position, gently pull the ends apart until the bore is of the desired size (Fig. 8, B). The glass jet is then formed by cutting the tube at B and rounding the edges in a flame.

EXERCISE 3

SOME PRELIMINARY MANIPULATIONS

Apparatus. Test tubes; glass rod; 2 beakers; funnel; ring stand; evaporating dish; burner.

Materials. Filter paper; piece of chalk; 2 or 3 g. of common salt.

a. Heating liquids in test tubes. Half fill a test tube with water and apply heat until the water boils rapidly. To do this, hold the test tube between the thumb and fingers

O

FIG. 9

FIG. 10

FIG. 11

(Fig. 9), constantly rotating it so as to apply the heat uniformly. The heat should be applied to the upper portion of the liquid, care being taken, however, that the flame does not strike the tube above the level of the liquid. In case the upper part of the tube becomes heated, it may be supported by a test-tube holder (Fig. 10) or by a band of paper wrapped about the upper part of the tube (Fig. 11). The sudden formation of vapor sometimes causes the contents of the tube to be thrown out; hence care must be taken not to point the tube toward anyone.

b. Pouring liquids from one vessel to another. In doing this care must be taken to prevent the liquid from running down the side of the vessel from which it is poured. A glass rod should be held lightly against the rim of the vessel, as shown in Fig. 14. The liquid will flow down the rod.

Fill a beaker with water and transfer it slowly to another vessel without using the glass rod; repeat, using the glass rod. In pouring liquids from bottles a glass rod may be used; or the neck of the bottle may be placed lightly against the rim of the vessel into which the liquid is being poured

(Fig. 13). This will prevent the liquid from running down the side of the bottle. The stopper must never be laid down on the desk (?). Catch it between the fingers, as shown in Fig. 12. This leaves the hand free to grasp the bottle, as shown in Fig. 13.

c. Decantation; filtration; solution; evaporation. It is often necessary to separate a liquid from a finely divided. solid which is suspended in it. This may be done by one of the two following methods.

1. Decantation. When the solid is heavy and readily settles to the bottom of the vessel the liquid may be carefully poured off or decanted. Thus sand suspended in water may be separated from the liquid in this way.

2. Filtration. Usually, however, the solid will not readily

settle or will do so only after long standing. In such cases the mixture is filtered; that is, poured on a filter paper, which allows the liquid to run through but retains the solid. To prepare the filter paper, fold it along a diameter into halves, then at right angles to the first fold into quarters. The folded filter is then opened so as to form a cone, half of which is composed of three thicknesses of paper and the remainder of one thickness. Now fit the cone into a funnel

of such a size that the paper does not quite reach the top. The paper must accurately fit the funnel; if it does not, make it do so by varying the fold. Place the paper in the funnel and thoroughly wet it with water. After the water has run through, press the paper firmly against the sides of the funnel with the finger so as to remove any air bubbles. The filter is now ready for use (Fig. 14). The process of filtration not only enables us to separate liquids from solids. but also certain solids from each other. To illustrate this, grind a piece of chalk to a powder in a mortar, and mix the product intimately with about an equal bulk of common salt. Place the mixture in a small beaker, add about 50 cc. of distilled water, and stir with a glass rod. (The ends of the

rod should be rounded by rotating them in a flame, otherwise the beaker may be scratched and broken.) The salt dissolves, forming a solution. Filter off the insoluble chalk, collecting the filtrate, that is, the clear liquid which passes through the filter paper, in a beaker (Fig. 14). The salt may be recovered from the filtrate by the process of evaporation. To perform this operation, pour the filtrate into an evaporating dish, support the dish on a ring stand (Fig. 15), and heat gently. The liquid may be made to simmer, but should not actually boil (?). Withdraw the heat as soon as the water is evaporated. Note the residue left. Convince yourself that it is common salt.

EXERCISE 4

A STUDY OF SOME OF THE CHANGES TAKING PLACE WHEN A SUBSTANCE BURNS

Apparatus. Porcelain crucible; burner; ring stand; pipe-stem triangle to support the crucible while being heated; pneumatic trough (or dish); wide-mouthed bottle or large beaker; candle.

Materials. 2 or 3 g. powdered iron; 2 or 3 g. granulated tin; pellet of phosphorus, size of a small pea (to be obtained from the instructor when needed); 5 cc. limewater (R.S.).

a. The burning of metals. Place 2 or 3 g. of powdered iron in a procelain crucible and accurately weigh. Now heat the crucible (Fig. 16) until the iron begins to glow (burn); then withdraw the heat. Does the iron continue to burn? After the crucible is cool, reweigh. Compare the weight of the product with that of the unburned iron (?). The experiment may be repeated, using tin.

b. The burning of phosphorus. (PRECAUTION: Phosphorus must be kept and handled only under water; otherwise it may ignite, and serious results follow.) Cover the bottom of a pneumatic trough with water to a depth of 2 or 3 cm. On the water float a porcelain crucible containing a piece of phosphorus the size of a small pea.

Ignite the phosphorus by touching it with a hot wire or the end of a hot file, and quickly invert over the crucible a large beaker (or a widemouthed bottle), being careful to keep the rim of the beaker below the surface of the water. The white powder formed by the burning phosphorus floats in the air in the beaker but is gradually dissolved by the

water.

FIG. 16

Leave the beaker in position until the powder has entirely disappeared. Note that the water has risen in the beaker. How do you account for this fact? Is your conclusion in accord with the results obtained by burning iron in air? Suppose it were possible for you to collect and weigh the white powder formed by the burning phosphorus,. how would you expect its weight to compare with that of the unburned phosphorus ?

c. The burning of a candle. Hold a cold, dry, wide-mouthed bottle over a candle flame, as shown in Fig. 17. Note the film of moisture collecting on the bottle. After one or two minutes remove the bottle quickly and pour into it 5 cc. of clear limewater. Place the palm of the hand tightly over the mouth of the bottle and shake the contents. Note any change in the appearance of the limewater.

FIG. 17

Clean and dry the bottle and repeat the experiment, but omit holding the bottle over the candle flame. What