Science: A History in 100 Experiments - John Gribbin

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English chemist, Joseph Priestley (1733–1804).

      In 1779, a Dutch physician and chemist, Jan Ingenhousz, settled in England after travelling widely in Europe. By then, Priestley had moved on, and Ingenhousz took over his laboratory in Calne, under the same sponsorship. Ingenhousz was also interested in the way air could be ‘revived’ by plants, and had independently carried out experiments similar to those Priestley had carried out at the beginning of the 1770s. At Calne, he took this work a stage further by putting green plants under water in transparent containers. He observed that bubbles of gas were produced from the underside of the green leaves when they were exposed to sunlight, but that in the absence of sunlight this bubbling stopped.

      It was a simple matter to catch the gas produced by the plants and test it. Ingenhousz found that a glowing taper plunged into the gas would relight, and this and other tests showed that it must be Priestley’s ‘pure air’ – what we now call oxygen. As a result of these experiments, Ingenhousz is credited with discovering photosynthesis, the chemical process by which plants use energy from sunlight and (among other things) carbon dioxide from the air to build their tissues, with oxygen released as a by-product. Animals use oxygen from the air to power their cells, releasing carbon dioxide as a waste product, so that there is a mutual interdependence between plants and animals. Although these details were worked out only later, the broad picture was clear to Ingenhousz in 1779.

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      © Biophoto Associates/Science Photo Library

      Photosynthesis in Canadian pondweed (Elodea canadensis). The bubbles around the plant contain oxygen, a by-product of photosynthesis. Photosynthesis is the process by which most plants convert sunlight into chemical energy.

      Ingenhousz summed up his discoveries in a book, Experiments upon Vegetables – Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring it in the Shade and at Night. He was fascinated by the interdependence between plants and animals, and at the end of the book he wrote: ‘If these conjectures were well grounded, it would throw a great deal of new light upon the arrangement of the different parts of the globe and the harmony between all its parts would become more conspicuous.’ This comes close to the idea of Gaia, the Earth as a single living organism, two centuries ahead of its time.

No. 18 OPENING UP THE SOLAR SYSTEM

      The scientific sensation of the 1780s was the discovery of a ‘new’ planet in the Solar System. This transformed the view of the heavens that had held since ancient times, and began the process of opening up astronomers’ images – of, first, the Solar System, and then of the whole Universe.

      The Ancients had observed five planets in the sky, named after Roman gods – Mercury, Venus, Mars, Jupiter and Saturn. By the 1780s, it was known that these planets orbit the Sun, with Mercury closest to the Sun and Saturn furthest out, and Earth was also known to be a planet, orbiting the Sun between Venus and Mars. Of course, the planet discovered in 1781, now know as Uranus, was not really new. It had been around for as long as the other planets, orbiting even further out than Saturn, and had even been observed many times, but it had been mistaken for a star or a comet. It is possible that one of the ‘stars’ identified by Hipparchos in his star catalogue in the second century BC was actually Uranus, although the planet is extremely difficult to spot with the naked eye. Telescopes made it easier to spot, and it is now certain that the planet was identified as a star by John Flamsteed in 1690. A French astronomer, Pierre Lemonnier, observed Uranus several times between 1750 and 1769, without realizing its true nature.

      The reason for those missed opportunities, even after the advent of the telescope, is that Uranus is so far from the Sun that it moves very slowly across the sky as seen from Earth. The other planets, as well as being brighter and easier to see, move noticeably against the background stars, which gives them their name, from the Greek word for a ‘wanderer’. But this also highlights the importance of applying the ‘experimental’ method to observations as well as to experiments. It is no good looking at the night sky casually from time to time and speculating about what you see. You have to make methodical observations over a long period of time, keeping careful records and comparing observations from different times to work out what is going on.

      That is exactly what William Herschel, assisted by his sister Caroline, was doing in the early 1780s. Herschel was a successful musician, living in Bath, who had developed a passion for astronomy and built his own telescopes, observing the skies from the garden of the house he shared with Caroline. He was actually carrying out a methodical search for double stars when, on 13 March 1781, he noticed an object that appeared in his telescope as a tiny disc, rather than a star-like point of light. (Stars do not appear as discs even in the best telescopes because they are so much further away than planets.) On 17 March, he looked for the object again, and found that it had moved against the background stars. The natural assumption was that he had found a comet, and he reported the discovery as such to the Royal Society. But when Herschel sent details of his discovery to the Astronomer Royal, Nevil Maskelyne, Maskelyne replied: ‘I don’t know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it.’

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      © New York Public Library Picture Collection/Science Photo Library

      William Herschel discovered Uranus in 1781 with this telescope.

      This was a crucial point. Planets move in roughly circular orbits around the Sun, staying at more or less the same distance. Comets dive in from the outer parts of the Solar System, swing past the Sun and head back out into the depths of space. Other observations confirmed Maskelyne’s speculation. In particular, the Russian astronomer Anders Johan Lexell calculated the orbit of the object from the available observations and showed that it was indeed nearly circular. In 1783, Herschel wrote to the Royal Society that ‘by the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System’. By then, he had already been appointed ‘King’s Astronomer’ (not to be confused with Astronomer Royal) by George III, with an income of £200 per annum, which enabled him to become a full-time astronomer.

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      © Royal Astronomical Society/Science Photo Library

      Letter by William Herschel (1738–1822), dedicating an engraving of the planet Uranus to King George III of Britain.

      In order to thank his patron, Herschel named the planet Georgium Sidus (George’s Star). But this did not go down too well outside the United Kingdom, and the astronomical community eventually settled on the name Uranus – with the stress on the first syllable. Ouranos was, in Greek mythology, the father of Cronus and grandfather of Zeus, who were Saturn and Jupiter in the Roman pantheon, fitting the place of the planet in the Solar System.

No. 19 ANIMAL HEAT, BUT NO ANIMAL MAGIC

      Although earlier experiments, such as those of Priestley and Ingenhousz, had shown the importance of some component of air in maintaining life, at the beginning of the 1780s the details of the process were still far from clear. Like many of his colleagues, the French chemist Antoine Lavoisier, a member of the French Academy of Sciences, speculated that the process resembled a slow form of combustion, with the life-giving component of air being converted into ‘fixed air’ (carbon dioxide) by,

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