[填空题]
A brass lid screws tightly ont.)bit51-.j vapvezq r ok1 toccu3 p e;:c)gjp, a glass jar at 20$^{\circ} $C. To help open the jar, it can be placed into a bath of hot water. After this treatment, the temperatures of the lid and the jar are both 60$^{\circ} $C. The inside diameter of the lid is 8.0 cm at 20$^{\circ} $C. Find the size of the gap (difference in radius) that develops by this procedure. cm