16. In 1848, gold began to disappear from a Ural gold mine. It was discovered in winter, shortly after jars of sodium cyanide had been brought in to kill rats. All attempts to find the thief were unsuccessful. The guards even carefully checked the trash taken out of the mine, but there was probably nothing suspicious there except for bottles of spent acid. A chance incident helped find the thief. On a slippery street in a prospectors' settlement, the manager's sleigh detained an employee of the "assay lab", and he dropped his suitcase. Something broke, smoke billowed out. ... The next morning, empty bottles with the inscription "spent acid" were found near the house of the owner of the suitcase, and an entire laboratory was discovered in the cellar: boxes of zinc dust, vials of hydrochloric acid, flasks, funnels, and even gold powder, packaged and prepared for shipment to the black market. A chemical expert has caught a thief. What is the expert's conclusion based on?

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