  The relativistic expression for the $n$-th order moment of the nuclear charge
density is presented. For the mean square radius(msr) of the nuclear charge
density, the non-relativistic expression, which is equivalent to the
relativistic one, is also derived consistently up to $1/M^2$ with use of the
Foldy-Wouthuysen transformation. The difference between the relativistic and
non-relativistic expressions for the msr of the point proton density is also
discussed. The $n(\ge 4)$-th order moment of the nuclear charge density depends
on the point neutron density. The 4-th order moment yields a useful information
on the msr of the point neutron density, and is expected to play an important
role in electron scattering off neutron-rich nuclei.
