Out of the Shadow of a Giant

by John Gribbin

  Boyle had reached Oxford by a circuitous route. Many accounts simply describe him as a rich aristocrat who had the time and money to indulge his interest in experimental philosophy. But things were never that simple in the England (and, especially in Boyle’s case, Ireland) of the middle decades of the seventeenth century. Boyle’s father, Richard Boyle, was indeed the Earl of Cork and filthy rich, but he was not the latest member of a long aristocratic line. Richard Boyle was what might now be called an entrepreneur, in the pejorative sense of the term. Born in England in 1566, into a respectable but unremarkable family, he became a penniless orphan before he was twenty and went to Ireland (then an English colony) to make his fortune. With the aid of marriages to a wealthy widow, and after she died to the daughter of the Secretary of State for Ireland, and financial dealings that were often on the shady side of legality, he succeeded in his aim so well that he became possibly the richest man in either Ireland or England, able to buy his title and the respectability that went with it.

  Wheeling and dealing didn’t take up all of Richard Boyle’s time. Along the way he fathered seven daughters and six sons, before the late addition of Robert, on 25 January 1627, when Richard Boyle was sixty and his wife Margaret forty years old. Yet another daughter was born three years later, but complications associated with the birth killed Margaret. The last girl was named after her.

  With no mother from the age of three, and far down the pecking order for any inheritance of either titles or money, the Honourable Robert Boyle (to give him the only title due to him) was initially brought up and educated at home, in the care of family retainers, but later went to Eton. There he was recognised as an outstanding scholar at a very young age, and first encountered the books of Nicolaus Copernicus and William Gilbert. But at the age of twelve, in 1639, Robert was plucked out of school and sent with his brother Francis, then fifteen, on the Grand Tour of Europe that was de rigueur for the sons of wealthy gentlemen. Their education was not forgotten. They were accompanied by a tutor, and visited many seats of learning – they were in Florence in 1642 when Galileo died. But when they were about to return home, rebellion broke out in Ireland. This was one of the early precursors to the Civil Wars, and although Francis was considered old enough to be summoned home to help suppress the rebellion, Robert was told to keep away until the fighting was over. The rebels, however, were not suppressed without serious consequences for the Boyle family. Two of Robert’s brothers (not Francis) were killed, and the grand old Earl of Cork lost most of his money and land. He died in 1643, soon after this phase of fighting finished. So when Robert returned to England in 1644, he had no money and had not been educated for any kind of useful career. Worse, by then the ‘proper’ Civil War was raging. He was saved by his sister Katherine, thirteen years older than Robert, who had married to become Viscountess Ranelagh, and lived in London but apart from her husband.

  At first, Robert lived in Katherine’s house. She was a known Parliamentarian sympathiser with many powerful friends in London, which was controlled by Parliament. Robert judiciously never gave any indication of preferring one side or the other in the Civil Wars, probably because he genuinely just wanted to be left alone to get on with his life in peace. Katherine helped him to find a retreat from the turmoil of the times. Their father had left to Robert a small estate in Dorset – not much compared with the large estates in Ireland once intended for the older brothers, but enough for the youngest son, and by chance one of the few possessions the Earl had left at his death. Thanks to Katherine’s connections, the estate was not confiscated by Parliament, and Robert was allowed to live there from 1645 onwards, setting up his own laboratory where he carried out chemical experiments. On visits to London, he stayed with Lady Ranelagh, and like-minded experimental philosophers used to gather to meet with him at her house. Boyle referred to this as an ‘invisible college’; we don’t know who was involved, but there must have been considerable overlap with the group we mentioned earlier, including Wilkins.

  Boyle’s fortunes improved in the 1650s, after the Civil Wars ended. One of his surviving brothers, now Lord Broghill, was in favour with Parliament for his part in crushing the Irish. This rubbed off on the rest of the Boyle family, and Robert was able to visit Ireland to pick up some of the threads of their former life. Horrified by the terrible conditions of the Irish people, after some soul-searching he got the estates running as a benevolent landlord (by the standards of the time) who used much of the income for charitable ends. This still left him enough to live on comfortably and continue in the role of gentleman scientist back in Dorset. But John Wilkins, who had met Boyle in London and knew his abilities, invited Robert to move to Oxford, where he could not just carry out his own experiments but be in the company of other people with similar interests. After mulling the offer over, Boyle made the move in 1655. He was never formally part of the university (as he put it himself, ‘never a Professor of Philosophy, nor a Gown-man’), but he had his own laboratory, and he also (as Wilkins had probably hoped) helped to finance the work of some of his fellow experimental philosophers. Boyle lived and worked in a house known as Deep Hall, on the High (convenient for the coffee shops!), and it was here, in 1656 (the year, incidentally, that Edmond Halley was born), that Robert Hooke came to live and work as Boyle’s paid assistant, although possibly they had already met.

  Back in September 1653, when Wilkins was already trying to persuade Boyle to move to Oxford, he had sent a letter to him by messenger. It read, in part:

  This bearer is the young man I recommended to you. I am apt to believe, that upon trial you will approve of him. But if it should happen otherwise, it is my desire he be returned, it not being so much to prefer him, as to serve you.fn5

  Lisa Jardine has suggested that the young man in question was Hooke, and that he was sent as part of the attempt to entice Boyle to Oxford, by showing that a skilled assistant would be available there:

  If it be not, Sir, prejudicial to your other affairs, I should exceedingly rejoice in your being stayed in England this winter, and the advantage of your conversation at Oxford, where you will be a means to quicken and direct us in our enquiries … shall be most ready to provide the best accommodation for you, that this place will afford.

  The immediate plan to persuade Boyle to Oxford that winter was aborted because he had to travel to Ireland to deal with the urgent business concerning the family estates we have already mentioned. The young man, presumed to be Hooke, returned to Oxford. But when Boyle did make the move some two years later, it seems that he was already aware of the abilities of the man who did indeed become his assistant.

  The greatest achievement of the Boyle–Hooke collaboration was an improved air pump, which made it possible for them to carry out experiments both at greatly reduced air pressure and at pressures greater than ordinary atmospheric pressure. That simple sentence, though, needs unpacking in order to put Hooke’s achievements, in particular, into perspective.

  First, although Hooke was a paid assistant to Boyle, this was a genuine collaboration. Hooke was more than a ‘mere’ technician who did things at Boyle’s direction. This was a very unusual – indeed, possibly unique – working relationship for the time, but it is made clear in Boyle’s published works, where Hooke is regularly mentioned by name as a co-experimenter. Other assistants are not so acknowledged. Secondly, an air pump might not sound like a dramatic invention today. But in the middle of the seventeenth century it was the highest of high-tech scientific equipment, equivalent, in terms of the insights it gave, to CERN’s Large Hadron Collider, or the Hubble Space Telescope, today. It was cutting-edge technology, leading to breakthrough science. And the man who made the air pump, and made it work, was Robert Hooke, still in his early twenties. If there had been Nobel Prizes in the seventeenth century, Hooke would have walked away with one, for this achievement alone.

  It all started with an experiment carried out by the Italian Evangelista Torricelli (one of Galileo’s pupils) in 1644. This seemed to she
d light on a puzzle that had vexed philosophers for centuries: was it possible for a vacuum, nothing at all, to exist? One school of thought held that matter must be continuous; a rival hypothesis described matter in terms of tiny particles (atoms) moving through the void (vacuum, or empty space). Torricelli took a glass tube, closed at one end, and filled it with mercury. He then put a finger over the open end, and submerged that end below the surface of a dish of mercury before taking his finger away and raising the closed end of the tube into the vertical. Instead of all the mercury flowing out of the tube, the level dropped only until there was a column nearly thirty inches high standing above the level of the liquid in the dish, with nothing at all in the space above the column. This seemed to be the definitive proof of the reality of the vacuum, and along the way the height of the mercury in the tube was explained as a result of the pressure of the weight of the air pushing down on the surface of the mercury in the dish. Torricelli had invented the barometer, for measuring atmospheric pressure, and similar instruments were soon tested by being carried up mountains, where the lower air pressure meant that the column of mercury was shorter than at sea level. Which suggested that if the air continued to thin out, then above the atmosphere there must be empty space.

  Instead of carrying the equipment up a mountain, Boyle wanted to try it out inside a vessel where air could be pumped out to lower the pressure. If he could make a vacuum inside the vessel, the level of mercury in the column would fall as the air was removed, until it would not be supported in a column at all. But first, he needed a way to make a vacuum in the laboratory. This is where Hooke came in. Otto von Guericke, in Saxony, had already made a reasonably efficient air pump, which he had used to suck air out of two large copper hemispheres that were placed together rim to rim to make a sphere, but with no mechanical fastenings at the join. With air pressure inside the sphere reduced, the pressure of the atmosphere outside squeezed the hemispheres together so tightly that in a famous demonstration made to Emperor Ferdinand III in 1654 thirty horses could not pull them apart.

  Von Guericke’s pump was large and cumbersome, needing two men to operate, and, of course, there was no way to see inside his copper sphere. Boyle needed something that could be operated by one man, with a chamber made of glass through which experiments could be observed. He first approached the greatest scientific instrument maker of the time, Ralph Greatorex, in London. But his forte was making precision instruments, and his attempt at the heavier machinery required for the pump was not up to Boyle’s needs. So it was Hooke, at the end of the 1650s, who designed and built the breakthrough instrument, using funds supplied by Boyle. He went to London to oversee the manufacture of the heavy components in the workshops there (we don’t know if he worked on these himself), then had them taken to Oxford, where he put the pump together and made it work.

  The vacuum chamber consisted of a glass sphere fifteen inches in diameter, known as the ‘receiver’, with a brass lid four inches in diameter, which could be opened to place apparatus inside the sphere. A tapering hole in the base of the sphere stood on top of a tight-fitting brass cylinder, sealed with a leather collar. The brass lid had a small tight-fitting stopper, sealed with oil (referred to by Hooke as ‘sallad oil’), that could be turned to tug a string attached to the stopper in order to set off an experiment inside the globe. The cylinder below the globe connected to a brass pump fitted with an ingenious rack-and-pinion system, which allowed air easily to be pumped out of or into the globe. Hooke’s pump sucked air from the cylinder using a piston that was connected to a rod 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. The piston could be pumped up and down repeatedly, sucking more and more air out of the glass vessel. This apparatus became known as ‘Boyle’s air pump’, which it was in the sense that he paid for it and owned it (just as Dolly Parton’s hair is her own). But as Boyle acknowledged, it was made by Hooke, and Hooke was the experimenter who operated it during the many investigations that followed. In the fragment of autobiography quoted by Waller, Hooke said:

  In 1658, or 9, I contriv’d and perfected the Air-pump for Mr Boyle, having first seen a Contrivance for that purpose made for the same honourable Person by Mr Gratorix, which was too gross to perform any great matter.

  Some idea of the significance of the pump is that, even by the end of the 1660s, there were only half a dozen comparable air pumps in Europe, and three of them had been made by Hooke.

  Boyle and Hooke carried out many experiments with their pump and vacuum chamber – Boyle later described forty-three of them in his book New Experiments Physico-Mechanical Touching the Spring of the Air, published in 1660. These included burning (or attempting to burn) substances such as candles, coal, charcoal and gunpowder in a vacuum, with results that convinced them that fire was not one of the ‘four elements’ (fire, earth, air and water) as the Ancient Greeks had taught, but involved a chemical process. Candles, for example, went out when air was removed from the globe, and burning coals died away, but, crucially, reignited when air was let back in. One of the other experiments showed that water boils at a lower temperature when the air pressure is reduced. But one of their most important discoveries is hinted at in the title of Boyle’s book. Every stroke of the handle of Hooke’s air pump demonstrated the ‘spring’ of the air, just like the springiness felt when using a bicycle pump today, and Hooke set out to measure this springiness – what we now call air pressure.

  Around this time, at the end of the 1650s, the Englishman Richard Towneley was carrying out experiments with a Torricelli barometer on Pendle Hill, in Lancashire. He was following the example of continental experimenters, notably Florin Périer. Like them, he found that the pressure of the air measured by the barometer is lower at higher altitude, and he surmised (without carrying out experiments to test the idea) that the pressure is less because the air is thinner – that is, less dense – at higher altitude. He mentioned this idea to Boyle, who asked Hooke to devise a way to test it.

  Hooke did this in 1660 or 1661, using a long glass tube shaped like the letter J, with the top open and the short arm of the J at the bottom sealed. He poured a little mercury into the top of the tube so that it partly filled the U-bend at the bottom but left some air trapped in the closed end. With the level of mercury the same on both sides of the U-bend, the trapped air was at atmospheric pressure. But Hooke could increase the pressure on the trapped air by pouring more mercury in, forcing some of it round the bend and squeezing the trapped air into a smaller volume. Boyle was short-sighted and bad at arithmetic, so we know for sure that it was Hooke who not only designed the experiment but also made the careful observations and records that showed that the volume of the trapped air was inversely proportional to the pressure applied. Double the pressure, and the volume halves; triple the pressure and the volume is reduced to one-third, and so on. These results were published in the second edition of Boyle’s book, in 1662, and became known as ‘Boyle’s Law’, although he did not use that name himself. Hooke’s own account appeared in his book Micrographia, published in 1665:

  Having lately heard of Mr. Townly’s Hypothesis, I shaped my course in such sort, as would be most convenient for the examination of that Hypothesis.

  After describing the experiment (Hooke tells us that the long arm of the J-tube was about fifty inches long), he concludes:

  and by making several other tryals, in several other degrees of condensation [compression] of the Air, I found them exactly answer the former Hypothesis.

  The discovery itself was significant. The measurements of the springiness of the air fed into the development of theoretical ideas about the nature of matter, leading up to the idea of atoms and molecules flying about in the vacuum and colliding with one another. It also had practical implications, because the idea of making vacuums using pistons, and using the weight of air (atmospheric pressure) to compress piston
s, found applications in steam engines. But from our point of view the most important thing about these experiments is the way they were carried out and reported. For the first time, experimental philosophers described their experiments in great detail, along with the way they overcame difficulties and how they interpreted their results. They not only gave a table showing the actual measurements of pressure made in the course of the investigation, but also included alongside these the numbers corresponding to ‘What the pressure should be according to the Hypothesis’. The match was not perfect; of course there were experimental errors. But they (or rather Hooke) had found that the accuracy of the hypothesis was confirmed within the limits of experimental error. And everything was laid out carefully so that other experimenters could repeat the whole process and see if their results agreed. It was only later, when many other experiments had indeed confirmed this, that the hypothesis was elevated to the status of a law, albeit with the wrong name attached to it.

  While in Oxford, Hooke also developed his interests in astronomy and timekeeping, which we have already mentioned. Some of his other activities can wait until we discuss the contents of Micrographia. But there was one interest in particular that Hooke at first eagerly investigated in Oxford but then (for sound scientific reasons) abandoned – flying. This change of heart is described in the autobiography:

  I contriv’d and made many trials about the Art of flying in the Air, and moving very swift on the Land and Water, of which I shew’d several Designs to Dr. Wilkins then Warden of Wadham College, and at the same time made a Module [model], which, by the help of Springs and Wings, rais’d and sustain’d itself in the Air; but finding by my own trials, and afterwards by Calculation, that the Muscles of a Mans Body were not sufficient to do anything considerable of that kind, I apply’d my Mind to contrive a way to make artificial Muscles; divers designs wherefore I shew’d also at the same time to Dr. Wilkins, but was in many of my Trials frustrated of my expectations.