Add an extra length to one arm and repeat. Are the levels different but are they equal distances above the water in their respective tumblers? Place the tumblers on the table, make one tube slanting, and repeat the experiment (Fig. 82). Are the levels again the same? When you suck air out of the tee, you decrease the air pressure in the two tubes, and the atmospheric pressure on the water in the tumblers lifts the water into the tubes. Experiment 63. Unequal columns. Put a large handful of salt into a tumbler partly filled with water and stir until the salt is dissolved. Now pour fresh water into another tumbler until it is at the same height as the salt water. Make the arms of equal length, put one arm in the salt water and the other in the fresh water, then suck a little air out of the top coupling and close it with a plug. Do you find that the column of salt water is shorter than the column of fresh water (1, Fig. 83)? It is shorter because salt water is heavier than fresh water. [Illustration: FIG. 83 UNEQUAL COLUMNS ] If you have gasoline or kerosene convenient fill one tumbler half full of either, and the other tumbler half full of water, then repeat the experiment. Do you find that the column of gasoline or kerosene is longer than the column of water (2, Fig. 83)? It is longer because gasoline and kerosene are lighter than water. Experiment 64. To fuse wire into glass. Find a piece of thin iron or copper wire about 4 inches long, heat the end of a piece of No. 2 tubing until it is nearly closed, insert the iron or copper wire into the small hole, and heat the glass around the wire until it shrinks and grips the wire firmly (Fig. 84). The glass then serves as a handle for the wire. [Illustration: FIG. 84 WIRE FUSED INTO GLASS ] It is difficult to make a secure joint between iron or copper wire and glass because they both expand and contract more than glass when heated and cooled. It is easy to make a secure joint between platinum wire and glass because platinum and glass expand and contract at practically the same rate when heated and cooled. Platinum, however, is too expensive to be used for ordinary experiments. Experiment 65. To cut window glass. The common glass cutter is a small very hard steel wheel mounted on a handle (Fig. 85). Practice with one on a pane of glass: place a ruler on the glass, draw the wheel along the ruler (Fig. 86) with sufficient pressure to scratch the glass, place the under side of the scratch exactly over the edge of the table, and press down on both sides. [Illustration: FIG. 85 A GLASS CUTTER ] Experiment 66. To bore a hole in glass. [Illustration: FIG. 86 CUTTING A PANE OF GLASS ]