More than 120 years ago, German chemist Emil Fischer said his lab had developed a crystal that seemed to defy the laws of physics. The solid form of acetaldehyde phenylhydrazone kept melting at two different temperatures. One batch might melt at 65° C; another at 100° C. Physics Today explains how the mystery was eventually solved:
"Colleagues and rivals at the time told him he must have made a mistake. Fischer didn’t think so. As far as he could tell, the crystals that melted at such different points were identical. A few groups in Britain and France repeated his work and got the same baffling results. But as those scientists died off, the mystery was forgotten, stranded in obscure academic journals published in German and French more than a century ago.
"There it would probably have remained but for Terry Threlfall, an 84-year-old chemist at the University of Southampton, UK. Stumbling across Fischer’s 1896 paper in a library about a decade ago, Threlfall was intrigued enough to kick-start an international investigation of the mysterious crystal. Earlier this year in the journal Crystal Growth and Design, Threlfall and his colleagues published the solution: APH is the first recorded example of a solid that, when it melts, forms two structurally distinct liquids. Which liquid emerges comes down to contamination so subtle that it’s virtually undetectable."