Do atoms exist? And what makes a crystal? Early in the nineteenth century, soon after John Dalton announced his atomic hypothesis, these long-running and contentious issues in physical science became dramatically linked. It is often forgotten that the study of certain types of crystal was one of the key verifications of the atomic hypothesis. The great French crystallographer Rene-Just Hauey (1743-1822), for whom the taxonomy of mineral species was a life's work, believed that crystal forms were determined by molecules integrantes, which he envisioned as tiny polyhedra. These were the smallest units into which a crystal could be broken down — the physical equivalent of the seventeenth-century view that a living embryo developed from a miniature 'homunculus' in the fertilized egg.
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