Air, nitrogen, oxygen, hydrogen, carbonic-acid gas, and carbon monoxide are gases that not even the sharpest eyes can see; and most other gases are of like character in this respect, so that gases as a class are thought of by us as invisible. Now, however, we have a gas that is as subtle and impalpable as the others and yet can be seen very well.

The Wonder Book of Chemistry - Chlorine, A. & C. Boni. 1930