Sep 062017
 

Today is the birthday (1766) of John Dalton FRS, English chemist, physicist, and meteorologist, best known (by people who know about these things) for proposing the basis of modern atomic theory. Dalton was born into a Quaker family in Eaglesfield, near Cockermouth, in the Lake District. His father was a weaver. He received his early education from his father and from Quaker John Fletcher, who ran a private school in the nearby village of Pardshaw Hall. Dalton’s family did not have enough money to support him in school for long, so he began to earn his living at the age of 10 in the service of a wealthy local Quaker, Elihu Robinson. It is said he began teaching at a local school at age 12, and became proficient in Latin at age 14.

When he was 15, Dalton joined his older brother Jonathan in running a Quaker school in Kendal, about 45 miles (72 km) from his home. Around the age of 23, Dalton may have considered studying law or medicine, but his relatives did not encourage him, perhaps because being a Dissenter, he was barred from attending English universities. He acquired most of his scientific knowledge from informal instruction by John Gough, a blind natural philosopher. At the age of 27 he was appointed teacher of mathematics and natural philosophy at the “New College” in Manchester, a dissenting academy. He remained there until the age of 34, when the college’s worsening financial situation led him to resign his post and take up a new career as a private tutor in mathematics and natural philosophy.

Dalton’s early life was strongly influenced by Elihu Robinson, who was a competent meteorologist and instrument maker, and who interested him in problems of mathematics and meteorology. In 1787 at age 21 he began his meteorological diary in which, during the succeeding 57 years, he entered more than 200,000 observations. He rediscovered George Hadley’s theory of global atmospheric circulation (now known as the Hadley cell) around this time. In 1793 Dalton’s first publication, Meteorological Observations and Essays, contained the seeds of several of his later discoveries but despite the originality of his treatment, little attention was paid to them by other scholars. His Elements of English Grammar, was published in 1801.

After leaving the Lake District, Dalton returned annually to spend his holidays studying meteorology and climbing mountains to measure their height. He took measurements of temperature and humidity at various altitudes which he estimated using a barometer. Until the Ordnance Survey published maps for the Lake District in the 1860s, Dalton was one of the few sources of information on altitudes in the region.

In 1794, shortly after his arrival in Manchester, Dalton was elected a member of the Manchester Literary and Philosophical Society, the “Lit & Phil”, and a few weeks later he communicated his first paper on “Extraordinary facts relating to the vision of colours”, in which he postulated that inability in color perception was caused by discoloration of the liquid medium of the eyeball. Both he and his brother were color blind, and so postulated (correctly) that the condition must be hereditary. He was able to recognize only blue, purple, and yellow. At the time (and still to an extent today), being colorblind was a severe handicap to being an analytic chemist.

The most important of all Dalton’s investigations concern atomic theory in chemistry. How he came up with the theory is not fully understood. The theory may have been suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote). These investigations may have led him to the idea that chemical combination (the production of definable compounds) consists in the interaction of atoms of definite and characteristic weight. Or the idea of atoms may have arisen in his mind as a purely physical concept, forced on him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper “On the Absorption of Gases by Water and other Liquids” where he says:

Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases. This is the germ of the idea that elements are composed of atoms and that the atoms of different elements have different weights.

The main points of Dalton’s atomic theory are:

Elements are made of extremely small particles called atoms.

 Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

 Atoms cannot be subdivided, created, or destroyed.

 Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton published his table of relative atomic weights containing six elements, hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. He provided no indication in this paper how he had arrived at these numbers but in his laboratory notebook, dated 6 September 1803, is a list in which he set out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

Compounds were listed as binary, ternary, quaternary, etc. (molecules composed of two, three, four, etc. atoms) in the New System of Chemical Philosophy depending on the number of atoms a compound had in its simplest, empirical form. Dalton hypothesized the structure of compounds can be represented in whole number ratios. So, one atom of element X combining with one atom of element Y is a binary compound; one atom of element X combining with two elements of Y is a ternary compound, and so on. Many of the first compounds listed in the New System of Chemical Philosophy correspond to modern views, although many others do not.

Dalton used his own symbols to visually represent the atomic structure of compounds (see above). They were depicted in the New System of Chemical Philosophy, where he listed 20 elements and 17 simple molecules.

He always objected to the chemical notation devised by Jöns Jakob Berzelius (the one using letters for elements that we use today), although most thought that it was much simpler and more convenient than his own cumbersome system of circular symbols.

Dalton never married and had only a few close friends. As a Quaker, he lived a modest and unassuming personal life. For the 26 years prior to his death, Dalton lived in a room in the home of the Rev W. Johns, a published botanist, and his wife, in George Street, Manchester. Dalton and Johns died in the same year (1844).

Dalton’s daily round of laboratory work and tutoring in Manchester was broken only by annual excursions to the Lake District and occasional visits to London. In 1822 he paid a short visit to Paris, and attended several of the earlier meetings of the British Association at York, Oxford, Dublin and Bristol.

Dalton suffered a minor stroke in 1837, and a second in 1838 left him with a speech impairment, although he remained able to perform experiments. In May 1844 he had another stroke. On 26th July 1844 he recorded with trembling hand his last meteorological observation. On 27th July 1844, in Manchester, Dalton fell from his bed and was found lifeless by his attendant. He was accorded a civic funeral with full honors. His body lay in state in Manchester Town Hall for four days and more than 40,000 people filed past his coffin. The funeral procession included representatives of the city’s major civic, commercial, and scientific bodies. He was buried in Manchester in Ardwick cemetery which was later converted to a playing field, and all the graves moved.

Dalton was a native of Cumbria but he spent all of his working life as a scientist in Manchester so a Manchester recipe is suitable to celebrate his birthday.  Manchester tart is a perennial favorite that used to be a mainstay of school lunches. It’s a rich mixture of raspberries, raspberry jam, and egg custard baked in a tart shell. Some people add sliced bananas as well.

Manchester Tart

Ingredients

butter, for greasing
500g shortcrust pastry
plain flour, for dusting
200g raspberry jam
3 tbsp plain desiccated coconut
3 tbsp desiccated coconut, toasted in a dry frying pan until golden-brown
300g fresh raspberries
500ml milk
1 vanilla pod, split, seeds scraped out with a knife
5 egg yolks
125g caster sugar
1 tbsp cornflour
2 tbsp icing sugar, for dusting
400ml double cream, whipped until soft peaks form when the whisk is removed

Instructions

Preheat the oven to 200˚C.

Grease a 24 cm tart tin with butter. Roll out the shortcrust pastry on to a lightly floured work surface to a 0.5cm thickness. Line the prepared tart tin with the pastry. Prick the pastry several times with a fork, then chill in the refrigerator for 30 minutes.

Place a sheet of baking parchment into the chilled pastry case and half-fill with dried beans. Transfer the pastry case to the oven and bake for 15 minutes, or until pale golden-brown. Remove the beans and baking parchment and return the pastry case to the oven for a further 4-5 minutes, or until pale golden-brown.

Spread the raspberry jam over the pastry base in an even layer. Sprinkle over the three tablespoons of non-toasted desiccated coconut and half of the fresh raspberries. Set the pastry base aside.

Bring the milk, vanilla pod and vanilla seeds to the boil in a pan, then reduce the heat to a simmer and simmer for 1-2 minutes. Remove the vanilla pod.

In a bowl, beat together the egg yolks and sugar until well combined. Pour the hot milk and vanilla mixture over the egg and sugar mixture, whisking continuously, until the mixture is smooth and well combined. Return the mixture to the pan over a medium heat. Whisk in the cornflour, gradually until well combined, then heat, stirring continuously until the mixture is thick enough to coat the back of a spoon. Transfer the custard mixture to a clean bowl and dust with the icing sugar to prevent a skin forming on the surface of the custard. Set aside to cool, then chill in the refrigerator for 30 minutes.

Fold the whipped double cream into the chilled custard mixture until well combined. Spoon the custard and cream mixture into the pastry case in an even layer. Sprinkle over the remaining fresh raspberries.

To serve, sprinkle over the three tablespoons of toasted desiccated coconut. Serve immediately.

Aug 302017
 

Today is the birthday (1871) of Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, PRS, a New Zealand-born British physicist who is one of the great pioneers of nuclear physics, yet, like so many other great experimental and theoretical physicists, his name is mostly unknown among the general public because it is not “Einstein.” In early work, Rutherford discovered the concept of radioactive half-life, proved that radioactivity involved the nuclear transmutation of one chemical element to another, and also discovered, differentiated and named alpha and beta radiation. This work was the basis for the Nobel Prize in Chemistry he was awarded in 1908. After the Nobel he performed his most famous work when he theorized that atoms have their charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model of the atom. He conducted research that led to the first “splitting” of the atom in 1917 in a nuclear reaction between nitrogen and alpha particles, in which he also discovered (and named) the proton. Under his leadership the neutron was discovered by James Chadwick in 1932 and in the same year the first experiment to split the nucleus in a fully controlled manner was performed by students working under his direction, John Cockcroft and Ernest Walton.

Rutherford was the son of James Rutherford, a farmer, and his wife Martha Thompson, a school teacher originally from Hornchurch, Essex in England. James had emigrated to New Zealand from Perth, Scotland, “to raise a little flax and a lot of children.” Ernest was born at Brightwater, near Nelson, New Zealand. His first name was mistakenly spelled ‘Earnest’ when his birth was registered.

Rutherford studied at Havelock School and then Nelson College and won a scholarship to study at Canterbury College, University of New Zealand. After gaining his BA, MA and BSc, and doing two years of research during which he invented a new form of radio receiver, in 1895 Rutherford was awarded a research fellowship from the Royal Commission for the Exhibition of 1851, to travel to England for postgraduate study at the Cavendish Laboratory, University of Cambridge. He was among the first of the ‘aliens’ (those without a Cambridge degree) allowed to do research at the university, under the leadership of J. J. Thomson, and the newcomers aroused jealousies from the more conservative members of the Cavendish fraternity. With Thomson’s encouragement, he managed to detect radio waves at half a mile and briefly held the world record for the distance over which electromagnetic waves could be detected, though when he presented his results at the British Association meeting in 1896, he discovered he had been outdone by another lecturer: his name was Marconi.

In 1898 Thomson recommended Rutherford for a position at McGill University in Montreal. He was to replace Hugh Longbourne Callendar who held the chair of Macdonald Professor of physics and was coming to Cambridge. In 1907 Rutherford returned to Britain to take the chair of physics at the Victoria University of Manchester. During World War I, he worked on a top secret project to solve the practical problems of submarine detection by sonar. In 1919 he returned to the Cavendish succeeding J. J. Thomson as the Cavendish professor and Director.

When he first went to Cambridge as a research student, Rutherford started to work with J. J. Thomson on the conductive effects of X-rays on gases, work which led to the discovery of the electron which Thomson presented to the scientific world in 1897. Hearing of Becquerel’s experience with uranium, Rutherford started to explore its radioactivity, discovering two types that differed from X-rays in their penetrating power. Continuing his research in Canada, he coined the terms alpha ray and beta ray in 1899 to describe the two distinct types of radiation. He then discovered that thorium gave off a gas which produced an emanation which was itself radioactive and would coat other substances. He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its “half-life” (11½ minutes in this case).

From 1900 to 1903, he was joined at McGill by the young chemist Frederick Soddy (Nobel Prize in Chemistry, 1921) for whom he set the problem of identifying the thorium emanations. Once he had eliminated all the normal chemical reactions, Soddy suggested that it must be one of the inert gases, which they named thoron (later found to be an isotope of radon). They also found another type of thorium they called Thorium X, and kept on finding traces of helium. They also worked with samples of “Uranium X” from William Crookes and radium from Marie Curie.

In 1902, they produced a “Theory of Atomic Disintegration” to account for all their experiments. Until then atoms were assumed to be the indestructable basis of all matter and although Curie had suggested that radioactivity was an atomic phenomenon, the idea of the atoms of radioactive substances breaking up was a radically new idea. Rutherford and Soddy demonstrated that radioactivity involved the spontaneous disintegration of atoms into other types of atoms (one element spontaneously being changed to another). The Nobel Prize in Chemistry 1908 was awarded to Ernest Rutherford “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances”.

In 1903, Rutherford considered a type of radiation discovered (but not named) by French chemist Paul Villard in 1900, as an emission from radium, and realized that this observation must represent something different from his own alpha and beta rays, due to its very much greater penetrating power. Rutherford therefore gave this third type of radiation the name of gamma ray. All three of Rutherford’s terms are in standard use today – other types of radioactive decay have since been discovered, but Rutherford’s three types are among the most common.

In Manchester, he continued to work with alpha radiation. In conjunction with Hans Geiger (of radioactive counter fame) he developed zinc sulfide scintillation screens and ionization chambers to count alphas. By dividing the total charge they produced by the number counted, Rutherford determined that the charge on the alphas was 2 (suggesting it was helium nuclei). In late 1907, Ernest Rutherford and Thomas Royds allowed alphas to penetrate a very thin window into an evacuated tube. As they sparked the tube into discharge, the spectrum obtained from it changed, as the alphas accumulated in the tube. Eventually, the clear spectrum of helium gas appeared, proving that alphas were at least ionized helium atoms, and probably helium nuclei.

Rutherford performed his most famous work after receiving the Nobel prize in 1908. Along with Hans Geiger and Ernest Marsden in 1909, he carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil. Rutherford was inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high deflection angles, of a type not expected from any theory of matter at that time. Such deflections, though rare, were found, and proved to be a smooth but high-order function of the deflection angle. It was Rutherford’s interpretation of this data that led him to formulate the Rutherford model of the atom in 1911 – that a very small charged nucleus, containing much of the atom’s mass, was orbited by low-mass electrons.

Before leaving Manchester in 1919 to take over the Cavendish laboratory in Cambridge, Rutherford became, in 1919, the first person to deliberately transmute one element into another. In this experiment, he had discovered peculiar radiations when alphas were projected into air, and narrowed the effect down to the nitrogen, not the oxygen in the air. Using pure nitrogen, Rutherford used alpha radiation to convert nitrogen into oxygen through the nuclear reaction 14N + α → 17O + proton. The proton was not then known. In the products of this reaction Rutherford simply identified hydrogen nuclei, by their similarity to the particle radiation from earlier experiments in which he had bombarded hydrogen gas with alpha particles to knock hydrogen nuclei out of hydrogen atoms. This result showed Rutherford that hydrogen nuclei were a part of nitrogen nuclei (and by inference, probably other nuclei as well). Such a construction had been suspected for many years on the basis of atomic weights which were whole numbers of that of hydrogen. Hydrogen was known to be the lightest element, and its nuclei presumably the lightest nuclei. Now, because of all these considerations, Rutherford decided that a hydrogen nucleus was possibly a fundamental building block of all nuclei, and also possibly a new fundamental particle as well, since nothing was known from the nucleus that was lighter. Thus, Rutherford postulated the hydrogen nucleus to be a new particle in 1920, which he dubbed the proton.

In 1921, while working with Niels Bohr (who later postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, (which he had christened in his 1920 Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons. The only alternative to neutrons was the existence of “nuclear electrons” which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery.

Rutherford’s theory of neutrons was proved in 1932 by his associate James Chadwick, who recognized neutrons immediately when they were produced by other scientists and later himself, in bombarding beryllium with alpha particles. In 1935, Chadwick was awarded the Nobel Prize in Physics for this discovery.

There you have it.  The current model of the atom – much refined by later physicists – of a dense nucleus consisting of protons and neutrons surrounded by electrons (along with the nature of radioactive decay as a nuclear process) is owed to the work of Rutherford. Rutherford died too early to see Leó Szilárd’s idea of controlled nuclear chain reactions come into being. However, a speech of Rutherford’s about his artificially-induced transmutation in lithium, printed in 12 September 1933 in The Times, was reported by Szilárd to have been his inspiration for thinking of the possibility of a controlled energy-producing nuclear chain reaction. Szilard had this idea while walking in London, on the same day.

Rutherford’s speech touched on the 1932 work of his students John Cockcroft and Ernest Walton in “splitting” lithium into alpha particles by bombardment with protons from a particle accelerator they had constructed. Rutherford realized that the energy released from the split lithium atoms was enormous, but he also believed that the energy needed for the accelerator, and its essential inefficiency in splitting atoms in this fashion, made the project an impossibility as a practical source of energy (accelerator-induced fission of light elements remains too inefficient to be used in this way, even today). Rutherford’s speech in part, read:

We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms.

Well, his work on releasing energy from the nuclei of light elements was, as he predicted, not practical, but his theorizing on radioactive elements led in the right direction.

Given that Rutherford was born in New Zealand and always thought of himself as a Kiwi first, a New Zealand recipe is in order, but, like Australian cooking, there’s not a lot to choose from. Australia and New Zealand continue to battle it out over who invented the pavlova (meringue shell with fruit and cream) as their signature dish, but I’ve given a recipe already. If you like you can make one with kiwi fruit.

I’ll go for something more modern, but very popular throughout New Zealand nowadays: Southland cheese rolls. They originated in the Southland region of the South Island but are now more or less universal. They are basically cheese and onion rolled in toasted bread and lightly baked. This recipe gives just one version, but you can vary the types of bread and cheese any way you want.  I recommend a hearty whole wheat for the bread. It’s common to use a commercial spread on top after broiling but I prefer butter.

Southland Cheese Rolls

Ingredients

1 loaf bread cut in slices
200 gm Colby or Cheddar cheese (grated)
150 gm Parmesan cheese (grated)
1 6 oz can evaporated milk
1 cup heavy cream (optional)
1 packet onion soup
1 onion, peeled and finely chopped
2 tsp dry English mustard
butter (or nasty commercial spread) for topping (optional)

Instructions

Mix the cheeses, onion, soup powder, mustard, cream (if used), and evaporated milk together in a small saucepan. Stir over low heat until the cheeses are melted and you have a thick smooth mixture. Let cool slightly.

Spread the cheese mixture generously over the bread slices and roll them into logs.  Place them on a baking tray and broil them, turning until all sides are evenly toasted.

Serve hot with a knob of butter on top.

Oct 072015
 

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Today is the birthday (1885) of Niels Henrik David Bohr, a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter of scientific research. Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another. Although the Bohr model has been supplanted by other models, its underlying principles remain valid. He also conceived the principle of complementarity: that items could be separately analyzed in terms of contradictory properties, like behaving as a wave or a stream of particles. The notion of complementarity dominated Bohr’s thinking in both science and philosophy.

I don’t want to delve too deeply into Bohr’s physics because I know I will lose a big chunk of my audience before I get started. But I will make one point before I fly off in different directions. Long-time readers know that I have a bee in my bonnet about certain superlatives – the BEST painting/painter or composer or mathematician or whatever. There are lots and lots of smart and talented people throughout history. If this were not so, this would be a very limited blog. My main limitation is that their birthdays are not spread evenly through the year, coupled with my intrinsic favoritism. In the latter case I am allowed because it is MY blog. I make the rules. What gets me wound up is the popular idea that the yardstick of hyper-genius is Einstein. He had a phenomenal mind – no question. He was, however, far from being the ONLY genius of the 20th century, yet his is the name that automatically comes to mind. I’ve always countered this prejudice when it comes up by mentioning Niels Bohr.

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The first half of the 20th century was almost wallpapered with brilliant mathematicians and physicists, not to mention anthropologists, writers, philosophers, painters, and all the rest of it. Niels Bohr is one of them, but he is far from being a household name. Yet he helped usher in the age of quantum mechanics (with other brilliant minds), the dominant model of atomic physics to this day.

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Bohr was born in Copenhagen, the second of three children of Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Adler Bohr, who came from a wealthy Danish Jewish family prominent in banking and parliamentary circles. He had an elder sister, Jenny, and a younger brother Harald. Jenny became a teacher, while Harald became a mathematician and Olympic footballer who played for the Danish national team at the 1908 Summer Olympics in London. Niels was a passionate footballer as well, and the two brothers played several matches for the Copenhagen-based Akademisk Boldklub (Academic Football Club), with Niels as goalkeeper. My son and I love goalies.

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In 1910, Bohr met Margrethe Nørlund, the sister of the mathematician Niels Erik Nørlund. Bohr resigned his membership in the Church of Denmark on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in Slagelse on 1 August. Their honeymoon was delayed, however, because Bohr had an insight into the nature of orbiting electrons within the atom that he felt could not wait. For reasons that are not clear to me, he was unable to sit and write the paper himself, so he dictated it to Margrethe. Maybe this was his idea of marital bliss?

Planetary models of atoms were fairly recent but not new. Bohr’s treatment was. The old planetary model could not explain why the negatively charged electron did not simply collapse into the positively charged nucleus. He advanced the theory of electrons travelling in nested orbits of different energies around the atom’s nucleus, with the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a quantum of discrete energy. This became a basis for what is now known as the old quantum theory. In 1922 Bohr received the Nobel Prize in physics for his work.

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Bohr became convinced that light behaved like both waves and particles, and in 1927, experiments confirmed the de Broglie hypothesis that matter (like electrons) also behaved like waves. He conceived the philosophical principle of complementarity: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework. He felt that it was not fully understood by contemporary philosophers. Einstein never fully accepted quantum mechanics and complementarity. Einstein preferred the determinism of classical physics over the probabilistic new quantum physics to which he himself had contributed. Philosophical issues that arose from the novel aspects of quantum mechanics became widely celebrated subjects of discussion. Einstein and Bohr had good-natured arguments over such issues throughout their lives.

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Bohr’s model of the atomic nucleus helped him explain the nature of nuclear fission which he published in a paper in 1939, “The Mechanism of Nuclear Fission,” along with John Wheeler. Thus the age of nuclear energy and the atom-bomb was born.

Bohr was aware of the possibility of using uranium-235 to construct an atomic bomb, referring to it in lectures in Britain and Denmark shortly before and after the war started, but he did not believe that it was technically feasible to extract a sufficient quantity of uranium-235 (fissionable material). In September 1941, Werner Heisenberg, who had become head of the German nuclear energy project, visited Bohr in Copenhagen. During this meeting the two men took a private moment outside, the content of which has caused much speculation, as both gave differing accounts. According to Heisenberg, he began to address nuclear energy, morality and the war, to which Bohr seems to have reacted by terminating the conversation abruptly while not giving Heisenberg hints about his own opinions.

In 1957, Heisenberg wrote to Robert Jungk, who was then working on the book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists. Heisenberg explained that he had visited Copenhagen to communicate to Bohr the views of several German scientists, that production of a nuclear weapon was possible with great efforts, and this raised enormous responsibilities on the world’s scientists on both sides. When Bohr saw Jungk’s depiction in the Danish translation of the book, he drafted (but never sent) a letter to Heisenberg, stating that he never understood the purpose of Heisenberg’s visit, was shocked by Heisenberg’s opinion that Germany would win the war, and that atomic weapons could be decisive.

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In September 1943, word reached Bohr and his brother Harald that the Nazis considered their family to be Jewish, since their mother, Ellen Adler Bohr, had been a Jew, and that they were therefore in danger of being arrested. The Danish resistance helped Bohr and his wife escape by sea to Sweden on 29 September. The next day, Bohr persuaded King Gustaf V of Sweden to make public Sweden’s willingness to provide asylum to Jewish refugees. On 2 October 1943, Swedish radio broadcast that Sweden was ready to offer asylum, and the mass rescue of the Danish Jews by their countrymen followed swiftly thereafter. Some historians claim that Bohr’s actions led directly to the mass rescue, while others say that, though Bohr did all that he could for his countrymen, his actions were not a decisive influence on the wider events. Eventually, over 7,000 Danish Jews escaped to Sweden.

When the news of Bohr’s escape reached Britain, Lord Cherwell sent a telegram to Bohr asking him to come to Britain. Bohr arrived in Scotland on 6 October in a de Havilland Mosquito operated by the British Overseas Airways Corporation (BOAC). The Mosquitos were unarmed high-speed bomber aircraft that had been converted to carry small, valuable cargoes or important passengers. By flying at high speed and high altitude, they could cross German-occupied Norway, and yet avoid German fighters. Bohr, equipped with parachute, flying suit and oxygen mask, spent the three-hour flight lying on a mattress in the aircraft’s bomb bay. During the flight, Bohr did not wear his flying helmet as it was too small, and consequently did not hear the pilot’s intercom instruction to turn on his oxygen supply when the aircraft climbed to high altitude to overfly Norway. He passed out from oxygen starvation and only revived when the aircraft descended to lower altitude over the North Sea.

On 8 December 1943, Bohr arrived in Washington, D.C., where he met with the director of the Manhattan Project, Brigadier General Leslie R. Groves, Jr,and went to Los Alamos in New Mexico, where the nuclear weapons were being designed. Bohr did not remain at Los Alamos, but paid a series of extended visits over the course of the next two years. Robert Oppenheimer credited Bohr with acting “as a scientific father figure to the younger men”, most notably Richard Feynman. Bohr is quoted as saying, “They didn’t need my help in making the atom bomb.”

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Bohr recognized early that nuclear weapons would change international relations. In April 1944, he received a letter from Peter Kapitza, written some months before when Bohr was in Sweden, inviting him to come to the Soviet Union. The letter convinced Bohr that the Soviets were aware of the Anglo-American project, and would strive to catch up. He sent Kapitza a non-committal response, which he showed to the authorities in Britain before posting. Bohr met Churchill on 16 May 1944. Churchill disagreed with the idea of openness towards the Russians to the point that he wrote in a letter: “It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes.”

Oppenheimer suggested that Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Soviets in the hope of speeding up its results. Bohr’s friend, Supreme Court Justice Felix Frankfurter, informed President Roosevelt about Bohr’s opinions, and a meeting between them took place on 26 August 1944. Roosevelt suggested that Bohr return to the United Kingdom to try to win British approval. When Churchill and Roosevelt met at Hyde Park on 19 September 1944, they rejected the idea of informing the world about the project, and the aide-mémoire of their conversation contained a rider that “enquiries should be made regarding the activities of Professor Bohr and steps taken to ensure that he is responsible for no leakage of information, particularly to the Russians”.

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Bohr died of heart failure at his home in Carlsberg on 18 November 1962. He was cremated, and his ashes were buried in the family plot in the Assistens Cemetery in the Nørrebro section of Copenhagen, along with those of his parents, his brother Harald, and his son Christian. Years later, his wife’s ashes were also interred there.

I’ve mentioned traditional Danish cuisine several times before. It shares features with the other Sandinavian countries, and, like them as well as Britain, tends to be unfairly disdained by foreigners. Danish food does not involve a lot of herbs and spices, but it is noted for combinations of flavors and colorful presentation. Historically lunch was usually an open faced sandwich known as smørrebrød. Smørrebrød (originally smør og brød, meaning “butter and bread”) usually consists of a piece of buttered rye bread (rugbrød), a dense, dark brown bread. Pålæg (“put on”), the topping, which can be cold cuts, pieces of meat or fish, cheese or spreads. More elaborate, finely decorated, varieties have contributed to the international reputation of the smørrebrød. A slice or two of pålæg is placed on the buttered bread and decorated with the right accompaniments to create a tasty and visually appealing lunch or snack. Standards include:

Dyrlægens natmad (Veterinarian’s late night snack). On a piece of dark rye bread, a layer of liver pâté (leverpostej), topped with a slice of saltkød (salted beef) and a slice of sky (meat jelly). This is all decorated with raw onion rings and garden cress.

Røget ål med røræg Smoked eel on dark rye bread, topped with scrambled eggs, herbs and a slice of lemon.

Leverpostej Warm rough-chopped liverpaste served on dark rye bread, topped with bacon, and sauteed mushrooms. Additions can include lettuce and sliced pickled cucumber.

Roast beef, thinly sliced and served on dark rye bread, topped with a portion of remoulade, and decorated with a sprinkling of shredded horseradish and crispy fried onions.

Ribbensteg (roast pork), thinly sliced and served on dark rye bread, topped with red cabbage, and decorated with a slice of orange.

Rullepølse, (rolled stuffed pork) with a slice of meat jelly, onions, tomatoes and parsley.

Tartar, with salt and pepper, served on dark rye bread, topped with raw onion rings, grated horseradish and a raw egg yolk.

Røget laks. Slices of cold-smoked salmon on white bread, topped with shrimp and decorated with a slice of lemon and fresh dill.

Stjerneskud (lit. shooting star). On a base of buttered toast, two pieces of fish: a piece of steamed white fish on one half, a piece of fried, breaded plaice or rødspætte on the other half. On top is piled a mound of shrimp, which is then decorated with a dollop of mayonnaise, sliced cucumber, caviar or blackened lumpfish roe, and a lemon slice.

Here’s a small gallery to get you thinking:

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