The problem of a pressure or volume increase due to the vertical redis
tribution of two substances with differing compressibilities is consid
ered and compared with a previous treatment. Modified expressions for
the pressure and volume changes are obtained using the general equatio
n of state with constant compressibility. The effects of gas and melt
compressibilities, and also of the solubility of gas in floating bubbl
es, on pressure and volume changes in magmatic systems are discussed.
Very different conclusions from those in the previous treatment are fo