Related Clues:

• 21-Across, for one
• Atmosphere makeup
• Station ration
• Nitrogen, for instance
• Balloon fill
• Swell time
• Stove fuel
• Rip-roaring time
• Blowtorch fuel
• Bang-up time
• Station purchase
• Highway sign
• See 78-Across
• Contents of some tanks
• It may be laughing