An interesting, slightly off topic aside here -- Daguerreotype photography (and 1840-ish process that preceded the wet plates used during the Civil War) has seen a resurgence since about 1990, and the canonical method of developing an exposed Daguerreotype plate is to "fume" it over heated mercury metal (there's an alternate method involving very long exposure to red light, but it gives much lower sensitivity and takes, literally, a week to develop a plate, where fuming takes a few minutes). Modern Daguerreotypists, of course, would prefer to avoid exposure to the resulting mercury vapor and for environmental reasons would also like to avoid venting it to the atmosphere; they found that they can capture the vapor from the fuming dish well enough that their exhaust contains no detectable vapor, by running the air from the fuming box through a cold trap where dry ice or liquid nitrogen condenses 99.999% of the vapor (which is already a very tiny amount). The primary consideration, other than ensuring there's enough cold material available in the cold trap to keep the temperature well below -40º, is to ensure that there's a forced air flow out of the fuming box into the cold trap (an exhaust fan behind the trap is the standard method of doing this). A bonus for these primitive photographers is that they cut their mercury consumption (i.e. the amount of make-up metal they have to buy) to virtually nothing; each plate consumes only a few milligrams, the rest is recovered from the trap.