The
reader may wonder why the volume of the shell is not taken as:
the difference in volume between two concentric spheres. When this expression
for the volume is expanded, we obtain (4/3)p(3*r*^{2}D*r*
+ 3*r*D*r*^{2}
+ D*r*^{3})
and for very small values of D*r* the terms
3*r*D*r*^{2}
and D*r*^{3}
are negligible in comparison with 3*r*^{2}D*r*.
Thus for small values of D*r* the two expressions
for the volume of the shell approach one another in value and when D*r*
represents an infinitesimal increment in *r* they are identical.