The United States and Great Britain made great strides in the manufacture of optical glass during the war and there are now many kinds on the market. They are used in making the lenses, prisms, and mirrors for optical instruments. Optical glass is made in much the same way as ordinary glass, but great care is taken: first, to see that the materials are pure; second, to stir the glass constantly, as it cools from the molten to the viscous state, to make it as uniform as possible; and third, to cool it very slowly in the annealing process, to avoid strains. QUARTZ GLASS [Illustration: FIG. 25 A POLLYWOG ] An entirely new glass has been placed on the market in quantity in recent years. It is made by melting very pure quartz sand at a temperature of 3000° F. and cooling it fairly rapidly. It has the very valuable property of expanding and contracting very, very slightly when heated and cooled. Thus there is practically no internal strain set up when it is heated or cooled quickly and it does not break. It can be heated red hot, for example, and then plunged into cold water without breaking. It is probable that this glass will be in universal use in a very few years. Experiment 12. To make an acrobatic pollywog. Smooth one end of a piece of No. 2 tube to put in your mouth, close the other end in the blowpipe flame, take it out and blow a bulb about ½ inch in diameter. Allow the bulb to cool, then heat the tube about ¼ inch from the bulb and draw it out into a thin tube. Now bend the thin tube at right angles near the bulb and break it off (Fig. 25). Place the bulb in water. Does it float? If not, blow another with a larger bulb. Experiment 13. Magic. [Illustration: FIG. 26 ACROBATS ] Place the pollywog in a bottle filled to overflowing with water, insert the solid rubber stopper, and press it down hard. Does the pollywog sink? Now release the stopper quickly. Does the pollywog turn somersaults in a most magical manner (1, Fig. 26), and also rise? Make one or two more pollywogs, place them all in the bottle together (2, Fig. 26), and entertain your friends with a pollywog circus. The pollywog sinks when you press down on the stopper because you compress the air in it and force water in until it weighs more than the water it displaces. [Illustration: FIG. 27 DANCING POLLYWOGS ] The pollywog rises when you release the stopper because the compressed air drives the water out until the pollywog weighs less than the water it displaces. The pollywog turns a somersault because the water rushes out sidewise in one direction and forces the nozzle in the other direction. Air may escape from the pollywog when it is turning a somersault; if so, water will take its place, and may make the pollywog too heavy to float. You can restore its buoyancy by sucking out the water. Experiment 14. A dancing pollywog. [Illustration: FIG. 28 DRAWING GLASS SPIDER-WEBS ]