Drunken mice are history in making

Hydrogen - The Ingredients

五月 2, 2003

John Rigden's Hydrogen is both broader and narrower than its title suggests. It contains next to nothing on the hydrogen economy or on hydrogen bonding and DNA. But such omissions are necessary because here hydrogen is the thread that weaves together a gripping survey of 20th-century atomic physics.

The hydrogen atom is so simple that the dominant theory of the moment can provide unusually precise predictions of its properties. So the hydrogen atom becomes an arena in which theories can be subjected to unusually severe tests. This process is very demanding both theoretically and experimentally. Experiments of great accuracy must be devised and theories modified in response to experimental challenge. Rigden shows how fertile this symbiosis has been, beginning in 1885 with Balmer's equation for a series of lines in the spectrum of atomic hydrogen. Bohr incorporated Balmer's equation into a quantum theory of the hydrogen atom that explained all line series in the spectrum and generated an accurate value of the Rydberg constant. Dissatisfaction with Bohr's theory led to the separate versions of quantum mechanics supplied by Heisenberg and Schrodinger, but in each case, agreement with the hydrogen spectrum was the touchstone of success. The fine structure of the spectral lines called for the inclusion of relativity, and Dirac's equation was the result. Like all great theories, this proved unexpectedly fertile; electron spin emerged naturally from it, and it opened doors to the new world of antimatter.

After the war, elegant beam experiments by Lamb and Rabi revealed new, tiny splittings of the hydrogen spectral lines. These challenged Dirac's theory and gave pre-eminence to quantum electrodynamics. Hansch's two-photon measurements of the Lyman alpha transition have reduced the uncertainty in the Rydberg constant to one-millionth of a wave number. Such precision allows experiments that may even reveal temporal changes in fundamental constants. The book ends with other two-particle analogues of hydrogen such as Rydberg atoms, muonium and positronium. These are currently putting quantum electrodynamics to the test.

All this is just one of the themes running through the 23 chapters of this ingenious book. Social questions are not neglected, such as the fact that the 1930s were crucial in bringing together the openness and vigour of America's young scientists with an extraordinary influx of talent provoked by European fascism. Thus began a westward drift of scientific hegemony.

Philip Ball's The Ingredients is his personal account of the chemical elements. Although it includes a chapter on the Periodic system, he argues that such theories are often at odds with our direct experience of the material world. So he begins with Aristotle's four elements and other compositional theories that appeared prior to 1700. These theories were compelling because they gave priority to sensual experience. Moreover, they were not so precise as to become prohibitive. Fanciful speculation was therefore possible about material change, and the theories entered poetry, philosophy and literature. Ball identifies connections with Platonic solids, Renaissance painting and T. S. Eliot's Four Quartets.

Gold also offers rich opportunities for cultural and historical allusions, so this element has a chapter to itself. We move from classical mythology to Ian Fleming, from alchemy to the death of the monetary gold standard. In another chapter, selected elements are connected to particular properties or uses. Silicon is represented by micro-electronics, the rare earths by europium phosphors, argon by its inertness, and palladium by catalysis and the cold fusion disaster. A chapter on isotopes tackles radiocarbon dating and its application to the Turin Shroud. It also shows how oxygen-isotope analysis of deep-sea carbonate sediments has detected Milankovitch cycles in global temperature.

Some chemical elements have contributed much more to science than others.

Lavoisier's revolutionary ideas appear in a chapter on oxygen. The emphasis is on combustion and the composition of air. Another of his transformations gets short shrift: he based the names of chemical compounds on those of their constituent elements. This omission is surprising given the subject of the book. Lavoisier represents an era when the chemical elements became immutable. Today, their transmutation is recognised in nature, and carried out in laboratories. Ball gives a lively account of these developments, starting with the earliest experiments on radioactivity. The story of nuclear fission and atomic weapons appears alongside that of an emerging theory of nucleosynthesis. The tale ends on the outer limits of the periodic table, where the expected upturn in half-lives has recently been detected near element 114.

The book contains some delightful anecdotes. I particularly enjoyed the story of G. N. Lewis getting mice drunk on heavy water, while an angry Ernest Lawrence went short of deuterium for his cyclotron. The author has also made good use of his consultant editorship with the journal Nature.

There are valuable current references to that journal on the nobility of gold, the chemistry of seaborgium and the existence of water on the primeval earth.

Scientists will read these books for anecdotal and biographical details, and for fresh insights into sometimes unfamiliar topics. They will be richly rewarded. But both works carry intimidating endorsements from eminent scientists assuring us that they will also suit the general reader.

I have my doubts. The general reader needs a little more didactic purpose.

In Rigden's book, the going sometimes gets tough. Oscillating fields, kaons and inertial mass drop into the text with little or no explanation. On page 52, fundamental constants appear in SI units; on page 38, the same constants appear beneath the Bohr equation as dimensionless quantities with different numerical values. Close inspection suggests that this is explained by a lingering affection for electrostatic units, but the general reader would be thrown by it. With Ball's book, the budding autodidact sometimes gets short-changed. The lively style is a great asset, but artistic licence can be carried a little far. We are told that niobium tends to form bonds to five other atoms, and that the noble gases do not form compounds with other elements, even though a compound of argon (not xenon) appears two pages later. One also gets little sense of the empowerment that chemists experienced when Lavoisier's definition of a chemical element was coupled with quantitative analysis. By 1820, it was clear that 17g of ammonia were composed of 14g of nitrogen and 3g of hydrogen. So ammonia might be made from air and water, but not from fire and sand. Such clarity was impossible with the four-element theory.

Once, books for the general reader on subjects such as these had a stronger sense of the mission to explain. Today, it seems more like a mission to inform and entertain. But there is nothing wrong with information and entertainment, and perhaps yesterday's general reader is today's graduate scientist.

David Johnson is reader in chemistry, Open University.

Hydrogen: The Essential Element

Author - John S. Rigden
ISBN - 0 674 00738 7
Publisher - Harvard University Press
Price - £18.50
Pages - 288



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