A strip of paper dipped in this and waved to and fro will emit a thick white smoke if the acid vapor be in the atmosphere.

The ammonia neutralizes the acid fumes.

By reversing the experiment we can determine whether vapor of ammonia be in the air, and also deprive these suffocating and dangerous gases of their injurious properties, and remove them from the air.

Every Daguerreotype operator should be furnished with, at least, a six ounce bottle of aqua ammonia.

Its operation is very nearly the same on bromine and iodine vapor.
Hydrofluoric Acid (Fluorohydric Acid) .-- This acid is used to form some of the most volatile and sensitive compounds employed in the Daguerreotype.