[Ill.u.s.tration: FIG. 149.--Wet and dry bulb hygrometer.]

[Ill.u.s.tration: FIG. 150.--A dial hygrometer.]

A form of hygrometer in common use is shown in Fig. 150. In this device, a thin strip of hygroscopic material (as a piece of goose quill) is formed into a spiral coil. One end of this is fastened to a post. The other end carried a hand or pointer. The latter moves over a printed scale and indicates directly the relative humidity.

Its indications should be tested by comparing its readings with the results of dew-point determinations. The position of the pointer may be adjusted by turning the post.

Important Topics

1. Water vapor in the air. Cause and effect.

2. Formation of dew, fog, rain, and snow.

3. Dew point, relative humidity.

4. Use of the dry- and wet-bulb hygrometer. Goose-quill hygrometer.

Exercises

1. How is the relative humidity of the air affected by warming it?

Explain.

2. How does the white cloud of steam seen about a locomotive in cold weather differ from fog? Explain.

3. In cold weather is the relative humidity of air out of doors and indoors the same? Explain.

4. Compare the relative humidity of air in a desert and near the ocean.

5. Look up the derivation of the term "hygrometer." Give the use of the instrument.

6. Find the relative humidity of air at 20C. if its dew point is at 10C.

7. How may the relative humidity of the air in a home be increased?

8. What is the effect of high humidity in the summer upon human beings?

How do you explain this?

9. Does dew fall? Explain how dew is formed?

10. In what respects is a cloud similar to a fog? In what respects different?

11. Why are icebergs frequently enveloped in fog?

12. Does dew form in the day time? Explain.

(7) EVAPORATION

=173. Effects of Evaporation.=--In Art. 19 the cooling effect of evaporation is mentioned and some explanation is made of the cooling effect observed. Since evaporation is employed in so many ways, and since its action is simply explained by the study we have made of molecular motions and molecular forces, it may be well to consider this subject further.

Take three shallow dishes, and place in one a little water, in another some alcohol, and some ether in the third, the liquids being taken from bottles that have stood several hours in the room so that all are at the same temperature. After a short time take the temperature of the three liquids. Each will be at a lower temperature than at first, but of the three the ether will be found to be the coolest, alcohol next, and the water nearest its first temperature. It will be noticed also that the ether has evaporated most in the same time. Similar effects may be observed by placing a few drops of each of these three liquids upon the back of one's hand, or by placing a few drops in turn upon the bulb of a simple air thermometer.

=174. Cooling Effect of Evaporation.=--The molecules that leave an evaporating liquid are naturally the swiftest moving ones, that is, the ones having the highest temperature, so their escape leaves the liquids cooler than before, and the one whose molecules leave fastest is naturally the one that becomes coldest, that is, the ether, in the experiment of Art. 173. If no air pressure were exerted upon the surface of the liquid, the escape of the molecules would be much increased and the temperature of the liquid would be lowered rapidly.

To test this, fill a thin watch gla.s.s with ether and place it over a thin slip of gla.s.s with a drop of cold water between the two. Now place this apparatus under the receiver of an air pump and exhaust the air. The rapid evaporation of the ether so lowers its temperature, that often the drop of water is frozen. The lowest temperatures are obtained by evaporating liquids at reduced pressure.

Onnes by evaporating liquid helium at a pressure of about 1.2 mm.

reached the lowest temperature yet attained, -456F., or -271.3C.

If four thermometers are taken, the bulbs of three being wetted respectively with ether, alcohol, and water the fourth being dry, on vigorously fanning these, the moistened thermometers show that they have been cooled while the dry one is unaffected.

This indicates that fanning a dry body at the temperature of the air does not change its temperature. Fanning does increase evaporation by removing the air containing the evaporated molecules near the surface of the liquid so that unsaturated air is continually over the liquid. If a pint of water is placed in a bottle and another pint in a wide pan the latter will become dry much sooner because of the greater surface over which evaporation can take place. Application of this is made at salt works where the brine is spread out in shallow pans.

=175. Rate of Evaporation.=--The rate of evaporation is affected by several factors. These have been ill.u.s.trated in the preceding paragraphs. To briefly summarize:

The rate of evaporation of a liquid is affected by--

(a) The nature of the liquid.

(b) The temperature of the liquid.

(c) The pressure upon its evaporating surface.

(d) The degree of saturation of the s.p.a.ce into which the liquid is evaporating.

(e) The rate of circulation of air over its surface.

(f) The extent of surface exposed to evaporation.

=176. Molecular Motion in Solids.=--Evidence of molecular motion in liquids is given by expansion on heating, evaporation, and diffusion. Do any of these lines of evidence apply to solids? It is a fact of common experience that solids do become larger on heating. s.p.a.ces are left between the ends of rails on railroads so that when they expand in summer they will not distort the track. Iron tires are placed on wheels by heating them until they slip on easily. Then on cooling, the iron shrinks and presses the wheel tightly. Many common demonstrations of expansion are found in lecture rooms. The fact of the evaporation of a solid is often detected by noticing the odor of a substance. The odor of moth b.a.l.l.s is one example. Camphor also evaporates. Heated tin has a characteristic odor noted by many. Ice and snow disappear in winter even though the temperature is below freezing. Wet clothes, "freeze dry,"

that is, dry after freezing, by evaporation. A few crystals of iodine placed in a test-tube and gently heated form a vapor easily seen, even though none of the iodine melts. Where the vapor strikes the side of the tube, it condenses back to dark gray crystals of iodine. This change from solid directly to gas and back again without becoming liquid is called _sublimation_. A number of solids are purified by this process.

Important Topics

1. Cooling effect of evaporation, rate of evaporation affected by six conditions.

2. Effects of molecular motion in solids: (a) Expansion, (b) Evaporation, (c) Sublimation.

Exercises

1. Does sprinkling the streets or sidewalks cool the air? Why?

2. Give an ill.u.s.tration for each of the factors affecting evaporation.

3. Give an ill.u.s.tration for each of the three evidences of molecular motions in solids.