EXPERIMENT 85. Take two test tubes with mouths of the same size so that you can fit them snugly against each other when you want to. Fill one to the brim with water and hold your thumb or a piece of cardboard over its mouth while you place it upside down in a pan of water. Take the free end of a rubber tube that is attached to a gas pipe and put it into the test tube a short distance, so that the gas will go up into the tube, as shown in Figure 149. Now turn on the gas gently. When all the water has been forced out of the tube and the gas bubbles begin to come up on the outside, turn off the gas. Put a piece of cardboard, about an inch or so square, over the mouth of the tube so that no air can get into it, and take the tube out of the water, _keeping the mouth down and covered_. Bring the empty test tube, which of course is full of air, mouth up under the test tube full of gas, making the mouths of the two tubes meet with the cardboard between them, as shown in Figure 150. Now have some one pull the cardboard gently from between the two test tubes, so that the mouths of the tubes will be pressed against each other and so that practically no gas will escape. Hold them quietly this way, the tube of gas uppermost, for not less than one full minute by the clock. A minute and a half is not too much time. Now have some one light a match for you, or else go to a lighted Bunsen burner. Take the test tubes apart gently and hold the lower one, which was full of air, with its mouth to the flame. What has the gas in the upper tube done? Now hold the flame to the upper test tube, which was full of gas. What happens? Has all the gas gone out of it? [Illustration: FIG. 149. Filling a test tube with gas.] [Illustration: FIG. 150. The lower test tube is full of air; the upper, of gas. What will happen when the cardboard is withdrawn?] As you well know, gas is much lighter than air; you can make a balloon rise by filling it with gas. Yet part of the gas went _down_ into the lower tube. The explanation is that the molecules of gas and those of air were flying around at such a rate that many of the gas molecules went shooting down among the air molecules, and many of the molecules of air went shooting up among those of gas, so that the gas and the air became mixed. DIFFUSION IN LIQUIDS. Diffusion takes place in liquids, as you know. For when you put sugar in coffee or tea and do _not_ stir it, although the upper part of the tea or coffee is not sweetened, the part nearer the sugar is very sweet. If you should let the coffee or tea, with the sugar in the bottom, stand for a few months, it would get sweet all through. Diffusion is slower in liquids than in gases, because the molecules are so very much closer together.