by John Gribbin
One of the key series of experiments that he hints at here was carried out as demonstrations at the Royal in January and February 1665. In a beautiful example of the scientific method at work, he showed first that gunpowder would still burn in the absence of air, and then that neither of two of the three ingredients of gunpowder, charcoal and sulphur, would burn on their own in the absence of air. But each of them could be reignited by adding the third ingredient, which he knew as saltpetre but which we call potassium nitrate. As Hooke says in Micrographia, it is clear from these experiments that combustion involves ‘a substance inherent, and mixt with the Air, that is like, if not the very same, with that which is fixt in Salt-peter.’ That substance is, of course, oxygen; the chemical formula for potasssium nitrate is KNO3.
Hooke’s idea is that something in the air is essential to combustion, which takes place when that something combines with something in the burning object. ‘There is no such thing as an Element of Fire’, he asserts, dismissing the idea that had held sway since the time of Ancient Greece. A flame ‘is nothing else but a mixture of Air and volatile sulphureous parts of dissoluble or combustible bodies, which are acting upon each other whilst they ascend, that is, flame seems to be a mixture of Air, and the combustible volatile parts of any body’. Further, the component of air that is essential for combustion is also, Hooke tells us, essential for life. In Observation 22, almost as an aside, he mentions that there is a ‘property in the Air which it loses in the Lungs, by being breath’d’. In being so close to the discovery of oxygen, Hooke was nearly a century and a half ahead of his time; right up until the end of the eighteenth century, the phlogiston theory of combustion (which said, flying in the face of experiments like those Hooke carried out with the air pump, that burning substances released phlogiston, rather than absorbing something from the air) held sway, and Hooke’s ideas were forgotten. In 1803, chemist John Robison wrote:
I do not know of a more unaccountable thing in the history of science, than the total oblivion of this theory of Dr. Hooke, so clearly expressed, and so likely to catch attention.
But it did catch the attention of one person, the serial plagiarist Isaac Newton. In an appendix to his book on optics, hurried into print immediately after Hooke’s death (see postscript to Chapter Seven), Newton presented a suite of ideas about combustion that chemist Clara de Milt has described, with admirable academic restraint, as ‘very, very much like those of Hooke’. As Private Eye might put it, could they by any chance be related?
The third great insight presented in Micrographia comes in Observation 17: Of Petrify’d wood, and other Petrify’d bodies. The petrified objects he refers to are what we now call fossils. Before Hooke, it was widely thought that these were, in his words, ‘Stones form’d by some extraordinary Plastick virtue latent in the earth’. In other words, that these were just curious stones that happened to resemble the forms of living things. But he dismissed this notion, and stated unequivocally (‘I cannot but think’) that they were ‘the Shells of certain Shel-fishes, which, either by some Deluge, Inundation, Earthquake, or some other such means, came to be thrown to that place’. ‘That place’, he was well aware, was high up in a mountain, or on the cliffs that he had walked as a boy on the Isle of Wight. So how did such things as wood and shells become petrified, or fossilised? Hooke’s description of the process could almost come from the pages of a modern textbook of geology:
this petrify’d Wood having lain in some place where it was well soak’d with petrifying water (that is, such water as is well impregnated with stony and earthy particles) did by degrees separate, either by straining and filtration, or perhaps, by precipitation, cohesion or coagulation, abundance of stony particles from the permeating water, which stony particles, being by means of the fluid vehicle convey’d, not onely into the Microscopical pores, and so perfectly stoping them up, but also into the pores or interstitia, which may, perhaps, be even in the texture or Schematisme of that part of the Wood, which, through the Microscope, appears most solid.
And as for shells, they must have been:
fill’d with some kind of Mudd or Clay, or petrifying Water, or some other substance, which in tract of time has been settled together and hardened in those shelly moulds.
Hooke clearly understood two things: that there were geological processes that transformed once-living things into ‘petrified’ rock, and that there were geological processes that transformed the structure of the Earth’s crust. Implicit in this was the understanding that the timescales involved (‘tract of time’) were far greater than the ‘official’ chronology of a few thousand years derived from the Bible.
Hooke even begins to hint at the kind of investigations that would lead to the idea of evolution:
It were therefore very desirable, that a good collection of such kind of Figur’d stones were collected; and as many particulars, circumstances, and informations collected with them as could be obtained, that from such a History of Observations well rang’d, examin’d and digested, the true original or production of all those kinds of stones might be perfectly and surely known.
Soon after the publication of Micrographia, a Dane, Niels Steensen (who used the Latinised version his name and is remembered as Steno), publicised very similar ideas, and suggested that different rock strata, containing fossils such as sharks’ teeth, had been laid down under water, far from the present-day seas, at different times during Earth’s history by a succession of floods. Coincidence? Hooke didn’t think so. He had developed these ideas further in his Cutlerian Lectures which we discuss later. Henry Oldenburg, the Secretary of the Royal Society and someone who often rubbed Hooke up the wrong way, was in correspondence with scientists across Europe as part of his job. Steno published his ideas in 1669, in Latin. Oldenburg promptly made the Royal aware of the book, and arranged for it to be translated into English, which helped to ensure that Steno became remembered as the inventor, or discoverer, of these ideas. Hooke was not exactly pleased and tried unsuccessfully to get recognition that he at the very least had the idea first. When it was suggested that he had borrowed his ideas from Steno, rather than the other way around, he was moved to write a letter, read to a meeting of the Royal on 27 April 1687, in which he said:
I must now add in my own vindication that I did long since prove Steno had much of his treatise from my Lectures, which some time before that I had read [in Gresham College] which Lectures Mr Old. Borrowed and transcribed and by Divers circumstances I found he had transmitted the substance of if not the very Lectures themselves [to Steno]. And he did as good as own it, and upon my challenging him with it he did in two of his transactions publish that I had Read A great part of that Doctrine & hypothesis in my Lectures in Gresham Colledge Some time before Mr Steno had published his Booke.
There is no reason to doubt Hooke’s version of affairs, and there is no doubt at all that his work preceded that of Steno, whether or not Steno got word of it via Oldenburg. Steno, by the way, never gave a clue one way or the other: he disappeared from the scientific scene after writing his book. He became a Catholic priest in 1675, and was ordained as a bishop in 1677, inflicting on his body such a harsh regime of fasting and self-denial that he died in 1686, at the age of forty-eight.
Hooke’s more extensive ideas about earthquakes, Earth history and geology will be covered in Chapter Nine. Now, we still have a fourth great insight from Micrographia to discuss, although here we diverge from ‘Espinasse’s assessment of which of the ideas Hooke presented there were most significant. She picks out his discovery of the structures he named cells (after the rooms occurred by monks in a monastery) in thin slices of cork (Observation 18). As Hooke puts it, no ‘Writer or Person’ had ‘made any mention of them before this’. But although the name was taken up and used by later biologists, it was in a slightly different context. The ‘pores’, as he also called them, that Hooke had found are not living cells, but non-living structures left over from the growth of the plant. The first person to see and study l
ive cells under a microscope was Hooke’s Dutch contemporary Antoni van Leeuwenhoek. In 1674 he described an algae, Spirogyra, and other organisms that moved of their own volition; he named them animalcules (‘little animals’). In this area, Hooke’s work was important, but not as important as the work of van Leeuwenhoek and others. In our estimation, his astronomical Observations were of far greater importance.
Hooke was a serious and highly respected astronomer. On 9 May 1664, using a twelve-foot-long refracting telescope, he had discovered the Great Red Spot of Jupiter, and used it to measure the rotation of the giant planet. Contemporary (and now more famous) astronomers such as the Italian Giovanni Cassini picked up on the discovery, and referred to the phenomenon as ‘Hooke’s Spot’. But it was observations of something much closer to home that led Hooke to important insights that appeared in Observation 60: Of the Moon. This is a short contribution that to a casual glance looks like a mere filler. That couldn’t be more wrong.
Observation 60 provides a nice example of the scientific mind – Hooke’s scientific mind – at work: making observations, devising hypotheses, testing them by experiment and further observation, and drawing general conclusions from specific cases. Remember that this was less than sixty years after Galileo, with the aid of one of the first astronomical telescopes, discovered that the Moon is not a perfect sphere but pockmarked with craters and scarred by mountain ranges. Hooke was intrigued by the nature of these craters, and puzzled over their origin. He described them as ‘almost like a dish, some bigger, some less, some shallower, some deeper, that is, they seem to be a hollow Hemisphere, incompassed with a round rising bank, as if the substance in the middle had been digg’d up, and thrown on either side’. Which establishes, as if we did not already know, that he was a good and accurate observer.
How could such craters be formed? Hooke came up with two hypotheses and set out to test them. The first was that the craters were caused by impacts. To test this, Hooke made a mixture of water and pipe-clay, ‘into which, if I let fall any heavy body, as a Bullet, it would throw up the mixture round the place, which for a while would make a representation, not unlike these of the Moon.’ So incoming objects (bodies) would do the trick. But Hooke found it ‘difficult to imagine whence those bodies should come’, so he turned to his other idea. In this experiment he heated a pot of alabaster to the boiling point, and then, while it was still bubbling, took it off the fire and allowed it to set. Then ‘the whole surface, especially that where some of the last Bubbles have risen, will appear all over covered with small pits, exactly shaped like these of the Moon, and by holding a lighted Candle in a large dark Room, in divers positions to this surface, you may exactly represent all the Phenomena of these pits in the Moon, according as they are more or less enlightened by the Sun’.
So Hooke plumped for volcanic activity as an explanation of lunar cratering, rather than impacts. This was a perfectly reasonable conclusion to draw at the time, and for the next four hundred years volcanic activity remained a viable explanation for lunar cratering. The idea was only finally laid to rest, in favour of the impact hypothesis, when astronauts visited the Moon and its geology could be studied first hand. We now know that the craters were indeed made by impacts, in which ‘the substance in the middle had been digg’d up, and thrown on either side’. But if it was thrown up, either by impacts or by volcanic activity, something must have pulled it back down on to the surface of the Moon to make the circular ramparts surrounding the craters. That something, Hooke reasoned, must have been gravity – the Moon’s own gravitational pull.
Developing his idea, Hooke said that it ‘is not improbable, but that the substance of the Moon may be very much like that of the Earth’ (which would have amounted to heresy a few decades earlier). And then he goes beyond Galileo, who noticed the imperfection of the Moon, to draw attention to the remarkable roundness of the Moon in spite of the small irregularities we see on its surface. The Moon, he points out:
we may perceive very plainly by the Telescope, to be (bating the small inequality of the Hills and Vales in it, which are all of them likewise shaped, or levelled, as it were, to answer to the center of the Moons body) perfectly of a Spherical figure, that is, all the parts are so rang’d (bating the comparatively small ruggedness of the Hills and Dales) that the outmost bounds of them are equally distant from the Center of the Moon, and consequently, it is exceedingly probable also, that they are equidistant from the Center of gravitation; and indeed, the figure of the superficial parts of the Moon are so exactly shap’d, according as they should bye, supposing it had a gravitating principle as the Earth has.
This is mind-blowing stuff. At a time when other people talked about vortices and whirlpools being responsible for the shape of the planets and their orbits, and Isaac Newton was an unknown student who would soon be eagerly devouring Hooke’s book,fn12 Hooke is suggesting the universal principle of gravitation (he can hardly have failed to notice that Jupiter and the other planets are also round!), that all objects possess this property, which makes the moons and planets round and (although he discusses this elsewhere) holds them in their orbits around the Sun. The very last paragraph of Micrographia begins with the words: ‘To conclude, therefore, it being very probable, that the Moon has a principle of gravitation … whereby it is not only shap’d round, but does firmly contain and hold all its parts united, though many of them seem as loose as the sand on the Earth’.
We emphasise that the idea of universal gravity is of key importance. This is the beginning of an understanding that the laws of physics which operate in the Universe at large – in the Heavens – are the same as the laws that apply here on Earth. That idea is often traced back to Newton; it should be traced back to Hooke. It’s a long way from the study of the point of a needle!
Any of these four ideas, or indeed his ideas about planetary orbits and gravity, which we have already discussed, should have ensured Hooke’s status as one of the greatest scientists of all time. And remember that there were dozens of lesser ‘observations’ in Micrographia, including some of the first observations of the tiny creatures that live in water and other liquids – the Fellows were particularly intrigued by the discovery of the creatures we call nematodes, but were referred to then as ‘eels’, living in vinegar (Observation 57). Perhaps Hooke would have been suitably recognised by posterity if he had been able to develop his ideas more fully, which he clearly intended to do. In his book, he says (especially in reference to his ideas about combustion, but undoubtedly with broader relevance):
In this place I have only time to hint at an Hypothesis, which, if God permit me life and opportunity, I may elsewhere prosecute, improve and publish.
But Hooke’s opportunities to ‘prosecute, improve and publish’ his revolutionary ideas were almost immediately restricted by plague, the Great Fire of London, and a change of career that occurred in the aftermath of the fire. While he was otherwise engaged, at least some of those ideas were taken up and developed by Isaac Newton, who had an early copy of Micrographia which he read and annotated extensively (the copy still exists), having ample opportunity to study it while he was away from Cambridge during the plague year of 1665 when he was twenty-two. He was particularly inspired at that time by Hooke’s ideas about light and colour, developed in Observation 10. Newton’s variation on this theme would soon come to the attention of the Royal and lead to a lifelong bitterness between Hooke and Newton. But Hooke would have ten more years – the happiest years of his life – before that controversy reared its head.
CHAPTER THREE
MONUMENTAL ACHIEVEMENTS
In the spring of 1665, following the publication of Micrographia, Hooke’s prestige was higher than ever, and he was, at the age of twenty-nine, at the peak of his abilities as a scientist and ‘mechanick’. But his immediate plans, and those of the Royal Society, were disrupted by the severe outbreak of plague that affected London as the weather got warmer. Plague was far from being unknown, and there had been lesser outbr
eaks from time to time, but this occasion was different. Cases had been reported in Holland in the spring of 1664, resulting in ships from the Netherlands being quarantined in the Thames; in February 1665, war broke out between England and the Dutch, so all trade ceased. In any case, the cold winter of 1664–1665 had slowed the spread of the disease, but the death toll began to rise in March 1665.
At the time, the Lord Mayor was Sir John Lawrence, who that same month had been instrumental in rectifying the irregularities involving Hooke’s election as Professor of Geometry, and he stayed in the City to keep control throughout the months that followed; Henry Oldenburg, the Secretary of the Royal, also stayed at his post, acting as a conduit (the seventeenth-century equivalent of the Internet) for the flow of information between scientists in Europe (including Holland, in spite of the war) and England.fn1 Samuel Pepys, of course, was another who stayed. But those who could leave the city did so. The King and his Court moved to Oxford, followed by many of the Fellows of the Royal, including Boyle. Queen Henrietta went to Paris, accompanied by a large party including Christopher Wren, whom the King had instructed to study the buildings programme of Louis XIV. But after the Royal suspended its Wednesday meetings following 28 June, its three leading experimenters – Hooke, Sir William Petty and Dr John Wilkins – moved (with a lot of experimental apparatus, and an assistant, known as an ‘operator’) to Durdans, an estate near Epsom in Surrey, owned by George, Lord Berkeley, himself an FRS. There, they carried out a full programme of experiments on behalf of the Royal, and many others of their own, particularly Hooke’s, devising.
