The history of scientific discovery is littered with mistakes, coincidences and serendipity. In chemistry, the formulation of a periodic table by Dmitri Mendeleev, which he presented to the Russian Chemical Society in 1869, enabled the search for new elements to gather pace by predicting their presence from supposed blank spaces in the table. Yet there are still instances of elements having been discovered fortuitously and unexpectedly. The story of one such element, indium, discovered by two German chemists shortly before the publication of Mendeleev’s table has a neat optical resonance with the researches of an English chemist over 50 years previously.
The English chemist is John Dalton (1766-1844), chiefly known in optics for his description of his own colour deficiency, the first person to describe the condition. He was not the first person to investigate colour deficiency but, after his account of his condition was given to the Manchester Literary and Philosophical Society (MLPS) in 1794, in a paper entitled Extraordinary facts relating to the vision of colours with observations, the term ‘Daltonism’ for colour deficiency stuck.
His theories about the cause of the condition were only half right; on discovering that his brother perceived colours the same way as himself he concluded that the colour deficiency they shared was hereditary, and he proposed that the condition was due to a blue colouration of the humour of the eye. He was correct in his first supposition, but wrong in the second. Ironically his own eyes betrayed him: at his request his eyes were removed after death and dissection of one of them disproved his ‘colouration’ theory.
For a man born into a humble Cumberland Quaker family and with only a grammar school education, it is something indeed to have been called the ‘father’ of two different branches of science. He did, though, have two important mentors who set him on his path of scientific enquiry. John Gough and Peter Crosthwaite, both amateur meteorologists, fired a lifelong interest in meteorology in their protégé that subsequently led him on to other speculations and discoveries in chemistry.
In 1793 he co-authored with these friends a book of essays on meteorological observations. While studying various atmospheric phenomena Dalton came to the then radical conclusion that the air comprised around 20% oxygen and 80% nitrogen rather than a being a specific compound in itself. Further studies of the properties of air led him to formulate what has become known as Dalton’s Law of Partial Pressures. This states that the total pressure of a gas mixture is equal to the sum of the pressures of each non-reacting constituent gas in the mixture. Dalton was, according to his contemporary chemist and meteorologist, John Frederic Daniell, the ‘father of meteorology’.
Dalton’s studies of gases led him onto perhaps his greatest achievement, his atomic theory, overturning the centuries-old idea, dating back to the Greek philosopher Democritus, that all atoms are alike. He postulated that each element comprised atoms of unique size and mass and assumed that elements combined as compounds in fixed proportions. He told the MLPS in October 1803 that, ‘An inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new; I have lately been prosecuting this inquiry with remarkable success.’ These inquiries enabled Dalton to formulate his Law of Multiple Proportions that asserted that when two elements combined to form more than one compound, they did so in fixed ratios of small whole numbers. The finer details of his theory were not perfect but, together with other advances such as Avogadro’s method of determining molecular masses, Dalton’s atomic theory enabled rapid progress in (especially, organic) chemistry. Thus was Dalton’s other popular title of ‘father of chemistry’ bestowed upon him.
The import and utility of Dalton’s atomic theory led to wide recognition and honours, including election to fellowship of the Royal Society, an honorary degree from Oxford University and the presidency of the MLPS. Modest and frugal as became his Quaker upbringing, he stayed in Manchester to teach students privately until his death. For the next link in the chain leading to the discovery of indium we must move forward 16 years and to Germany.
Up until 1860 the elements that had been discovered since antiquity had been found by techniques involving the use of chemical reactions or electrolysis. It was suspected that by the employment of novel methods of investigation other, trace, elements might be tracked down. Robert Bunsen (pictured) – he of the Bunsen burner – and Gustav Kirchhoff came up with the idea of looking at the light from flames produced by burning samples of compounds.
Bunsen was an intrepid explorer and experimenter, sometimes dangerously so. He once daringly managed to make temperature measurements of the water in Iceland’s Great Geyser just before it erupted. He was less fortunate when experimenting with a dangerous arsenic compound, cacodyl cyanide, causing an explosion in which he lost his right eye. Bunsen and Kirchhoff became friends when both teaching at the University of Breslau and, when Bunsen was offered a position at the University of Heidelberg, he engineered a move for Kirchhoff too.
Kirchhoff suggested that analysis of their emission spectra through a prism could enable similarly coloured flames to be differentiated. This was the basis of spectroscopy, which reveals the signature bright lines present in the emission spectra of different elements. Within a year of their invention of the spectroscope in 1860, Bunsen and Kirchhoff had discovered two new alkali metal elements, caesium and rubidium. They chose these names for the colours of the spectral lines that led to their identification. Caesium was isolated from a sample of mineral water from which known salts were removed. Spraying the remaining liquid through a flame revealed two blue emission lines previously unseen; caesius is Latin for sky blue. As for rubidium, the collaborators were investigating a lithium-containing mineral called lepidolite, supposing that it might harbour another alkali metal. Sure enough, spectroscopy revealed two novel ruby lines in the emission spectrum. They named the new element from the Latin, rubidius, ‘deepest red’. Spectroscopy had really opened the way for more elemental discoveries; and it is interesting to note that Mendeleev was one of those scientists who flocked to Heidelberg to study under or work with Bunsen.
Another German chemist, Ferdinand Reich (1799-1882, pictured), was also making use of this new technology, but he had a problem. In 1863 his assistant, Hieronymus Theodor Richter (1824-1898), and he were studying a sulphide ore of zinc called black sphalerite that was local to their laboratory in Freiberg. They suspected that it might contain thallium (Greek, thallos, ‘green twig’) another element recently discovered via spectroscopy by the British chemist, Sir William Crookes. Reich’s difficulty was that he was colour deficient, so he asked Richter to examine the spectrum of their sample for the hoped-for characteristic green line of thallium. To their great surprise, what Richter actually saw was an intense indigo blue line, never previously observed, that was, in their words, ‘of such bright glow, sharpness and consistency that we concluded it to be a so far unknown metal that we would call indium [from the Latin, indicum, ‘violet’ or ‘indigo’].’
Richter (pictured) and Reich’s discovery is today commemorated by a two-metre diameter bronze disc set in the ground of Freiberg’s palace square at the entrance to the city’s castle. Although they managed to isolate a small amount of indium, and a half-kilogram ingot of the element was displayed at the World’s Fair in Paris in 1867, there was not much use for it until the Second World War, when it was used for coating bearings in high-performance aircraft. Sitting as it does between gallium and thallium in its periodic table group, it is unsurprising that it has properties that has made it an important part of the semiconductor industry. But the greatest demand for indium now is in the form of its tin oxide as a transparent conductive coating for touch screens and LCD televisions; so it is quite possible that you are looking through or interacting with indium if you are reading this article on your tablet. John Dalton would no doubt be gratified to know that such an important element, with its unique atoms that create its own spectrum, was co-discovered by a chemist with Daltonism.
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