The great London freeze of January had turned the River Thames into a magnificent highway of solid ice. Merchants had erected a bustling "Frost Fair" directly over the frozen tides, filling the air with the scents of roasting chestnuts, hot gin, and boiling tallow. Inside 221B Baker Street, the cold pressed hard against our windows, but Sherlock Holmes was utterly absorbed. He sat at his chemical table, using a pipette to drop a reagent onto a tray of ice crystals.