B

desk and draw the edge of a triangular file across the point at which you wish to cut the glass. After the glass has been scratched, take the tube in the hands with the thumbs placed near together just back of the scratch (Fig. 9), and gently pull the glass apart, at the same time exerting a slight pressure with the thumbs. If the tube does not yield readily to a gentle pressure, a deeper scratch must be made. In the case of large tubing it may be found necessary to file a groove around the glass. The edges of the cut tube will be sharp, and should be rounded by being rotated in the tip of the Bunsen flame.

A

FIG. 8

To bend the glass tubing, first heat it at the point where you wish to bend it, in the luminous Bunsen flame spread out by means of the so-called " wing-top" burner (Fig. 10). Hold the tube lengthwise in the flame, gently rotating it so that all sides may be equally heated. Continue the heating until the glass bends easily, then remove it from the flame and quickly bend it to a right angle B (Fig. 8). Great care must be taken

to heat the tube uniformly, otherwise the bore of the tube will be contracted (A, B, Fig. 11), forming a bend which is not only unsightly but easily broken.

Next select a cork of such a size that the smaller end will just enter the hard-glass test tube. Soften the cork by rolling it between the desk and a block of wood. Now insert into the cork the glass tube prepared as directed above. To do

this, select a borer slightly smaller than the tube. Place the cork on the desk and cut half through it with the borer, not by punching but by rotating the borer under gentle pressure (Fig. 12); then reverse the cork and bore through from the other end. Care must be taken to keep the borer at a right angle to the top and base of the cork. The hole should be straight and smooth.

The glass tube, rounded at the edges, is now inserted in the cork by a gentle, screwlike motion. If the hole is too small to

admit the tube when a gentle pressure is applied, it may be slightly enlarged with a round file. Now put the cork in the test tube and set the apparatus aside for use in preparing oxygen (Exercise 6).

If time permits, the pupils may also construct the apparatus (Fig. 18, Exercise 8) used in preparing hydrogen.

b. To make glass stirring-rods. These should be about 15 cm. in length. Round the ends by heating in the Bunsen flame. Place the finished rods in the desk for future use.

EXERCISE 5

THE COLLECTION OF GASES; THE PREPARATION OF OXYGEN

Apparatus. Three 250-cc. wide-mouthed bottles; 3 pieces of glass (window) 10 cm. square; pneumatic trough; rubber tubing; burner; 2 test tubes.

Materials. 4 g. potassium chlorate; 1 g. manganese dioxide; wooden splints.

a. Collection of gases. Fill a 250-cc. wide-mouthed bottle with water. Cover its mouth with a glass plate, being careful to exclude all air bubbles. Hold the plate firmly in place, invert the bottle, and dip its mouth into the water in a pneumatic trough. Remove the glass plate. Why does the water remain in the bottle? Now, without changing the position of the bottle, fill it with exhaled air by placing one end of a piece of glass or rubber tubing under the mouth of the bottle and blowing gently through the other end.

Before the bottle, so filled, is removed from the trough, cover its mouth tightly with a glass plate. The bottle so covered may then be placed on the desk either right side up or in an inverted position. (When should it be placed in an inverted position ?)

Fill a bottle with exhaled air and then transfer the air to another bottle. Draw a diagram to show the method of doing this.

b. Preparation of oxygen from mercuric oxide. Recall Exercise 3, Experiment a.

c. Preparation of oxygen from potassium chlorate (preliminary experiment). Select two test tubes of the same size, and clean and dry them thoroughly. Into the one introduce 2 g. of potassium chlorate; into the other introduce 2 g. of potassium chlorate mixed intimately with 1 g. of manganese dioxide.

so that both tubes are equally heated. Repeatedly thrust a glowing splint into each tube in order to detect any oxygen that may be evolved. Note the results. What effect has the manganese dioxide? From which tube is the greater amount of oxygen evolved (p. 16 of text)?

EXERCISE 6

THE PREPARATION AND PROPERTIES OF OXYGEN

Apparatus. Hard-glass test tube fitted with cork and delivery tube as shown in Fig. 13 (use apparatus constructed in Exercise 4); four 250-cc. wide-mouthed bottles; 4 pieces of glass 10 cm. square; pneumatic trough; ring stand and burner; deflagrating-spoon.

Materials. 6 g. potassium chlorate; 3 g. manganese dioxide; wooden splints; bit of sulfur; picture-frame wire 20 cm. long; bit of cotton.

a. Usual laboratory method for preparing oxygen. Mix intimately on paper 6 g. of potassium chlorate and 3 g. of manganese dioxide. The presence of impurities in the materials may lead to a serious explosion when heat is applied; hence test a small portion of the mixture, say 0.5 g., by heating it in a test tube. In the absence of impurities the oxygen is evolved quietly, unaccompanied by any very marked sparking in the materials.

If pure, transfer the remainder of the mixture to a hardglass tube and insert the cork (Fig. 13); then, holding the burner in the hand, heat the mixture gently with a small flame, applying the heat at first to the upper part of the mixture. The flame must not strike the upper part of the test tube, as the cork may be ignited. At first the heat expands the air and a few bubbles of air escape; then the oxygen is evolved. Regulate the heat so as to secure a uniform and not too rapid evolution of the gas. Collect three or four 250-cc. wide-mouthed bottles of the gas. Before the heat is withdrawn, remove the cork from the tube (?). What is the

source of the oxygen? What is the function of the manganese dioxide? Place the tube and contents aside for use in Exercise 7.

b. Note the physical properties (color, odor, taste, solubility in water) of the gas. (The slight cloud that is often present when oxygen is prepared from potassium chlorate is due to an impurity and will disappear if the gas is allowed to stand over water.)

c. Repeatedly thrust a glowing splint into a bottle of the

gas.

d. Heat some sulfur in a deflagrating-spoon until it begins to burn. Note the color and size of the sulfur flame. Now lower the burning sulfur into a bottle of oxygen and note the change.

e. Tip the piece of picture-frame wire with sulfur by wrapping a very small bit of cotton about the end of the wire and dipping this into melted sulfur (for this purpose melt a little sulfur in a deflagrating-spoon). Ignite the sulfur by holding it in a Bunsen flame for an instant, and then thrust the wire into a bottle of oxygen.

Describe the results obtained in c, d, and e. What becomes of the oxygen?