National Geographic : 1968 Aug
and sugar, for example. Diamonds and rubies. Ice and the delicate, fanciful snowflake. The rocks and minerals of the earth's crust, and most metals and ceramics-all these show crystalline structure inside when broken or etched. In fact, glass and plastics are about the only nonorganic solid substances of everyday life that normally are noncrystalline. What is it that sets crystals apart? Look at a few grains of table salt under a simple mag nifying glass, and you will begin to see the answer. Each unbroken grain appears as a cube, with sharp edges and corners and flat faces (opposite and next page). Other kinds of single crystals reveal more complex shapes, but all naturally possess the same flat planes. Home-grown Crystal Resembles a Gem This orderliness on the outside reveals a significant orderliness inside as well. The se cret of every crystal is that its atoms respond to their inner electrostatic forces by arranging themselves in a highly regular order to achieve stability. Except where a flaw interrupts it, this geometric pattern, called a lattice, repeats itself endlessly as the crystal grows. With a bit of patience, you can actually observe the growing process. While I was working on this article, my secretary, Miss Heather Burridge, experimented with crystals of ordinary potash alum. She dissolved the chemical in warm water, let it cool to form seed crystals, and then suspended one tiny seed in the saturated solution. Two days later the seed crystal had obvi ously grown. It showed the unmistakable octahedral shape of an alum crystal-two pyramids with their bases together. The weeks went by, the solution slowly evaporated, and the crystal became larger and larger. As I write, it glistens on my desk -more than half an inch across, and as beau tifully faceted as a diamond (above).* As a single alum crystal grows, its double pyramid structure stays the same. It obeys the laws of the crystal world in repeating its atomic patterns over and over. It is because of this regularity that carefully controlled amounts of impurities may be add ed to many crystals to give them seemingly magical properties that man can use to his advantage. And nowhere have crystals be come more important than in electronics. "We do not believe in miracles; we rely on them." I saw this sign on the wall at Texas Instru ments Incorporated, in Dallas, Texas, where KODACHROMEBYJAMESE. RUSSELL(C) N.G.S. How to grow a crystal: Heather Burridge, the author's secretary, made this single eight-sided crystal of alum by dissolving one part of alum by weight in five parts of warm water. This produced a heavily satu rated solution which, after cooling, yielded an initial seed of alum. Suspended in the remaining solution, the seed gradually grew into this gleaming octahedron. chips of silicon crystal by the millions are turned into integrated circuits, the newest and most glamorous product of the electronics in dustry. A miracle they are indeed; integrated circuits squeeze scores of electronic compo nents into a shimmering flake so thin it can slip through the eye of a needle or vanish in a puff of air (page 291). To appreciate the miracle, you need to re call a bit of electronics history. If you ever looked inside an old-style radio or television set, you know that much of the space was filled with bulky, fragile glass tubes used to control or amplify the flow of electricity. They required a great deal of electric power and gave off annoying amounts of heat. Fre quently they went bad and had to be replaced. *For those who would like to repeat the experiment, see Crystals and Crystal Growing, by Alan Holden and Phylis Singer, Anchor Books, Doubleday & Company, Inc., Garden City, New York, 1960, $1.45.