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>>	To perhaps start another thread, I have a question for our on line
>>doctors: I crack my knuckles and sometimes my neck. I have heard that the
>>popping noise may be due to the creation of a low pressure area in the joint,
>>which cavitates a bubble out of solution in the fluid of the joint. 
>
>It is.  Get a syringe, half fill it with water, seal the end and pull back 
>on the plunger.  What do you see?
>
>>to do it after diving, but bad habits are hard to break. Since this bubble, if
>>that's what it is, normally goes back into solution, 
>
>Look closely in the syringe.  Does all the gas made by this 
>decompression/cavitation go back into solution?

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.

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

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