Errr Natural gas (CH4) is lighter than air, so will rise, not fall. The molecular weight of air is 29, but the molecular weight of natural gas is 16. So it is lighter.
This is calculated like this. Air is about 80% nitrogen which has a molecular weight of 28 (N2=2X14=28) and 20% oxygen which has a molecular weight of 32 (O2=2X16=32). 0.8X28=22.4 and 0.2X32=6.4. 22.4+6.4=28.8 which rounds to 29.
Natural gas is methane, CH4, which has a molecular weight of 12+4X1=16
Joss
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