US Patent 4219945 is an invention by Marion Frank Rudy It relates to a "pressurized gas encapsulated in polyurethane" . If the gas leaks out of the capsule, it goes flat. Therefore, the X design uses a dense gas, so the gas stays inside the capsule. In US Patent 4219945, Mr. Rudy mentions a large number of possible gases, which he describes as "supergases." Generally, the "supergases" are halogenated hydrocarbons, with the exception of sulfur hexafluoride. The patent mentions ethane hexafluoride and sulfur hexafluoride. as being the best two choices. Since "denseness" is proportional to molecular weight, calculations with a periodic table would suggest that the gas is sulfur hexafluoride.
Gimme X .