Poisonous gases were methods for mass killings since ancient times. In times of world wars, civilians were trained for detecting poisonous gasses in air and most distinct observable feature of these gasses were of course their smells.
This card showing properties and observable characteristic odors of various kinds of gasses was a part of a series of precautions. As seen in the example (second window on top), smelling geraniums meant presence of Lewisite gas in the environment.
