We show that when fermions in field theories of coupled fermions and b
osons are treated as a perfect fluid, the equations for spherically sy
mmetric non-topological solitons have the form of potential motion. Th
is gives a straightforward method for proving the existence of of non-
topological solitons in a given theory and for finding saturating solu
tions, which are constant-density solitons of any size.