Science: A History in 100 Experiments

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William Gilbert’s illustration of the angle of dip of a magnetic field surrounding the Earth; the line AB is the equator, C is the North Pole, and D is the South Pole.

As a result of these experiments, Gilbert was the first person to appreciate that, because magnetic opposites attract, the end of a magnet that points northwards (towards the north magnetic pole of the Earth) ought to be called the south pole. (In modern language, scientists sometimes refer to the ‘north-seeking’ pole and the ‘south-seeking’ pole of a magnet to avoid this confusion.) Gilbert said: ‘All who hitherto have written about the poles of the loadstone, all instrument-makers, and navigators, are egregiously mistaken in taking for the north pole of the loadstone the part of the stone that inclines to the north, and for the south pole the part that looks to the south: this we will hereafter prove to be an error. So ill-cultivated is the whole philosophy of the magnet still, even as regards its elementary principles.’

Indeed, it was Gilbert who introduced terms such as ‘magnetic pole’ and ‘electric force’ into the language. He was the first person to realize that magnetism and electricity (a word he invented) are separate phenomena, and his work on magnetism was not improved upon for two centuries, until the work of Michael Faraday.

Gilbert’s book caused a sensation in its day, and was highly influential. Galileo was one of its readers, and commented favourably on it in a letter to a friend. Indeed, Galileo described Gilbert as the founder of the scientific method. In Gilbert’s own words: ‘In the discovery of secret things, and in the investigation of hidden causes, stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators.’ That is science in a nutshell, and in his book Gilbert was careful to spell out every detail of his experiments, so that other people could carry them out and see the results for themselves. But he cautions whoever does this ‘to handle the bodies carefully, skilfully and deftly, not heedlessly and bunglingly; when an experiment fails, let him not in his ignorance condemn our discoveries, for there is naught in these Books that has not been investigated and again and again done and repeated under our eyes’.

Galileo Galilei is famous for an experiment he did not carry out – but it was a real experiment, inspired by his work. He was Professor of Mathematics in Padua from 1592 until 1610, and during that time he worked in mechanics and astronomy as well as mathematics.

To put Galileo’s achievements in perspective, at the time he was working there were many people – educated people – who thought that a bullet fired horizontally from a gun, or a ball fired from a cannon, would fly a certain distance in a straight line, then stop and drop vertically to the ground. It was Galileo who first appreciated that the trajectory followed when a bullet is fired from a gun, or when an object such as a ball is thrown up in the air, is a parabola – and he carried out tests to prove this.

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Nineteenth century illustration showing a glamourised version of Galileo’s experiment rolling balls down inclined planes. Although the scene depicted is fictional, Galileo really did carry out such experiments.

Among the many experiments he carried out in the years around the turn of the century there was a series of studies in which he rolled balls with different weights down inclined planes. He timed how quickly the balls moved using his pulse, and reached two important conclusions. The first was that the ‘natural’ state of a ball rolling off the slope was to continue horizontally (literally ‘towards the horizon’), unless it was stopped by friction. Without friction, he reasoned, the ball would roll on forever. This was an early insight into what Isaac Newton, following Robert Hooke, developed as his ‘First Law’ of mechanics, that an object stays at rest or moves in a straight line at a steady speed unless it is acted upon by an outside force.

Galileo’s second discovery was that the speed with which the balls rolled down the slope did not depend on their weight. For any particular slope, it took the same amount of time for any of the balls to get from the top to the bottom. This applied no matter how steep he made the slope. So he concluded – without actually dropping things vertically – that, apart from the effects of wind resistance, all falling objects would accelerate downwards at the same rate.

This infuriated some of his colleagues, philosophers of the old school who believed that Aristotle, who said that heavy objects fall faster than light objects, could not be wrong. So, in 1612, two years after Galileo moved from Padua to Pisa, one of them really did drop two weights from the leaning tower in a public demonstration intended to prove that Aristotle was right. The balls hit the ground very nearly at the same time, but not exactly. The Aristotelians said that this proved Galileo was wrong. But Galileo had an answer: ‘Aristotle says that a hundred-pound ball falling from a height of one hundred cubits hits the ground before a one-pound ball has fallen one cubit. I say they arrive at the same time. You find, on making the test, that the larger ball beats the smaller one by two inches. Now, behind those two inches you want to hide Aristotle’s ninety-nine cubits and, speaking only of my tiny error, remain silent about his enormous mistake.’

Among other things, this true story highlights the power of the experimental method. If you carry out an experiment honestly, it will tell you the truth, regardless of what you want it to tell you. The Aristotelians wanted to prove Galileo was wrong, but the experiment proved he was right – within, as we would now say, the limits of possible experimental error.

By 1612 Galileo was nearly 50, and his days as an experimental physicist were essentially over. His famous clash with the Church authorities in Rome did not take place until the 1630s, and led to his spending his final years, from 1634, under house arrest at his own home (a relatively lenient sentence considering that he had been forced to confess to heresy). There, he summed up his life’s work on mechanics and promoted the scientific method pioneered by Gilbert in a great book, Discourses and Mathematical Demonstrations Concerning Two New Sciences, usually known as Two New Sciences, published in 1638 in Holland. The book was enormously influential, the first real scientific textbook, and an inspiration to scientists across Europe – except, of course, in Catholic Italy, where it was banned. As a direct result, from being a leading light in the scientific renaissance, Italy became a backwater, while the real progress was made elsewhere.

Even after the publication of the Fabrica (see here (#ue3d8c422-9ff7-530f-ab81-ffe501586b64)), in the second half of the sixteenth century and early in the seventeenth century there was still strong opposition to the idea that classical teachers such as Galen could be wrong. So, although the English physician William Harvey was born in 1578 and studied the circulation of the blood in the early decades of the seventeenth century, he did not publish his discoveries until 1628, by which time he had gathered an overwhelming weight of evidence to support his ideas. (He did, however, give lectures on his work in 1616.) The result was a book, De Motu Cordus et Sanguinis in Animalibus (On the Motion of the Heart and Blood in Animals, known as De Motu), which presented an open-and-shut case based on a series of genuinely scientific experiments carried out over the previous two decades. All this was done in Harvey’s spare time as a successful physician who had studied in Cambridge and Padua and, like William Gilbert, became (in 1618) one of the Court Physicians to James I, and later personal physician (a much more important post) to Charles I. Both Williams were contemporaries of another William, Shakespeare, who died in 1616.

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Title page from William Harvey’s De Motu.

Before Gilbert, following the teaching of Galen, it was thought that veins and arteries carried two different kinds of blood. One kind, supposedly manufactured in the liver, was thought to be carried through the veins to nourish the tissues of the body, getting used up in the process and being replaced by new blood from the liver. The other kind of blood was thought to be carried in the arteries, conveying a mysterious ‘vital spirit’ from the lungs to the tissues of the body.

As with Gilbert (see here (#u68ccc399-6d30-52d8-914f-c3f0025c67b3)), the way in which Harvey worked and presented his results was as important as the discoveries themselves. He did not base his ideas on abstract philosophising, but on direct measurements and observations. The key insight came when he measured the capacity of the heart and, by taking a typical pulse rate, worked out how much blood it was pumping each minute. He found that, in modern units, a human heart pumps about 60 cubic centimetres with each beat, adding up to nearly 260 litres in an hour. That much blood would weigh three times as much as a human, so it was clearly impossible that it was all being manufactured in the liver (or anywhere else) every hour. The only alternative was that there was a lot less blood, and that it was continuously circulating around the body, out from the heart through the arteries and back through the veins. An equivalent system circulates blood between the lungs and the heart, carrying not ‘vital spirit’ but oxygen. All this was born out by Harvey’s observations of the tiny valves in veins (discovered by one of Harvey’s teachers in Padua, Hieronymous Fabricius) which allow venous blood to flow towards the heart, but not away from it.

Having reached this conclusion by observation, Harvey then established his case with a series of experiments, one of which stands out for its simplicity and clarity. If he was right, there must be a connection between arteries and veins. As arteries lie deeper below the surface of the skin than veins, he tested this by tying a cord (ligature) around his own arm, tight enough for it to cut off the flow of blood in his veins, but not in his arteries. As blood continued it flow from the arteries into the blocked-off veins, the veins behind the ligature swelled up dramatically. He also pointed out that arteries near the heart are thicker than those further away from it, because they have to be strong in order to cope with the powerful pumping action of the heart.

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Woodcut from William Harvey’s book, De Motu Cordis et Sanguinis in Animalibus. The illustrations show the valves in the superficial veins of the forearm. On the left, a finger has been passed along the vein from O to H (away from the heart). The stretch of vein is emptied and remains so because of the valve at O.

There were still elements of mysticism in Harvey’s thinking, and he saw the heart as not merely a pump but a place where the blood was made perfect by ‘the foundation of life, and author of all’. It was René Descartes who took the next step, drawing on Harvey’s work, and said, in 1637, that the heart is simply a mechanical pump.

Although his book caused great interest in England, Harvey’s ideas about the circulation of the blood were not fully accepted during his lifetime, except by pioneers such as Descartes. One reason was that blood-letting was then (and would long remain) a treatment for illness, and the rationale for such treatment would be undermined if there was a limited amount of blood in the body.

Harvey died in 1657 and soon afterwards the development of the microscope (see here (#u5da144d9-87d9-5358-936d-87cf3d5ecaca)) made it possible to see the tiny connections between veins and arteries, establishing once and for all that Harvey had been correct.

In the early 1640s, the Italian Evangelista Torricelli investigated the problem that water could not be pumped up (by a suction pump) from a well more than about 30 feet (roughly 9 metres) deep. The way these pumps work is similar to the way it is possible to suck water into a bicycle pump if the open end is placed below the surface of the water. If you had a very long bicycle pump standing upright in a swimming pool, you would be able to suck water up to just over 30 feet, but no further, no matter how hard you pulled on the handle. Torricelli reasoned that the weight of the air pressing down on the surface of the water in the well could push the air in a pipe up this far, but no further. So he set out to test the idea using a denser liquid, mercury, instead of water. Mercury is roughly fourteen times denser than water, so Torricelli worked out that a column of water 30 feet high must exert the same pressure at its base as a column of mercury a bit more than 2 feet (more than 60 centimetres) high. He found that if a glass tube sealed at one end and full of mercury was stood upright with its open end in a dish of mercury, the level of the mercury in the tube would fall to 30 inches (76 centimetres), leaving a gap above the top of the mercury in the tube, and matching his calculation. The gap contained nothing at all, and became known as the Torricelli Vacuum.

Torricelli noticed that the exact height of the column of mercury in his tubes changed from day to day, and he realized that this was because the pressure of the atmosphere weighing down on the mercury in the dish was changing. He had invented the barometer. Torricelli died in 1647, but his discoveries were taken up and developed by the Frenchman Blaise Pascal, who studied the way the pressure of the air, measured by this kind of early barometer, varied with the weather. Another Frenchman, René Descartes, visited Pascal in 1647, and suggested that it would be interesting to take a barometer up a mountain, to find out how the pressure of the air changed with altitude. Pascal lived in Paris, but his brother-in-law Florin Périer lived near a mountain, the Puy-de-Dôme, and in 1648 Pascal persuaded him to do the experiment. Périer write to Pascal to describe what happened:

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Imaginative illustration of Florin Périer measuring the air pressure as he ascends the Puy-de-Dôme, a volcanic mountain in France with a height of 1464 metres.

‘The weather was chancy last Saturday … [but] around five o’clock that morning … the Puy-de-Dôme was visible … I decided to give it a try. Several important people of the city of Clermont had asked me to let them know when I would make the ascent … I was delighted to have them with me in this great work.

… at eight o’clock we met in the gardens of the Minim Fathers [monastery], which has the lowest elevation in town … First I poured 16 pounds of quicksilver … into a vessel … then took several glass tubes … each four feet long and hermetically sealed at one end and opened at the other … then placed them in the vessel … the quick silver stood at 26" and 3½ lines above the quicksilver in the vessel … I repeated the experiment two more times while standing in the same spot … [it] produced the same result each time …

I attached one of the tubes to the vessel and marked the height of the quicksilver and … asked Father Chastin, one of the Minim Brothers … to watch if any changes should occur through the day … Taking the other tube and a portion of the quick silver … I walked to the top of Puy-de-Dôme, about 500 fathoms higher than the monastery, where upon experiment … found that the quicksilver reached a height of only 23" and 2 lines … I repeated the experiment five times with care … each at different points on the summit … found the same height of quicksilver … in each case …’

Equally importantly, the priest at the bottom of the mountain reported that the reading on his barometer had not changed during the day. There was less weight of air pressing down at the top of the mountain than at the bottom. So the experiment revealed that the atmosphere gets thinner as you go higher, and suggests that if you go high enough it will thin out entirely, with a vacuum above it, like the vacuum above the mercury in Torricelli’s tubes. Pascal then carried out a mini-version of the experiment by carrying a barometer up about 50 metres to the top of the bell tower at the church of Saint-Jacques-de-la-Boucherie. The mercury dropped by two ‘lines’. Many people, including Descartes, refused to accept Pascal’s interpretation of the evidence, and insisted that there must be some invisible substance filling the ‘empty’ space in the tubes and (presumably) the space above the atmosphere. But further experiments eventually proved that Pascal was right (see here).

Following the experiments of Torricelli, Pascal, and his brother-in-law, the study of the vacuum became one of the hottest topics in science. In order to investigate this phenomenon, scientists needed very efficient pumps that could suck air out of glass bottles and other vessels. These pumps were hi-tech by the standard of the day – the seventeenth-century equivalent of modern particle accelerators such as the Large Hadron Collider. The very best air pumps available in the 1660s were made by the British scientist Robert Hooke, who was working at the time as an assistant to Robert Boyle. Boyle was a pioneering scientist (he helped to found the Royal Society) inspired by the work of Galileo, and once said that in investigating the world ‘we assent to experience, even when its information seems contrary to reason’.

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Robert Boyle’s experiment to demonstrate the greatest height to which water could be raised by pumping. Boyle stood on a roof approximately 10 metres above a barrel of water and used a pump to suck water from the barrel up a pipe. From ‘A continuation of new experiments physico-mechanical, touching the spring and weight of the air, and their effects’, by Robert Boyle (1669).

Hooke’s design for a vacuum pump was based upon a cylinder with one end closed, containing a piston that stuck out from the open end of the cylinder. The end of the piston was cut with teeth which engaged with a gear wheel that could be wound with a handle to push the piston up, forcing air out through a one-way valve, then pull the piston down, leaving a vacuum in the tube (there is a replica of Hooke’s pump in the Science Museum in London). When a glass vessel was attached to the pump via another one-way valve, the piston could be pumped up and down repeatedly, sucking more and more air out of the glass vessel.

At about the time Hooke was developing his pump, in the late 1650s, another Englishman, Richard Towneley, was repeating the experiments carried out by Florin Périer, using a Torricelli barometer, on Pendle Hill in Lancashire. He surmised that the lower pressure of air at higher altitude is because the air is thinner (less dense) there, and mentioned this idea, which became known as Towneley’s Hypothesis, to Boyle. Boyle was intrigued, and gave Hooke the task of carrying out experiments to test the hypothesis.

The simplest of these experiments did not involve the air pump. Hooke took a glass tube shaped like the letter J, with the top open and the short end closed. He poured mercury into the tube to fill the U-bend at the bottom (just like the U-bend in a kitchen sink), sealing off the air trapped in the short arm of the J. When the mercury was at the same level on both sides of the U-bend, it meant that the trapped air was at atmospheric pressure. But when more mercury was poured in to the tube, because of its extra weight the pressure increased and forced the air in the closed end into a smaller space. Boyle was not a great one for calculations, but Hooke was, and he made careful measurements of the amount of mercury being added and the amount by which the trapped air was squeezed, which showed that the volume of the trapped air was inversely proportional to the pressure. In other words, if the pressure doubles, the volume is halved; if the pressure triples, the air is squeezed into a third of its original volume, and so on.

Other experiments carried out by Boyle and Hooke did use the air pump, and showed, for example, that water boils at a lower temperature when the air pressure is reduced (which explains why it is hard to make a good cup of tea on top of a mountain). This was a very tricky experiment, as it involved placing a mercury barometer inside a sealed glass vessel where the water was being heated, to monitor the pressure as air was pumped out.

The experimental results were first announced to the world in Boyle’s book, New Experiments Physico-Mechanical Touching the Spring of the Air, published in 1660. But at that time he did not explicitly spell out the inverse law relating volume and pressure. That appeared in the second edition of his book, published in 1662, and as a result it became known as Boyle’s Law, even though Hooke had done the experiments and made the calculations on which the law was based.

All of this was important to scientific thinking, because it supported the idea that the air is made of atoms and molecules, flying around and colliding with one another. It was also important in practical terms, because the realization that air has weight, and that it can be extracted using pistons to leave a vacuum, fed directly into the idea of the steam engine (see here (#u341b0065-ad75-5bcb-a045-021d9d621204)).

Robert Hooke may have missed out on getting his name attached to ‘Boyle’s’ Law, but he soon achieved an even greater experimental success as a pioneer of the use of the microscope. In the second half of the seventeenth century other experimenters also studied the world of the very small using lenses to magnify tiny objects, but it was Hooke who did the most thorough job, and explained his discoveries in a book, Micrographia, which was published in 1665. It was written in English, unusually for the time (most learned tomes were written in Latin), and easy for any educated person to understand. Samuel Pepys called it ‘the most ingenious book that ever I read in my life’.

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Drawing of the head of a fly, from Micrographia.

There were actually two ways to achieve the kind of magnification needed to make microscopy worthwhile. One was pioneered by Hooke’s contemporary, the Dutch draper and amateur scientist, Antoni van Leeuwenhoek. His ‘microscopes’ were single tiny lenses, some no bigger than a pinhead, mounted in strips of metal. They had to be held very close to the eye, and acted as powerful magnifying glasses – so powerful that they could enlarge images 200 or 300 times. These were very difficult to work with, but van Leeuwenhoek made many important discoveries, including tiny living creatures in droplets of water. Hooke used this kind of lens when he needed to pick out the tiniest details in the objects he studied, but he also used a different experimental setup, the forerunner of modern microscopes. These microscopes were made from combinations of lenses, mounted in tubes 6 or 7 inches long (approximately 15–18 centimetres), similar to the way in which telescopes are made. They were easier to work with, but did not give such powerful magnification as the tiny single lenses.

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Seventeenth-century drawing of a flea observed through a microscope by Robert Hooke (1635–1703).

But ‘easier’ does not mean ‘easy’. Anyone who has used a microscope knows that it is important to have a bright light shining on the object being studied at the focus of the instrument. But there was no electric light (or even gaslight) in the 1660s. Candles were not bright enough to do the job. So Hooke used an ingenious arrangement of glass lenses and round containers filled with water to act as spherical lenses to focus light from the Sun (or sometimes a candle) on the object of interest. This worked well for inanimate objects. But he also wanted to study living things, such as ants. These would soon crawl out of the focus of the microscope, but if he killed them then their bodies shrivelled up. He tried sticking them down with wax or glue, but they still wriggled about too much to be studied. Then he hit on the idea of dosing them with brandy to make them unconscious. The ant, he said, was soon ‘dead drunk, so that he became moveless’.

Of course, there was no way to photograph the objects under the microscope, and Hooke’s book is full of beautiful, detailed drawings of what he saw. A seventeenth-century experimenter had to be an artist as well as a scientist. Hooke showed his readers how much irregularity there is in something as seemingly perfect as the point of a needle or the edge of a razor, and he also showed them how much regularity there is in crystals. This, he said, must result from a regular arrangement of the particles making up crystals – an early hint at the existence of atoms.