Related Clues:

• Hydrogen and helium
• Nebula make-up
• Eg, O, H, Ne, CO, etc.
• Eg, helium, nitrogen
• Neon, oxygen, etc
• Eg, oxygen, helium
• Such as radon and neon
• Compressible fluids
• Noble __
• Some inert elements
• Vapours
• Waffles producing much flatulence
• They may be noble
• Volcanic emissions
• Pneumatics is the study of them