The liquid is the cooling fluid. Not sure what it is - in my other lasers it’s either distilled water (for inside units) or water mixed with Dow Frostguard (a special non-reactive anti-freeze).
Rumor has it that the coolant is tears from a drunk unicorn. Or, alcohol based. Take your pick.
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I think that is the reason they degrade while not being used but I think there is also a chemical reaction due to the ionisation when it is running that also degrades the gas mixture. I.e. more than simply the increase in glass temperature lets the helium out faster.
Did you know that ordinary glass bulbs can loose their vacuum if the glass gets too hot? Glass becomes more porous to gas when it is hot.
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The PTB have never said what the fluid is, but liquid Nitrogen is cryogenic (approx -300F / -200C) so definitely not suitable for use in the GF (see video below). I’m betting on water with alcohol or other anti-freezing agent. 
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… I hope they can see well enough to eat their Tide Pods. Ha!
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If they’re smart, the liquid is the same stuff I use in my computers, which is basically Mineral Oil. It works well as a coolant and most importantly, it does not cause short circuits or corrosion if it leaks on to the electronics package. I watched the GF when I first started it up and could see bubbles in the laser tube as the coolant system purged the air. They seemed consistent with something more viscous that water. But distilled water deionized and so is also not conductive if it leaks. But it will still corrode…
People with leaks have said it seems oily but oil is not as efficient at cooling as water because it has a much lower thermal conductivity.
The usual coolant for CO2 tubes is deionised water and ethylene glycol anti-freeze. The antifreeze also reduces the cooling efficiency but of course it would be needed to survive shipping conditions. An advantage of making my own is I won’t need anti-freeze because the inside of my house never freezes.
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I like to watch my laser, or at least check up on it to make sure it hasn’t gone awry, but the bright spot hurts my eyes. So I have a camera pointed at it. It seems like that would be a good approach for a school. Put it up on a monitor – then everybody can see without crowding around, and there’s no eye damage.
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Keep some sunglasses nearby maybe? It’s super bright.
I do that when I bring it somewhere for a display or mini-makerfaire. Let’s everyone see without crowding all over. Stole the idea from the GF booth at NYMF 
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