  We have performed specific heat and electric resistivity measurements of
Na$_{x}$CoO$_{2}$ ($x=0.70$-0.78). Two anomalies have been observed in the
specific heat data for $x=0.78$, corresponding to magnetic transitions at
$T_{c}=22$ K and $T_{k}\simeq 9$ K reported previously. In the electrical
resistivity, a steep decrease at $T_{c}$ and a bending-like variation at
$T_{b}$(=120K for $x=0.78$) have been observed. Moreover, we have investigated
the $x$-dependence of these parameters in detail. The physical properties of
this system are very sensitive to $x$, and the inconsistent results of previous
reports can be explained by a small difference in $x$. Furthermore, for a
higher $x$ value, a phase separation into Na-rich and Na-poor domains occurs as
we previously proposed, while for a lower $x$ value, from characteristic
behaviors of the specific heat and the electrical resistivity at the
low-temperature region, the system is expected to be in the vicinity of the
magnetic instability which virtually exists below $x=0.70$.
