From "Lost in Space: Attack of the Monster Plants (#1.14)"
Prof. John Robinson
He tricked you, Will. He gave you this fake and kept the real deutronium.
Maj. Don West
Because he knows we can't go back to Earth unless we have enough deutronium, and he's trying to blackmail us into taking him, but what he doesn't know, Will, is that while you were gone we found our last little pocket in that vein and already have enough deutronium without his canister.