8. A cylindrical container with a base diameter of $16 \mathrm{~cm}$ contains 2 spheres of the same size. The height from the bottom of the container to the top of the upper sphere is $18 \mathrm{~cm}$.
（1）Find the radius of the spheres;
（2）If a third sphere is added, find the height from the bottom of the container to the top of the third sphere.
8. ( 1 ) $5 \text{ cm} ;$ (2) $26 \text{ cm}$.