Possible Clues:

• The first two periodic table entries
• Compressible fluids
• Argon and xenon, for two
• Neon and xenon, for two
• Eleven of the elements, under normal conditions
• Xenon and radon, for two
• Such as radon and neon
• Aerostatics subject
• Krypton and xenon
• Symbolically N and H, for instance