Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
and positions one to another.
By Density and Rarity, I understand a property of a transparent body that does either more or less refract a Ray of light.
And
The redness of the Sun, Moon and Stars, will be found to be caused by the inflection of the rays within the Atmosphere … it is not merely the colour of the Air interpos’d.
In other words, the colour is inherent in the original white light and is not some kind of pollution, or corruption, caused by the passage of light through the intervening medium – another discovery later attributed to Newton.
The second great insight in Micrographia comes in Observation 16, where Hooke presents his ideas on combustion. The microscopic justification for including these ideas comes from his studies of charcoal and burnt vegetables, but the experiments from which his most impressive insights are drawn do not really involve the microscope at all. These included his observations of the way flames went out when a lit candle was shut in a sealed chamber, how small animals collapsed and died after a certain time in such a chamber, the gruesome vivisection of a dog, and the experiments with candles and living things involving the air pump. Having already, in Observation 9, asserted that heat is ‘a motion of the internal parts’ of a substance (also mentioned in Observations 7 and 8), he now draws a clear distinction between heat and combustion. ‘This Hypothesis,’ he says, ‘I have endeavoured to raise from an Infinite of Observations and Experiments, the process of which would be much too long to be here inserted.’ But as he tells us, the idea ‘has not, that I know of, been publish’d or hinted, nay, not so much as thought of, by any.’ He was right.
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