How the Atmos clock functions

I wonder why Chloroethane / ethyl chloride was chosen. I don't think I've ever seen this for anything, other than a synthesis precursor to useful stuff.

I got my sister a Atmos VIII as a present a few years back, and it took us a few attempts to get it running at the proper speed via the adjustment lever. Now it gloriously stands there, never needing to be touched to keep time.