 Complete the following phrase:  The Germans had extensive experience with hydrogen as a lifting gas. Accidental hydrogen fires had never occurred on civilian Zeppelins, so the switch from helium to hydrogen did not cause much concern. Hydrogen also increased lift by about 8. After the Hindenburg disaster Eckener vowed to never use hydrogen again in a passenger airship. He planned to use helium for the second ship and went to Washington, D.C. to personally lobby President Roosevelt, who promised to supply the helium only for peaceful purposes. After the 