If the bubbles do not form rapidly, ask the teacher to pour a little strong hydrochloric acid into the flask; but this will probably not be necessary. Let the balloon keep filling until it is as large as you blew it. But if the bubbles stop coming before it gets as large as that, close the neck of the balloon by pinching it tightly, and take the stopper out. Let some one add more zinc shavings and more acid to the flask; put the stopper back in, and stop pinching the neck of the balloon. _In this and all other experiments when you use strong acids, pour the used acids into the crockery jar that is provided for such wastes. Do not pour them into the sink, as acids ruin sink drainpipes._ When the balloon is full, close the neck by slipping the rubber band up from the part of the neck that is over the glass tube on to the upper part of the neck. Pull the balloon off the glass tube and pinch the neck firmly shut. Take the stopper out and rinse the flask several times with running water. Any zinc that is left should be rinsed thoroughly, dried, and set aside so that it may be used again. Now tie one end of a long thread firmly around the mouth of the balloon and let the balloon go. Does it rise? If it does not, the reason is that you did not get it full enough. In that case make more hydrogen and fill it fuller, as explained above. [Illustration: FIG. 162. Filling a balloon with hydrogen.] [Illustration: FIG. 163. Adding more acid without losing the gas.] Here is another experiment with hydrogen: EXPERIMENT 92. Put a wad of zinc shavings, about the size of the end of your little finger, into the bottom of a test tube. Cover it with hydrochloric acid (HCl) diluted one to three, as in the preceding experiment. After the bubbles have been rising for a couple of minutes, take the test tube to the side of the laboratory where the burners are, and hold a lighted match at its mouth. Will hydrogen burn? Remember that the hydrogen which the zinc is driving out of the acid is exactly the same as the hydrogen you drove out of water with an electric current. There is a metal called _sodium_ (Na) and another called _potassium_ (K) which are as soft as stiff putty and as shiny as silver; if you put a tiny piece of sodium (Na) or potassium (K) on water, it will drive the hydrogen out of the water just as zinc drove it out of the acid. The action is so swift and violent and releases so much heat that the hydrogen which is set free catches fire. This makes it look as if the metal were burning as it sputters around on top of the water. There is so much sputtering that the experiment is dangerous; people have been blinded by the hot alkaline water spattering into their eyes. So you cannot try this until sometime when you take a regular course in chemistry. [Illustration: FIG. 164. Trying to see if hydrogen will burn.]