The ability to manufacture rudimentary chemical weapons existed during the period of the ACW. I'm curious how either side using gas weapons, both on the battlefield and as strategic weapons against cities would have affected the outcome of both the war and the way it was fought.
My first thought is that if the US as a whole entity develops them prior to the ACW and the confederates and union troops both have them ab initio, it is entirely possible that Lincoln would be unwilling to resort to force to hold together the union: The prospect of conflict would be bad enough, add in the risk of a chemical shelling of major cities might set the US back 100 years and the entire war might be avoided and left as a local "Cold War".
The second possible thought would be one side or the other developing them and using them, which in turn would result in some pretty horrific effects for the civilian and military casualties, making the reconstruction efforts pretty nasty.
The third possibility that I've considered is a latent chemical weapon program by the confederates that is used late in the war as a desperation move: Lob chemical weapons into a major US city via clandestine efforts and publically declare that unless the union pulls back to their prewar borders, the confederates will continue to use chemical weapons against the civilian populations.
Thoughts?