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>> Hold on a second here - I don't believe there is any significant amount of 
>> gas in this "bubble" in the syringe in the above example.  That is a 
>> "vacuum" bubble.  Given time the bubble will fill with gas from the liquid 
>> beneath, but there is hardly time for this to occur in this example or in 
>> joint cracking.  If you see bubbles forming within the liquid volume (as in 
>> boiling), then you have gas.  A bubble temporaily formed at the top surface 
>> is essentially vacuum.
>
>But, if the "vacuum bubbles" were truely vacuums, they would instantaneously
>vanish when exposed to any ambient pressure.  Aren't the bubbles in the above
>example formed by "pulling" dissolved gas molecules out of solution?

No, if this were true you would be seeing bubbles forming within the volume 
of the liquid, coming to the top and breaking at the surface.  I've seen the 
phenomenon with the syringe, and the bubble just appears at the top of the 
liquid.  Also, the bubble disappears instantly no matter how fast the plunger 
is pushed back in.

This is also verified by the following:  I often need to degass liquids at 
work.  I do this by placing them in a vacuum.  Bubbles appear all through the 
volume of the liquid.  Any bubbles remaining in the volume of the liquid, 
when re-exposed to atmosphere, do not instantly return to solution.  It 
takes awhile for gas to evolve or be absorbed in a liquid.

Barry Lanier
lanier@ex*.cs*.ti*.co*

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