bit of filter paper previously moistened with coal oil. Raise the inverted test tube until its mouth dips just below the surface of the water in the beaker, and quickly insert the sodium. Stand at arm's length, as a slight explosion sometimes occurs. Notice that the sodium decomposes the water, liberating a gas which is caught in the tube. Place your thumb tightly over the mouth of the tube to prevent the gas from escaping, and bring the tube to an upright position. Light a splint, remove the thumb from the tube, and quickly bring the flame to the mouth of the tube. Does the gas act like

What other

oxygen? What is the source of the gas? methods may be employed for obtaining it from the same source?

b. Preparation from acids (usual laboratory method). Prepare a hydrogen generator according to Fig. 18. D represents a wide-mouthed bottle of about 250-cc. capacity. The gasdelivery tube B, C is the same as that used in the preparation of oxygen (Fig. 13). A rubber stopper for the bottle D is preferable, although a good cork stopper will do. The funnel tube A must extend nearly to the bottom of the bottle (?). Put 10 g. of granulated zinc (why granulated?) into D, and add a few drops of a solution of copper sulfate. Now pour just enough water through the funnel tube to cover the zinc.

delivery tube C until the water is forced nearly to the top of the funnel tube; then quickly close the rubber tube either by tightly pinching it or by placing the tongue firmly against its end. If the apparatus is air-tight, the water in the funnel tube will not fall.

Prepare some dilute sulfuric acid by slowly pouring 15 cc. of concentrated acid into a beaker containing 50 cc. of water. Stir the water with a glass rod while the acid is being added. Notice that the acid is poured into the water—never

the reverse.

Cool the mixture and pour a few drops of it through the funnel tube. Hydrogen is at once evolved. Enough of the acid must be added from time to time to cause a gentle and continuous evolution of the gas. An excess of the acid should be avoided, however, or the action will become too violent and a large quantity of zinc will have to be added at the close of the exercise.

It is evident that the gas which passes over first is a mixture of hydrogen and air. The student must remember that such a confined mixture of hydrogen and air explodes with great violence if ignited. Hence see that the end of the delivery tube is not brought near any flame. Determine when the hydrogen is free from air by repeatedly collecting a test tube full of gas and igniting it, holding the tube mouth downward (?). If pure, the gas burns quietly; otherwise there is a slight explosion. After all the air has been expelled from the generator, collect four bottles (250-cc., wide-mouthed) of the gas.

What is the source of the hydrogen? What is the use of the zinc? Why is the copper sulfate solution added (p. 28 of text)?

Remove the cork from the generator, add a few more pieces of zinc, and set aside. Sufficient zinc should be used so that at least a small portion of it remains undissolved.

c. Thrust a lighted splint into a bottle of the gas held mouth downward. Slowly withdraw the splint and again thrust it into the gas. Describe the results. What do they prove?

d. Fill a small (60-cc.) wide-mouthed bottle or test tube one-third full of water and invert it in a pneumatic trough. Displace the remaining water with hydrogen from one of the bottles. What does the bottle now contain? Withdraw it from the water and, holding it at arm's length, quickly bring it, mouth downward, over a flame. What do the results prove?

e. Uncover a bottle (mouth upward) of the gas. After a minute, test for the presence of hydrogen with a lighted splint. Repeat, holding the bottle mouth downward. Describe the results. Is the gas heavier or lighter than air?

*f. Without removing the fragments of undissolved zinc, pour the liquid set aside in Experiment b into an evaporatingdish and boil gently on a ring-stand support. As soon as white crusts begin to form on the side of the dish, just above the liquid, filter the hot liquid into a beaker and set it aside to cool. How does the product which separates from the filtrate compare in properties with the original zinc ? Has the zinc undergone any change? Explain (p. 27 of text).

EXERCISE 9

THE COMBUSTION OF HYDROGEN; THE OXYHYDROGEN BLOWPIPE

Apparatus. Hydrogen generator A (Fig. 19) attached by rubber tubing C to a drying-tube B. This tube is filled with granulated calcium chloride, held in place by loose plugs of cotton placed at each end of the tube. D is a glass tube drawn to a jet.

Materials. Granulated calcium chloride; cotton; 8 g. zinc; 2 or 3 drops copper sulfate solution (R. S.); dilute sulfuric acid; picture-frame wire 10 cm. long; bit of charcoal.

a. Charge the generator A (Fig. 19) with 6 or 8 g. of zinc, add the solution of copper sulfate, cover with water, and add

rubber tubing over the tube D, collect samples of the gas in test tubes over water, and test with a flame to see whether the hydrogen is free from air. After all the air has been expelled, wrap a towel carefully about the generator and cautiously ignite the hydrogen. The flame is nearly invisible and is very hot. Test the heat of the flame by holding in it different objects, such

FIG. 19

as a splint, a piece of picture-frame wire, a bit of charcoal. Finally, hold over the flame a cold, dry beaker or bottle and note the liquid deposited on the sides of the vessel. Explain.

b. Examine the structure of the oxyhydrogen blowpipe. Draw a diagram representing a cross section of it.

EXERCISE 10

MEASUREMENT OF GAS VOLUMES (OPTIONAL)

Apparatus. Graduated tube and cylinder, as shown in Fig. 20; thermometer.

a. (It is suggested that the instructor arrange one or more pieces of apparatus as shown in Fig. 20. The students will then take the readings and solve the problems.)

Partially fill a graduated tube with water and invert it in a cylinder (or other vessel) of water as shown in Fig. 20. Adjust the tube until the level of the liquid inside and outside of the tube is the same; then take the reading of the

c. Thrust a lighted splint into a bottle of the gas held mouth downward. Slowly withdraw the splint and again thrust it into the gas. Describe the results. What do they prove?

d. Fill a small (60-cc.) wide-mouthed bottle or test tube one-third full of water and invert it in a pneumatic trough. Displace the remaining water with hydrogen from one of the bottles. What does the bottle now contain? Withdraw it from the water and, holding it at arm's length, quickly bring it, mouth downward, over a flame. What do the results prove?

e. Uncover a bottle (mouth upward) of the gas. After a minute, test for the presence of hydrogen with a lighted splint. Repeat, holding the bottle mouth downward. Describe the results. Is the gas heavier or lighter than air?

*f. Without removing the fragments of undissolved zinc, pour the liquid set aside in Experiment b into an evaporatingdish and boil gently on a ring-stand support. As soon as white crusts begin to form on the side of the dish, just above the liquid, filter the hot liquid into a beaker and set it aside to cool. How does the product which separates from the filtrate compare in properties with the original zine? Has the zinc undergone any change? Explain (p. 27 of text).

EXERCISE 9

THE COMBUSTION OF HYDROGEN; THE OXYHYDROGEN BLOWPIPE

Apparatus. Hydrogen generator A (Fig. 19) attached by rubber tubing C to a drying-tube B. This tube is filled with granulated calcium chloride, held in place by loose plugs of cotton placed at each end of the tube. D is a glass tube drawn to a jet.

Materials. Granulated calcium chloride; cotton; 8 g. zinc; 2 or 3 drops copper sulfate solution (R. S.); dilute sulfuric acid; picture-frame wire 10 cm. long; bit of charcoal.

a. Charge the generator A (Fig. 19) with 6 or 8 g. of zinc, add the solution of copper sulfate, cover with water, and add