Re: Is the earth within a container, considering basic gas laws?

All around us we have gas at a reasonably high pressure. In outer space 
there is a vacuum. The gas laws say that gas under pressure ought to expand 
into a vacuum. So what is going on? Is the Earth in some sort of container 
that stops this expansion? Or is there continuing expansion going on, with 
the atmosphere being replenished somehow?

The Earth is not within a container, in any sense.

The gases in the Earth's atmosphere are not undergoing continuing expansion 
either.

The gas laws you are thinking of are ideal gas laws, and they leave quite a 
number of things out of consideration. They work very well for smallish 
samples of gases that are well above their condensation points. But they do 
not take into account many other effects, and gravity is one of these 
effects. 

The Earth's atmosphere is held to the Earth by gravitational forces. It is 
under pressure, because each bit of atmosphere is "squashed down" by the 
weight of the column of gas above it. An important "gas law" when you are 
thinking about the atmosphere is the barometer equation, which deals with 
this situation:

ln (p/p0) = Mgh/RT   

--where p is the pressure at altitude h above some reference altitude (e.g. 
sea level) where the pressure is p0; M is molar mass of the gas; g 
acceleration due to gravity; R gas constant; and T absolute temperature.

As you go higher, the air very rapidly gets a lot thinner. If you calculate 
the terms on the right hand side using M = 29 g/mol, 
g = 9.8 N/kg, R = 8.314 J/mol/K, T = 240 K, you come up with

ln (p/p0) = h / 7 km.

This means that the Earth's atmosphere has a pressure which halves for each 
5 km of height, or falls by a factor of 10 for each 16 km of height. This 
is roughly correct. It is not accurately correct because

1) The atmosphere is not at equilibrium, but is turbulent -- especially the 
bottom bit where all the weather systems are.

2) The temperature of the atmosphere is different at different heights, and 
varies in quite a complicated way. In the equation I substituted 240 K -- 
about -33 deg C or -28 deg F -- as an average value. But different parts of 
the atmosphere can be a lot hotter or a lot colder than this.

3) The atmosphere is made up of a mixture of gases, not a single gas. Below 
about 80km, it is well mixed by air currents, and no separation of gases 
occurs. But above that height mixing occurs mainly by diffusion, and 
lighter gases move to the top, so that in the very outermost parts of the 
atmosphere there are layers of lighter gases like helium and hydrogen.

In the solar system away from planets, there is a fairly good vacuum. 
Scientists do know what the pressure and composition of the "interplanetary 
medium" is. It contains much less that a trillionth as much gas per cubic 
metre as the air that we breathe. And it is about 95% hydrogen, 5% helium, 
with traces of water and methane.