The phrase "passing the acid test" gained popularity in the gold-rush years of the 1850s when miners used strong acids to determine whether the metal they had found was real gold or not. If it bubbled and frothed on contact with acid, it wasn't gold. But even these failures produced something interesting and beautiful.rnWhen pure metals cool, they solidify into intricately interlocked crystals. You can't see the crystals because they fit together perfectly to form what appears to be a uniform mass with a smooth, solid surface. But acid can reveal the structure inside.rnWhen you dip pure metals into strong acid (the muriatic acid available in hardware stores, for example), they dissolve slowly by releasing hydrogen gas (gold, being inert, doesn't give off hydrogen; thus, no frothing). Instead of breaking down the metal uniformly, the acid etches out microscopic ridges and valleys aligned to the crystal structure of the metal. This creates light and dark patches on the surface that change with the direction of the light. As you turn the sample, the patches sparkle on and off, an effect known as diffraction that you can also see on the backs of CDs.
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