Many references state that pumice is formed by gases in lava, but I have yet to find a reference of which gases they are. If a piece of pumice floats, then conceivably it still contains the original gases from the magma, right?
Breaking the pumice would release the gas trapped in the vesicles.
According to https://www.tulane.edu/~sanelson/Natural_Disasters/volcan&magma.htm, the gases in magma are
- Mostly H2O (water vapor) & some CO2 (carbon dioxide)
- Minor amounts of Sulfur, Chlorine, and Fluorine gases
But is this true for pumice? If it's mostly water vapor in the gas, then when it cools, the pumice holes would contain liquid water, albeit small amounts compared to the size of the vesicles.
Otherwise, does pumice typically have other gases?
Edit: Here's some background for this question. My 7yo son asked me where the bubbles in pumice came from, when I showed him that this "rock" can float. Google didn't give a good answer, so after he went to bed I started searching... Kids ask the best questions!