Related Clues:

• Scientific discovery of 1894
• Vacuum tube filler
• Radio tube gas
• Number 18
• About 1% of the atmosphere
• Light bulb gas
• One of the noble gases
• Noble gas
• Incandescent lamp gas
• An inert gas
• Inert gas
• Fluorescent lamp filler
• Light gas
• Fluorescent tube gas
• About 1% of the Earth's atmosphere