This sketch is linked from the archive page production order columns but not chronological order columns because I remove a cartoon from the chronological order columns when it is not part of The Hero of Three Faces continuity.

When The Zygon Invasion/The Zygon Inversion (2015) debuted, many viewers objected to the idea of gentle medical man Harry Sullivan inventing a gas that would kill Zygons. This sketch was drawn to suggest Harry would, but would only, create the gas if the effects were reversible, something supported by nothing in the text. It's been removed from Three Faces continuity because the climax of The Zygon Inversion reveals that the device purported to contain the gas is empty; there's an implication, therefore, that the gas doesn't actually exist (though admittedly in the two-parter there are several instances of Zygons being dispatched with some weapon with a never-before-seen effect, being reduced to a tumbleweed as portrayed here). Forthcoming there'll be a cartoon which clarifies all this. EDIT Here it is.