A spherical gas bubble accelerates to steady motion in an irrotational flow of a viscous liquid induced by a balance
of the acceleration of the added mass of the liquid with the Levich drag. The equation of rectilinear motion is linear
and may be integrated giving rise to exponential decay with a decay constant 
where 
is the kinematic viscosity of the liquid and a is the
bubble radius. The problem of decay to rest of a bubble moving initially when
the forces maintaining motion are inactivated and the acceleration of a bubble
initially at rest to terminal velocity are considered. The equation of motion follows
from the assumption that the motion of the viscous liquid is irrotational. It is an elementary example of how potential
flows can be used to study the unsteady motions of a viscous liquid suitable
for the instruction of undergraduate students. Another example, considered
here, is the purely radial irrotational motion of a
viscous liquid associated with the motions of a spherical gas bubble. This gives
rise to an exact potential flow solution of the Navier-Stokes
equations in which the jump of the viscous component of the normal stress is
evaluated on the potential flow. The equation of motion for the liquid is
almost always called the Reyleigh-Plesset equation
but the viscous terms were introduced by Poritsky
(1951) and not by Plesset (1949). We show that when
the normal stress equation is converted into energy equation in the
conventional way used for inviscid fluid, the viscous
normal stress term is converted into the viscous dissipation in the liquid
evaluated on potential flow.