  We use a recently proposed method [Berthier L.; Biroli G.; Bouchaud J.P.;
Cipelletti L.; El Masri D.; L'Hote D.; Ladieu F.; Pierno M. Science 2005, 310,
1797.] to obtain an approximation to the 4-point dynamic correlation function
from derivatives of the linear dielectric response function. For four liquids
over a range of pressures, we find that the number of dynamically correlated
molecules, Nc, depends only on the magnitude of the relaxation time,
independently of temperature and pressure. This result is consistent with the
invariance of the shape of the relaxation dispersion at constant relaxation
time and the density scaling property of the relaxation times, and implies that
Nc also conforms to the same scaling behavior. For propylene carbonate and
salol Nc becomes constant with approach to the Arrhenius regime, consistent
with the value of unity expected for intermolecularly non-cooperative
relaxation.
