Why is it possible to pass current to another insulated human while touching a plasma tube? I work in a museum and we have a large plasma tube (I do not know any details of the current used within the tube). When I place my hand on the tube and then hold out my finger and touch another human being (who's not touching the tube) we can pass a current at times large enough to make a small spark. Both of us are insulated from the ground with rubber shoes.
How is this possible?
Does it have anything to do with the fact that even though we are somewhat insulated we are not completely insulated and current can dissipate into the air around us and thus we are at a relatively close to a grounding potential compared to potential on the outer surface of the plasma tube?
 A: If the plasma tube works on the same principle as plasma bowl, then there must be Tesla transformer which generates high voltage at high frequency. The high voltage (like 10kV to 1MV) would be enough to travel through your body. However there is also so called skin effect which ensures that high frequency current is conducted in a thin layer on the surface of your skin and not through muscles and internal organs.
The fact that you wear shoes is irrelevant first because the floor itself is not conductive and second because you care about the current between you and the other person not you and ground.
As for the dissipation it is kinda like that. The Tesla transformer creates high frequency alternating current which moves electrons in the gas towards it and back so that you see sparks but you don't get electricaly charged when you touch it. You can read about it on wikipedia: https://en.wikipedia.org/wiki/Tesla_coil#Air_discharges
You don't even need plasma tube to pass spark to another person try rubbing your rubber shoes on the carpet. You will genenerate static electrical charge which you can then pas to another person. It's a little different principle but it is similar.
A: 
When I place my hand on the tube and then hold out my finger and touch another human being (who's not touching the tube) we can pass a current at times large enough to make a small spark. Both of us are insulated from the ground with rubber shoes.  How is this possible?

The most likely explanation is an electrostatic discharge, not a net current flowing from inside the plasma tube.  If a current were flowing, you would have health and safety issues and plasma tubes would probably not be for public entertainment.  Insulators can accumulate electric charge on their outer surfaces but charges cannot freely flow throughout their volume like conductors.
I am guessing when you shut the tube off for the day and come back the next morning that it has accumulated slightly more dust than other surfaces near by.  Unless you discharge the excess charge on the surface of the glass, it will remain for relatively long times.  You might even be able to "feel" the static fields (you actually feel this because of the response of small hairs on your skin) when it is off and no one has touched it.

Does it have anything to do with the fact that even though we are somewhat insulated we are not completely insulated and current can dissipate into the air around us and thus we are at a relatively close to a grounding potential compared to potential on the outer surface of the plasma tube?

No, I do not think there is a net current flowing from within the tube.  The most likely cause is a static discharge similar to what occurs if your rub your feet on a carpet and then touch someone.  During the static discharge, yes current flows through the arc but you are not conducting a current from the ground into the next person and back into the ground.  During the rubbing of your feet on the carpet, you are creating electric charges that do work to redistribute themselves evenly over your body.  Since neither your outer skin or the carpet are good conductors, the charge remains (i.e., it does not flow back to the carpet just by standing still... well it does slowly dissipates but it is very slow).

Can glass at high frequencies behave like a conductor?

The short answer is yes, if the frequency is high enough.
The long answer is that for most purposes, glass (i.e., SiO2) is a really good insulator.  It was used for years for high voltage power lines and other purposes because of its high resistivity, but tends to attract condensation and had other issues so the utilities switched to ceramics.
To illustrate that glass is a good insulator across a broad range of frequencies, I Googled the phrase SiO2 electrical conductivity vs frequency.  I found the image below, which is taken from Hassan et al. [2015].

It shows that the conductivity of silica (i.e., SiO2) has a conductivity that ranges from ~10-8 S/m at ~20 Hz to ~10-3 S/m at ~3 MHz.  For comparison, germanium has a conductivity of ~2.17 S/m or roughly three orders of magnitude higher than glass at ~3 MHz.  Germanium is considered a semi-conductor and is used in transistors and diodes.  For comparison, good conductors have conductivities above ~107 S/m like silver and copper.
Thus, unless the plasma tube power supplies are opperating above the THz range, I do not think the glass is acting like a conductor.  A quick Google search shows that plasma tubes typically operate in the ~few MHz range, not above THz.
References


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*Hassan, S.A., H.M. Gobara, M.M. Gomaa, R.S. Mohamed, and F.H. Khalil "Can microwave assisted in situ reduction of supported Pt nanoparticles challenge the chemical method in controlling the dispersion profile-catalytic performance relationship?" RSC Adv. 5, pp. 54460--54470, doi:10.1039/C5RA08116E, 2015.


Update
In an answer to another question CRDrost shared a link to a Princeton Plasma Physics Lab paper on plasma globes, which can be found at http://w3.pppl.gov/~szweben/Papers/coauthorpapers/Plasma%20Ball_POP_final.pdf.  In there they state:

The current is capacitively coupled through the inner glass bulb to the plasma filaments, and from the outside glass sphere back to the power supply in the base.  The current inside the plasma ball is presumably carried by the visible filaments.  The filament structure inside the ball can be changed by touching the outer glass sphere with a grounded wire or a finger, which produces a single bright filament.  The current through such a filament was measured by an external AC Pearson meter to be ~1 mA.

It appears that the surrounding air is acting like a ground and when you place your finger on the glass, your finger is a much better ground than the air.  Each filament is actually a superposition of thousands of filaments flashing at upwards of 25 kHz and the total current is rather small at ~1 mA.
From these observations it does appear that each filament carries a net current, and when an individual touches the glass, the glass sphere acts as a dielectric to capacitively couple the finger and the inner source.  Thus, it appears that polwel was correct.  When a finger is not on the glass tube, the electrons flow back-and-forth between the inner source and the glass tube/wall.  Not all electrons will make it back and the outer glass enclosure can charge up, similar to rubbing a balloon on hair.  Thus, my original post was not inaccurate.
