mercury
汞 centers on the liquid metal mercury, often associated with its fluid, water-like appearance and its use in traditional contexts.
汞 combines 工 (work/craft) and 水 (water), likely reflecting the liquid, metallic nature of mercury and its use in alchemy or crafts. The exact historical development is uncertain.
Picture a craftsman (工) pouring liquid metal (水) that flows like water: that's mercury.
For コウ, imagine a craftsman working with mercury and saying 'Coat!' as he coats a surface: 'Coat' -> コウ.