The ethna bromide induces asphyxia, but in order to delay the erodent action of the exnide, C2HC was added.
12 seconds sound clip from the Monsieur Verdoux movie soundboard.
You can hear this line at 00:54:57 in the Blu-ray version of the movie.
Quote context
[...]
- Do you remember some time ago discussing a humane method of doing away with dumb animals?
- A lethal formula for veterinary work? No pain. No convulsions. Absolutely tasteless.
- The creature drinks it and in about an hour curls up into a comfortable sleep and dies.
- Yes. That was very interesting, pharmaceutically.
- The results are interesting.
- What have you done about it?
- Nothing.
- The State Medical Department banned it.
- Why?
- Too dangerous for public use.
[...]