This question is related to Deadly, Heavier than Air Gas, and Is a world with two different types of air possible? but has several different criteria. I've also looked at https://chemistry.stackexchange.com and not found anything
I'm building an alternate Earth in which, some time in the 18th or 19th century, the world was covered to a depth of ~100m by a certain gas.
I need a gas with the following properties:
- Denser than air (necessary)
- Will remain separate from the air above it (necessary, with a few metres of interface)
- Gaseous down to at least 0°, preferably -50° or less (semi-necessary,would need to be gaseous at the very least in the daytime)
- Not flammable (covering the planet in a flammable gas sounds like it would bring a whole new meaning to the phraseGreat Balls of Fire (optional, would settle for only flammable with difficulty - I don't want to watch the world burn)
- Relatively safe (would only kill you by asphyxiation, unlike say Chlorine) (optional)
- Contains Hydrogen or Helium atoms, which can extracted with c19th technology (alternatively, something else which can be extracted and burned as fuel) (optional)
- Opaque or visible (optional)
Unfortunately, it's been a while since I last did any Chemistry and I can't think of any gases that quite fit the bill. The noble gases are out because they don't help with the fuel element, while gases like Benzene fail the 'not flammable' test.
Is there a real gas with the above properties, or will I have to go with Handwavium?