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How is this fountain different from that shown in the preceding experiment? What are the chief facts shown by this experiment?

Suggestions for report. Make a cross-section drawing, onefourth size, of the apparatus used. Name the parts. Write a paragraph which describes the action of the fountain.

Reference work. Make a list of all the uses of a vacuum that you have seen. Explain how the pump shown in figure 3 of the text can be used as a vacuum pump.

Optional problems. Pinch off the tip of a lamp bulb while it is under water. Make a full-size sectional drawing and explain the action of the apparatus.

THE WEIGHT OF AIR (1-4)

The problem. From the preceding exercises we have learned that air is a substance that fills space. Other familiar substances that fill space — as wood, water, and iron — have weight, and it is natural to question whether air may not likewise have weight. On the other hand, we are living at the bottom of a great ocean of air, but we are not conscious of any weight due to its pressure upon us, and this might lead us to believe that air does not have weight. Which of these suppositions is correct?

What to use. Large flask or large bottle of one-half gallon capacity or more, onehole rubber stopper, Y-tube, five feet of extra strong three-sixteenths-inch rubber tubing, three clamps, pump (filter pump may be used), five feet of three

Fig. 4 sixteenths-inch glass tubing, meter stick, ring stand, wire, mercury, scales, and set of weights.

What to do. 1. Study the metric system given in the appendix to learn what is meant by meter, centimeter, liter, cubic centimeter, kilogram, and gram. 2. Assemble apparatus as in figure 4.

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3. Weigh carefully the bottle after it is tightly closed and record the weight.

4. Pump air from the bottle until it seems probable that no more will be removed. Close the clamp next to the pump and watch the mercury gauge to see if the connections leak. Vaseline may often be used to advantage where a glass tube joins a rubber tube. If there is no leak close the clamp next to the bottle. Remove the bottle, weigh, and record the weight. Measure the height of the mercury.

5. Answer the questions in the following paragraph.

Questions. What are the English equivalents of the meter, liter, and kilogram? What is the proper method for weighing? What occurs in the bottle of mercury? Why? Why is the change rapid at first? Why does the mercury column change when the clamp is released ? Does the bottle change in weight? Why? What does the mercury column show ?

Suggestions for report. After a class discussion of the answers, write them in correct form in the notebook. Write answers to Questions 13, 14, 15, and 22 in section 2 of the text.

Optional problems. Determine whether all the air was exhausted from the bottle by putting the neck under water before opening the clamp. Does this result agree with the readings on the gauge ? About what fraction of the air was exhausted ? Measure the size of the bottle and compute its volume in cubic centimeters, or measure its volume by means of water and a measuring glass. Knowing the volume of the air contained in the bottle and the weight of the air, calculate the weight of air per thousand cubic centimeters. Make the proper correction for the error due to the air not pumped out. How does your

result compare with the weight given in the text ? Find the English equivalents of gram and centimeter and express your results in ounces and inches. Make a diagram of a vacuum cleaner to show how the air moves. By means of a bell jar, rubber stopper, rubber balloon, glass tube, and pan of water make a model of the human lungs.

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