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Why gallium? Not a common element, not one likely to be lying around the house, but one I was introduced to quite early on, by my Uncle Tungsten (as we used to call my Uncle Dave, who owned a tungsten lightbulb factory). Uncle possessed what was, to my eyes, a quite wonderful thermometer that he used for testing the temperature in the furnaces in his factory. This, he showed me, contained gallium; it was the only stuff such a thermometer could contain, for gallium had the widest temperature range of any metal in the liquid state: It would melt in the heat of the hand but not boil until well over 2,000 °C (higher than the melting point of platinum or of the quartz of which the thermometer itself was made).
Uncle gave me a lump of gallium to play with, and I can feel to this day the intense surprise I experienced when this lump, by no means soft, started to melt and trickle through my fingers as I held it. I later used a mold to make a teaspoon from it, and I would give this to unsuspecting guests and watch gleefully as they tried to stir their tea with it, only to find the seemingly solid spoon getting shorter and shorter and ending up as a glittering puddle at the bottom of the glass. And glitter it did--one had only to slosh liquid gallium round a hemispherical bowl to get an instant, brilliant, concave mirror.
Sacks
Photo By Peter Cutts
GALLIUM AT A GLANCE
Name: From the Latin Gallia, an old name for France.
Atomic mass: 69.72.
History: Discovered spectroscopically in 1875 by French chemist Paul-Emile Lecoq de Boisbaudran. In the same year, he obtained the free metal by electrolysis of a solution of the hydroxide in KOH.
Occurrence: Gallium minerals are rare, but up to 1% gallium occurs in the ores diaspore, sphalerite, germanite, and bauxite. It is also recovered as a by-product of burning coal.
Appearance: Silvery metal. Extremely soft and can be cut with a knife. The metal expands upon solidifying.
Behavior: One of the few metals (with mercury, cesium, and rubidium) that can be a liquid at room temperature. It has one of the longest liquid ranges of any metal and has a low vapor pressure even at high temperatures. Gallium salts generally have low toxicity.
Uses: Gallium arsenide is capable of converting electricity directly into coherent light and is a key component of LEDs (light-emitting diodes) and some integrated circuits. Gallium is also used in semiconductors and solid-state devices, microwave equipment, low-melting alloys, mirrors, and high-temperature thermometers. Radioactive gallium is used in medical imaging.
A strange optical illusion appears if one melts gallium in a cup: There seems to be a transparent liquid skimming and floating above a silver background. Is this due to the strongly concave meniscus of liquid gallium? I do not think I ever saw this, by contrast, with mercury, which has a strongly convex meniscus. And once melted, gallium may remain liquid, superfluid, for many hours, even if the room temperature is well below its melting point. The liquid may form a skin, wrinkled with fine lines, and when it finally solidifies, it may do so in shallow quadrangular prisms and zigzags like medieval fortifications.
I also had a little stick of indium, another element that intrigues me, partly because, like tin and zinc, it emits a "cry" or "squeal" when bent. (With my little bar, it was more like a crackling.) One day, just recently, I carelessly left the indium on top of some gallium I had in a bowl, and I was startled to find, within hours, that the bar seemed to have partly dissolved and that there was now a pool of liquid metal at the bottom of the bowl, despite its being a rather cold day. Clearly, the two elements, merely by being in contact, had fused together to form a eutectic alloy with a substantially lower melting point than that of pure gallium. I was reminded of how Berzelius had been sent samples of metallic sodium and potassium, which were put together in the same container for convenience, and when he opened the package, he found only a pool of liquid metal, the two elements having spontaneously alloyed at room temperature, just as my indium and gallium had.
When I came to learn about the periodic table and its history, I was intrigued to learn that gallium was the first element to be predicted by Mendeleev based on its place in Group III (he called it "eka-aluminum"), and how this prediction was vindicated, just six years later, helping to convince Mendeleev's critics of the fundamental truth of his periodic law.
Many decades later, I was fascinated to learn that there was a huge pool containing 200 tons of ultrapure liquid gallium deep beneath the Caucasus, an essential part of the Soviet solar neutrino detector. A passionate swimmer, I had fantasies of swimming, or rather floating, on this unique lake of metal. I was shocked when I read, a few years ago, that thieves had come by with siphoning equipment one night and almost managed to steal the whole lot. The great gallium heist was foiled only at the last moment.
Oliver Sacks is a neurologist practicing in New York City. He is the author of "Uncle Tungsten: Memories of a Chemical Boyhood," as well as "Awakenings" and "The Man Who Mistook His Wife for a Hat."
Chemical & Engineering News
Copyright © 2003 American Chemical Society
