors had begun referring to their Royal Navy counterparts as ‘lime-juicers’ – later shortened to ‘limeys’ and applied to anyone from Britain.

It was not until the early 1930s that the active ingredient was identified and named ascorbic acid, or vitamin C. Most animals can make their own vitamin C, but monkeys and apes (including humans), guinea pigs and bats are among the few who cannot do so and have to get vitamin C through their diet.

In the middle of the eighteenth century, there was no way to generate electric currents (see here (#u3371ff7d-69e9-5468-83dd-ed7479698c22)), but scientists were familiar with the ‘static’ electricity that can be made by friction, for example by rubbing a glass rod with a silk cloth. This is the same kind of electricity that crackles on a dry day when you pull up a sweater made of synthetic material, and a rod charged up in this way will produce a spark when it is touched to another object. These sparks are like miniature lightning bolts, which led people to speculate that lightning might be a form of electricity. The person who took up the challenge of proving this was the American savant Benjamin Franklin. In 1746 Franklin acquired a glass rod from Peter Collinson, a merchant with an interest in science, who was a Fellow of the Royal Society. He carried out a series of experiments with it.

© Print Collection, Miriam and Ira D. Wallach Division of Art, Prints and Photographs/New York Public Library/Science Photo Library

Peter Collinson (1694–1768).

These experiments convinced Franklin that storm clouds must be electrically charged, like a glass rod rubbed with silk, and that lightning occurred when this static electricity was discharged to the ground, like the sparks that flew when the rod came near another object. But how could this idea be tested? Franklin wrote about his experiments in letters he sent to Collinson in England, and suggested that a tall metal rod, or spike, might be erected during a thunderstorm to draw off electricity from the clouds. The idea was not to encourage lightning to strike the metal rod, but to draw the electricity off gently and capture it in a special kind of glass bottle, known as a Leyden (or Leiden) Jar. Apart from Collinson, nobody at the Royal Society was enthusiastic about Franklin’s ideas. But Collinson had them published in the early 1750s, and they were widely read by scientists in mainland Europe.

One of those scientists, Thomas-François d’Alibard, decided to put Franklin’s idea to the test. In May 1752 he set up a 40-foot (12-metre) high metal spike in a garden in Marly-la-Ville, in northern France, and drew off sparks from it during a storm. The rod was not, however, struck by lightning, which probably would have killed d’Alibard. Before news of this success could reach America, Franklin had carried out his own version of the experiment in Philadelphia, by flying a kite during a thunderstorm in June 1752.

Like d’Alibard, Franklin knew that it would be dangerous to be struck by lightning, and was simply trying to draw off some of the electric charge from the clouds, along the wet string of the kite to a key attached to the string. To encourage this, a pointed wire was attached to the kite itself, but as a safety precaution he held the kite by a silk ribbon, attached to the string of the kite, as an insulator. Sure enough, electricity was drawn down to the key, and, when an object was moved close to the key, sparks flew across the gap. Franklin even let the sparks leap across, painfully, to his own knuckles.

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French scientist Thomas-François d’Alibard (1709–1799) carrying out his lightning experiment on 10 May 1752, at Marly, France.

Soon after he had carried out his own experiment, Franklin heard of d’Alibard’s success in France. In October 1752, he wrote to Collinson with directions on how to repeat his own experiment: ‘When rain has wet the kite twine so that it can conduct the electric fire freely, you will find it streams out plentifully from the key at the approach of your knuckle, and with this key a phial, or Leiden jar, may be charged: and from electric fire thus obtained spirits may be kindled, and all other electric experiments [may be] performed which are usually done by the help of a rubber glass globe or tube; and therefore the sameness of the electrical matter with that of lightening completely demonstrated.’

Other experimenters were not so careful – or rather, not so lucky – as Franklin, and several people were killed by lightning while trying to copy what he had done. In Franklin’s case, the electricity was drawn down gradually from the clouds, but he was wrong to think that lightning itself could not strike via the kite. The same flawed thinking was behind his invention of the lightning conductor, in the form of a metal rod attached to the highest point of a building and connected to the earth by a wire. He thought that such a rod would draw off electricity gradually, and prevent a lightning strike. In fact, such a lightning conductor encourages the lightning to strike, but protects the building by acting as a direct route to earth for the lightning, which strikes the metal rod rather than the building itself. But either way, it does work! And all of this proved that lightning is indeed the same phenomenon as static electricity, but on a larger scale.

Ice has an intriguing property, which fascinated scientists studying the nature of heat in the eighteenth century. As well as being intrinsically interesting, these studies had practical implications; it was just at the time steam power was beginning to be harnessed to drive the Industrial Revolution. The curious property is that when ice at the freezing point (0 ºC, or, in the units used in Britain then, 32 ºF) is heated, its temperature stays the same until all the ice has melted into water. Only then does the temperature of the water increase as more heat is applied. The same sort of thing, of course, happens when other substances, such as metals, are melted, but ice is much easier to study.

Other people had thought that if a lump of ice at the melting point were heated by a tiny amount it would all melt. But the person who studied what was really going on in a careful series of experiments in the 1760s was a professor at Glasgow University, Joseph Black. Whenever Black did experiments, he measured everything that could be measured, as accurately as possible. He had made his name by studying the amount of gas produced or absorbed in chemical reactions. In one of his experiments, a carefully weighed amount of limestone was heated, to produce quicklime, which was then weighed. The quicklime weighed less, because the gas we now call carbon dioxide had been driven off. A weighed amount of water was added to the quicklime to produce slaked lime, which was weighed. Then, a weighed amount of a mild alkali was added to convert the slaked lime back into what weighing proved to be the same amount of limestone that he had started with. Along the way, the differences in weight told him how much gas had been lost or absorbed at each stage. This was quantitative science, as opposed to qualitative science, in which the changes in the character of the substances (their quality) was noted, but there were no measurements of how much they had changed (the quantity).

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Joseph Black (1728–1799).

© Sheila Terry/Science Photo Library

Joseph Black giving a practical demonstration of latent heat to students of Glasgow University in the 1760s.

Black carried over this quantitative approach – a cornerstone of modern experimental science – into his studies of heat. He found that the amount of heat needed to melt a certain amount of ice at 32 ºF into water at the same temperature was enough to raise the temperature of the water from 32 ºF all the way to 140 ºF (or 60º C). He also studied the way water turns into steam, showing that when a mixture of water and water vapour at the boiling point (212 ºF, or 100 ºC) is heated, the temperature does not increase until all the water has been turned into vapour. And if a certain weight of water – say, a pound – at 32 ºF is added to the same quantity of water at 212 ºF, the resulting liquid has a temperature of 122 ºF (or 50º C), halfway between boiling and freezing. This led him to the idea of ‘specific heat’, which is the amount of heat required to raise the temperature of a certain amount of stuff by one degree (in modern units, the heat required to raise the temperature of 1 gram by 1 ºC). Black coined the term ‘specific heat’, and also gave the name ‘latent heat’ to the heat absorbed by a melting substance. And when a liquid such as water freezes, the same amount of latent heat is released as it does so; similarly, latent heat is released when vapour condenses into liquid. In Black’s own words: ‘I, therefore, set seriously about making experiments, conformable to the suspicion that I entertained concerning the boiling of fluids … I imagined that, during the boiling, heat is absorbed by the water, and enters into the composition of the vapour produced from it, in the same manner as it is absorbed by ice in melting, and enters into the composition of the produced water. And, as the ostensible effect of the heat, in this last case, consists, not in warming the surrounding bodies, but in rendering the ice fluid; so, in the case of boiling, the heat absorbed does not warm surrounding bodies, but converts the water into vapour. In both cases, considered as the cause of warmth, we do not perceive its presence: it is concealed, or latent, and I give it the name of LATENT HEAT.’

These discoveries were noted by a certain James Watt, an instrument maker at the university, who built experimental apparatus for Black, and who went on to develop steam engines.

There is no clear distinction between experiment and invention. Inventors have to experiment to find out what works, and experimenters often have to be inventors, as the example of the development of the vacuum pump for use in scientific investigation highlights (see here (#u7824ccbf-b10e-59fd-a67e-f1f1b2282452)). Although this book concentrates on the more obviously experimental end of this spectrum, there is one beautiful example of the synergy between experiment and invention that is so important historically that it simply cannot be overlooked. This is the way in which investigations of the relationship between heat and temperature led to the development of the steam engine, which in turn powered the Industrial Revolution.

In 1763, James Watt was working as an instrument maker in Glasgow, where he became familiar with Black’s work, but not, at first, with all of his discoveries concerning latent and specific heat. Watt was asked to repair a scale model of a kind of steam engine developed by Thomas Newcomen, often referred to as an ‘atmospheric’ engine, because air pressure was just as important in its operation as steam. Such engines had a vertical cylinder, made of metal, containing a metal piston attached at the top (which was open to the air) by a beam to a counterweight. When the space beneath the piston was filled with steam, pressure would increase and the piston would rise. Then, cold water was sprayed into the cylinder, making the steam condense and reducing the pressure so that atmospheric pressure would push the piston down. By repeating this process over and over again, the resulting rocking motion of the beam could be used to drive a pump sucking water out of a mine.

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