  Two percolation transitions are observed in Ag$_x$(SnO$_2)_{1-x}$
nanogranular films with Ag volume fraction $x$ ranging from $\sim$0.2 to
$\sim$0.9. In the vicinity of each percolation threshold $x_{ci}$ ($i$$=$1, 2),
the variation in $\sigma$ with $x$ obeys a power law for $x$$>$$x_{ci}$. The
origin of the first percolation transition at $x_{c1}$ ($x_{c1}$$>$$x_{c2}$) is
similar to that of the classical one, while the second transition is explained
as originating from the tunneling to the second-nearest neighboring Ag
particles. These observations provide strong experimental support for the
validity of current theories concerning tunneling effect in conductor-insulator
nanogranular composites.
