- It's kryptonite, a meteorite fragment of the planet from which Superman came. It evidently gives off a ray that renders him powerless.
- We've seen it work. It really knocks him out.
10 seconds sound clip from Superman (1948) - Into The Electric Furnace (Chapter 7).
You can hear this line at 00:04:10.901 in the Blu-ray version of the episode.
Quote context
[...]
- We did it because we need your scientific knowledge.
- You're safe here, and you won't lose by helping us.
- You'll share in our ultimate triumph.
- You still have Superman to contend with.
- You can forget about him, as I have. I have the one thing that renders him powerless.
- Very interesting if true. Do you mind telling me what it is?
- We've seen it work. It really knocks him out.
- No, but perhaps if he were exposed to it long enough...
- I'll study it. Perhaps some way may be found to use it against Superman, wherever he is.
- No, I haven't yet. But I hope to.
- In the meantime, why not trap Superman again and expose him to the kryptonite?
[...]