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Uncle Tungsten
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p223 Through Uncle Abe, I was drawn into the history of “cold” light -- luminescence -- which started perhaps before there was any language to record things, with observations of fireflies and glowworms and phosphorescent seas; of will-o’-the-wisps, those strange, wandering, faint globes of light that would, in legend, lure travelers to their doom. And of Saint Elmo’s fire, the eerie luminous discharges that could stream in stormy weather from a ship’s masts, giving its sailors a feeling of bewitchment. There were the auroras, the Northern and Southern Lights, with their curtains of color shimmering high in the sky. A sense of the uncanny, the mysterious, seemed to inhere in these phenomena of cold light -- as opposed to the comforting familiarity of fire and warm light.
There was even an element, phosphorus, which glowed spontaneously. Phosphorus attracted me strangely, dangerously, because of its luminosity -- I would sometimes slip down to my lab at night to experiment with it. ...I put a piece of white phosphorus in water and boiled it, dimming the lights so that I could see the steam coming out of the flask, glowing a soft greenish blue. Another, rather beautiful experiment was boiling phosphorus with caustic potash in a retort... and this produced phosphoretted hydrogen (the old term), or phosphine. As the bubbles of phosphine escaped, they took fire spontaneously, forming beautiful rings of white smoke.
p224 I could ignite phosphorus in a bell jar (using a magnifying glass), and the jar would fill with a “snow” of phosphorus pentoxide. If one did this over water, the pentoxide would hiss, like red-hot iron, as it hit the water and dissolved, making phosphoric acid. Or by heating white phosphorus, I could transform it into its allotrope, red phosphorus, the phosphorus of matchboxes. I had learned as a child that diamond and graphite were different forms, allotropes, of the same element. Now, in the lab, I could effect some of these changes for myself, turning white phosphorus into red phosphorus, and then (by condensing its vapor) back again. These transformations made me feel like a magician. (Footnote: Phosphorus, oxidizing slowly, was not the only element to glow when exposed to air. Sodium and potassium did this too, when they were freshly cut, but lost their luminosity in a few minutes as the cut surfaces tarnished. I found this by chance as I was working in my lab late one afternoon, as it gradually darkened into dusk...)
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p225 Hennig Brandt of Hamburg had been the first to obtain this marvelous element, in 1669. He distilled it from urine (apparently with some alchemical ambition in mind), and he adored the strange, luminous substance he had isolated, and called it cold fire (kaltes Feuer), or, in a more affectionate mood, mein Feuer.
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...though all the early researchers burned themselves severely with phosphorus, they also saw it as a magical substance...
p226 No one was more intrigued by this than Boyle, who made detailed observations of its luminescence -- how it, too required the presence of air... Boyle had already made extensive investigations of “luciferous” phenomena, from glowworms to luminous wood and tainted meat, and had made careful comparisons of such “cold” light with that of glowing coals (both, he found, needed air to sustain them).
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...Chemical luminosity could indeed be dazzlingly brilliant; the only problem was that it was ephemeral, transient, by nature, disappearing as soon as the reactants were consumed -- unless there could be (as with fireflies) a continued production of the luciferous chemicals. If chemistry was not the answer, then one needed some other form of energy, something that could be transformed into visible light.
Abe’s interest in luminescence had been stimulated, when he was growing up, by a luminous paint used in their old house in Leman Street -- Balmain’s Luminous Paint, it was called -- for painting keyholes, gas and electric fixtures, anything that had to be located in the dark. Abe found these glowing keyholes and switches wonderful, the way they glowed softly for hours after being exposed to light. This form of phosphorescence had been discovered in the seventeenth century, by a shoemaker in Bologna who had gathered some pebbles, roasted them with charcoal, and then observed that they glowed in the dark for hours after they had been exposed to daylight. This “phosphorus of Bologna,” as it was called, was barium sulfide, produced by the reduction of the mineral barytes. Calcium sulfide was easier to procure -- it could be made by heating oyster shells with sulfur -- and this, “doped” with various metals, was the basis of Balmain’s Luminous Paint...
p227 While some substances emitted light slowly in the dark after being exposed to daylight, others glowed only while they were being illuminated. This was fluorescence (after the mineral fluorite, which often showed it). This strange luminosity had been originally discovered as early as the sixteenth century... ...whether a substance was fluorescent or phosphorescent (many were both), it required blue or violet light or daylight... to elicit the luminescence... The most effective illumination, indeed, was invisible -- the ultraviolet light that lay beyond the violet end of the spectrum.
My own first experiences of fluorescence occurred with the ultraviolet lamp my father kept in the surgery -- an old mercury vapor lamp with a metal reflector, which emitted a dim bluish violet light and an invisible blaze of ultraviolet. It was used to diagnose some skin diseases (certain fungi fluoresced in its light) and to treat others -- though my brothers also used it for tanning.
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p228 Uncle Abe’s house, a short walk from ours, was a magical place, filled with all sorts of apparatus: Geissler tubes, electromagnets, electric machines and motors, batteries, dynamos, coils of wire, X-ray tubes, Geiger counters and phosphorescent screens, and a variety of telescopes, many of which he had built with his own hands...
Abe had racks and racks of phosphors in his attic, which he would blend like an artist with his palette -- the deep blue of calcium tungstate, the paler blue of magnesium tungstate, the red of yttrium compounds. Like phosphorescence, fluorescence could often be induced by ‘doping,” adding activators of various sorts, and this was one of Abe's chief research interests, for fluorescent lights were just beginning to come into their own, and subtle phosphors were needed to produce a visible light that was soft and warm and agreeable to the eye. (Footnote: Equally important were cathode-ray tubes, which were now being developed for television. [He never uses the abreviation, CRT which most people know them by.] Abe himself had one of the original television sets of the 1930s, a huge, bulky thing with a tiny circular screen. Its tube, he said, was not much different from the cathode-ray tubes that Crookes had developed in the 1870s, except that its face was coated with a suitable phosphor. Cathode-ray tubes in use for medical or electronic apparatus were often coated with zinc silicate, willemite, which emitted a brilliant green light when bombarded, but for television one needed phosphors that would give a clear, white light -- and if color television was to be developed, one would need three separate phosphors with exactly the right balance of color emissions, like the three pigments in color photography. The old dopants used in luminous paints were quite unsuitable for this; much more delicate and precise colors were needed.) Abe was especially attracted to the very pure and delicate colors which could be made if one added various rare earths as activators -- europa, erbia, terbia. Their presence in certain minerals, he told me, even in minute quantities, lent these minerals their special fluorescence.
p229 But there were also substances that would fluoresce even when absolutely pure, and here uranium salts (or, properly speaking, uranyl salts) were preeminent. Even if one dissolved uranyl salts in water, the solutions would be fluorescent -- one part in a million was sufficient. The fluorescence could also be transferred to glass, and uranium glass or “canary glass” had been very popular in Victorian and Edwardian houses (it was this which so fascinated me in the stained glass in our front door). Canary glass transmitted yellow light and was usually yellow to look through, but fluoresced a brilliant emerald green under the impact of the shorter wavelengths in daylight, so it would often appear to shimmer, shifting between green and yellow depending on the angle of illumination. And though the stained glass in our front door had been shattered by a bomb blast during the Blitz... its colors, intensified by nostalgia perhaps, still remained preternaturally vivid in my memory -- especially now that uncle Abe had explained its secret to me.
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p231 Using the powerful discharges from the induction coils invented in the 1850s, it was found that a long column of mercury vapor could be set glowing (Alexandre-Edmond Becquerel suggested, early on, that coating the discharge tube with a fluorescent substance might make it more suitable for illumination). But when mercury-vapor lamps were introduced, for special purposes, in 1901, they were dangerous and unreliable, and their light -- in the absence of a fluorescent coating -- was too blue to allow domestic use. Attempts to coat such tubes with fluorescent powders before the First World War collapsed before a multitude of problems. Other gases and vapors, meanwhile, were being tried: carbon dioxide gave a white light, argon a bluish light, helium a yellow light, and neon, of course, a crimson light. Neon tubes, for advertising became common in London by the 1920s, but it was only in the late 1930s that fluorescent tubes (using a mixture of mercury vapor with an inert gas) started to become a commercial possibility, a development in which Abe played a considerable part.
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...I understood a certain amount about light: that colors were how we saw different frequencies or wavelengths; and that the color of objects came from the way they absorbed or transmitted light, obstructing some frequencies, letting other through. I understood that black substances absorbed all the light, letting nothing through; and that with metals and mirrors it was the opposite -- the wave front of light particles, as I imagined it, hit the mirror like a rubber ball and was reflected in a sort of instant bounce.
p132 But none of these notions was helpful when one came to the phenomena of fluorescence and phosphorescence, for here one could shine an invisible light, a “black” light, on something and it would glow white or red or green or yellow, emitting a light of its own, a frequency of light not present in the illuminant.
And then there was the question of delay. The action of light normally seemed instantaneous. But with phosphorescence, the energy of sunlight, seemingly, was captured, stored, transformed into energy of a different frequency, and then emitted in a slow dribble, over hours (there were similar delays, Uncle Abe told me, with fluorescence, though these were far shorter, just a fraction of a second). How was this possible?
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