  Cavitation bubbles collapsing and rebounding in a pressure gradient grad(p)
form a "micro-jet" enveloped by a "vapor jet". This letter presents
unprecedented observations of the vapor jets formed in a uniform
gravity-induced grad(p), modulated aboard parabolic flights. The data uncovers
that the normalized jet volume is independent of the liquid density and
viscosity and proportional to zeta=grad(p)*R0/p, where R0 is the maximal bubble
radius and p is the driving pressure. A derivation inspired by "Kelvin-Blake"
considerations confirms this law and reveals its negligible dependence of
surface tension. We further conjecture that the jet only pierces the bubble
boundary if zeta>0.0004.
