Since glass is acid-proof, marbles are also useful in oil and acid filters. Glass balls are being used as well in ball valves for ball bearings and ball sockets. Paper mills use them in connection with their paper-feeding devices.
Marbles are with us in one way or another virtually all the days of our lives. They accompany us all our days from just beyond the cradle to the grave—and even after. How's that? In many modern mausoleums a dozen or so aggies are placed under coffins in the crypts so that they will roll more easily across the floor in case they need to be moved.