  The analysis of numerical simulations describing the collapse of capillary
cavities reveals that the jets originated from the bursting of bubbles are
driven by the condition that the dimensionless liquid flow rate per unit length
directed towards the axis of symmetry, $q_\infty$, remains nearly constant in
time. This observation, which is justified in physical terms because liquid
inertia prevents appreciable changes in $q_\infty$ during the short time scale
characterizing the jet ejection process, together with the fact that bubble
bursting jets are produced from the bottom of a conical cavity, justify the
purely inertial scalings for the jet width and velocity found here,
$r_{jet}\propto\sqrt{q_\infty\tau}$ and $v_{jet}\propto \sqrt{q_\infty/\tau}$,
with $\tau$ indicating the dimensionless time after the jet is ejected, a
result which notably differs from the common belief that the jet width and
velocity follow the inertio-capillary scaling $r_{jet}\propto \tau^{2/3}$ and
$v_{jet}\propto \tau^{-1/3}$. Our description reproduces the time evolution of
the jet width and velocity for over three decades in time, obtaining good
agreement with numerical simulations from the instant of jet inception until
the jet width is comparable to that of the initial bubble.
