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ANY sufficiently advanced technology, as Arthur C. Clarke once observed, is indistinguishable from magic. And one that seems routinely to be ascribed magical properties is graphene. It has been proposed for the manufacture of transistors and light bulbs, as a replacement for bone and a way of delivering drugs, for storing power and for transmitting it, and for lubricating things and waterproofing them. Its latest suggested role, though, is to help turn heat directly into electricity. The Seebeck...