  We investigate experimentally within a two-dimensional cell the distribution
of the free volume associated either to single grains or to clusters of grains.
Our main result is that the logarithm of the probability to find a free volume
per grain v_N^f in a cluster of N grains scales as N^alpha g(v_N^f), with alpha
= 0.75. We interpret this non extensive scaling factor N^alpha as an evidence
for the onset of long range correlations between the free volumes of individual
grains. We also discuss the possible relation between g(v_N^f) and Edwards
entropy.
