c. Space transmits the energy of hot objects by the method called
_radiation_.
=151. Conduction.=--To illustrate conduction, place in a gas flame the ends of same metal wires supported as in Fig. 130. In a short time the other ends of the wires become hot enough to burn one's hand. This may be explained as follows: The hot gas flame contains molecules in violent vibration and those striking the wire set its molecules rapidly vibrating. Since, in a solid, the molecules are held in the same relative positions, when one end of a wire is heated the rapidly vibrating molecules at the hot end set their neighbors vibrating and these the next in turn and so on until the whole wire is hot. It is a fortunate circumstance that different substances have different rates of conductivity for heat. To realize this, suppose that our clothing were as good a conductor as iron, clothing would then be very uncomfortable both in hot and in cold weather. The best conductors for heat are metals. It is interesting to note that, as a rule good conductors of heat are also good conductors of electricity, while poor conductors of heat are also poor electric conductors. Careful experiments in testing the rate that heat will be conducted through different substances show the following rates of conductivity.
These figures are averages taken mainly from the Smithsonian Physical Tables:
Silver 100 Copper 74 Aluminum 35 Brass 27 Zinc 26 Iron 15 Tin 14.7 German silver 8.4 Mercury 1.7 Granite 0.53 Limestone 0.52 Ice 0.5 Glass 0.2 Water 0.124 Pine, with grain 0.03 Pine, across grain 0.01 Felt 0.008 Air 0.005
To test the conductivity of _liquids_, take a test-tube nearly full of cold water, hold the lower end in the hand while the tube is inclined so that the upper end is heated by a gas flame until the water boils. The lower end will be found to remain cold. (See Fig. 131.) Careful measurements of the conductivity of water show that heat is transmitted through it only {1/800} as rapidly as in silver, while air conducts but {1/25} as rapidly as water.
=152. Non-conductors and Their Uses.=--Many solids, however, are poor conductors, as leather, fur, felt, and woolen cloth. These substances owe their non-conductivity mainly to the fact that they are porous. The air which fills the minute spaces of these substances is one of the poorest conductors known and hinders the transfer of heat through these solids. For the same reason loosely packed snow is a protection to vegetation covered by it during a period of severe cold in winter. The efficiency of storm sash or double windows, and of the double and triple walls of ice-houses and refrigerators (see Fig. 132) in preventing the conduction of heat is also largely due to the poor conductivity of the air confined in the spaces between the walls. To prevent the circulation of the air, sawdust, charcoal, and other porous material is often loosely packed into the space between the walls of such structure.
Other illustrations of effective non-conductors will occur to every one; such as _woolen_ clothing, _wooden_ handles for hot objects, and the _packing_ used in fireless cookers. A _Thermos_ bottle is effective as a non-conductor of heat because the space between the double walls has the air exhausted from it (Figs. 133 and 134).
Of several objects in a cold room, some feel much colder to the touch than others, thus iron, marble, oil cloth, and earthenware will feel colder than woolen cloth, carpet, feathers, or paper. The first four objects feel cold because they are conductors, and conduct the heat away from the hand rapidly. The other substances named are non-conductors and hence remove heat from the hand less rapidly, and therefore do not feel so cold. In a similar way, if several hot objects are touched by the hand, the good conductors are the ones which will burn one most quickly by conducting heat rapidly to the hand. The non-conductors, however, will rarely burn one. Why are the handles of hot utensils often made of non-conducting materials such as wood, cloth, asbestos, etc.?
=153. Radiation= is the method by which heat comes to us from the sun across space containing no tangible matter. It is also the method by which heat gets to us when we stand near a fire. Everyone has noticed that this heat is cut off by holding an object between the person and the fire. This fact indicates that radiant heat travels in _straight_ lines.
_The radiation of heat_ is believed to be accomplished by means of waves in a medium called _ether_, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are _first, heat is transferred by radiation with the speed of light_, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. _Second, radiant heat[I] travels in straight lines_, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. _Third, radiant heat may pass through an object without heating it._ This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of glass may not be heated appreciably by the heat and light from the sun which passes through it.
[I] Radiant heat is really _radiant energy_ and becomes heat when it is absorbed by a body.
When radiant energy falls upon any object it may be (a) _reflected_ at the surface of the object, (b) _transmitted_ through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. _Well-polished surfaces are good reflectors._ Rough and blackened surfaces are _good absorbers_. Transparent objects are those which transmit light well, but even they absorb some of the energy.
=154. The Radiometer.=--Radiant heat may be detected by means of the radiometer (Fig. 135). This consists of a glass bulb from which the air has been nearly exhausted. Within it is a wheel with four vanes of mica or of aluminum mounted on a vertical axis. One side of each vane is covered with lampblack, the other being highly polished. when exposed to radiant heat from any source the vanes revolve with the bright side in advance.
The bulb is so nearly exhausted of air that a single molecule remaining may travel from the walls of the bulb to the vanes without coming in contact with another molecule.
The blackened sides absorb more heat than the highly polished sides. The air molecules striking these blackened sides receive more heat and so rebound with greater velocity than from the other side, thus exerting greater pressure. The blackened sides therefore are driven backward. If the air were not so rarified the air molecules would hit each other so frequently as to equalize the pressure and there would be no motion.
_Sun's Radiation._--Accurate tests of the amount of the sun's radiation received upon a square centimeter of the earth's surface perpendicular to the sun's rays were made at Mt. Wilson in 1913. The average of 690 observations gave a value of 1.933 calories per minute. These results indicate that the sun's radiation per square centimeter is sufficient to warm 1 g. of water 1.933°C. each minute. Although the _nature_ of _radiation_ is not discussed until Art. 408-411 in light, it should be said here that all bodies are radiating heat waves at all temperatures, the heat waves from cool bodies being much longer than those from hot bodies. Glass allows the short luminous waves to pass through freely but the longer heat waves from objects at the room temperature pass through with difficulty. This is the reason why glass is used in the covering of greenhouses and hot beds. Water also absorbs many of the longer heat waves. It is therefore used in stereopticons to prevent delicate lantern slides from being injured by overheating.
Important Topics
1. Conduction in solids, liquids, gases.
2. Non-conductors; uses, best non-conductors.
3. Radiation, three characteristics.
4. The sun's radiation, amount. The radiometer.
Exercises
1. Does clothing ever afford us heat in winter? How then does it keep us warm?
2. Why are plants often covered with paper on a night when frost is expected?
3. Will frost form in the fall of the year sooner on a wooden or a cement sidewalk? Why? On which does ice remain longer? Why?
4. Why in freezing ice-cream do we put the ice in a wooden pail and the cream in a tin one?
5. Is iron better than brick or porcelain as a material for stoves? Explain.
6. Which is better, a good or a poor conductor for keeping a body warm? for keeping a body cool?
7. Should the bottom of a teakettle be polished? Explain.
8. How are safes made fireproof?
9. Explain the principle of the Thermos bottle.
10. Explain why the coiled wire handles of some objects as stove-lid lifters, oven doors, etc., do not get hot.
(5) TRANSMISSION OF HEAT IN FLUIDS. HEATING AND VENTILATION
=155. Convection.=--While fluids are poor conductors, they may transmit heat more effectively than solids by the mode called _convection_. To illustrate: if heat is applied at the _top_ of a test-tube of water, the hot water being lighter is found at the top, while at the bottom the water remains cold. On the other hand, if heat is applied at the _bottom_ of the vessel, as soon as the water at the bottom is warmed (above 4°C.) it expands, becomes lighter and is pushed up to the top by the colder, denser water about it. This circulation of water continues as long as heat is applied below, until all of the water is brought to the boiling temperature. (See Fig. 136.)
When a liquid or a gas is heated in the manner just described, the heat is said to be transferred by _convection_. Thus the air in the lower part of a room may receive heat by conduction from a stove or radiator. As it expands on being warmed, it is pushed up by the colder denser air about it, which takes its place, thus creating a circulation of the air in the room. (See Fig. 137.) The heated currents of air give up their heat to the objects in the room as the circulation continues. These air currents may be observed readily by using the smoke from burning "touch paper" (unglazed paper that has been dipped into a solution of potassium nitrate ["saltpeter"] and dried).
=156. Draft of a Chimney.=--When a fire is started in a stove or a furnace the air above the fire becomes heated, expands, and therefore is less dense than it was before. This warm air and the heated gases which are the products of the combustion of the fuel weigh less than an equal volume of the colder air outside. Therefore they are pushed upward by a force equal to the difference between their weight and the weight of an equal volume of the colder air.
The chimney soon becomes filled with these heated gases. (See Fig. 138.) These are pushed upward by the pressure of the colder, denser air, because this colder air is pulled downward more strongly by the force of gravity than are the heated gases in the chimney.
Other things being equal, the taller the chimney, the greater the draft, because there is a greater difference between the weight of the gases inside and the weight of an equal volume of outside air.
=157. Convection Currents in Nature.=--Winds are produced by differences in the _pressure_ or _density_ of the air, the movement being from places of high toward places of low pressure. One of the causes of a difference in density of the air is a difference in temperature. This is illustrated by what are called the _land_ and _sea breezes_ along the sea shore or large lakes. During the day, the temperature of the land becomes higher than that of the sea. The air over the land expands and being lighter is moved back and upward by the colder, denser air from the sea or lake. This constitutes the _sea breezes_ (Fig. 139). At night the land becomes cooler much sooner than the sea and the current is reversed causing the _land breeze_. (See Fig. 140.)
The _trade winds_ are convection currents moving toward the hot equatorial belt from both the north and the south. In the hot belt the air rises and the upper air flows back to the north and the south. This region of ascending currents of air is a region of heavy rainfall, since the saturated air rises to cool altitudes where its moisture is condensed. The _ocean currents_ are also convection currents. Their motion is due to prevailing winds, differences in density due to evaporation and freezing, and to the rotation of the earth, as well as to changes in temperature.
=158. The heating and ventilation of buildings= and the problems connected therewith are matters of serious concern to all who live in winter in the temperate zone. Not only should the air in living rooms be comfortably heated, but it should be continually changed especially in the crowded rooms of public buildings, as those of schools, churches, and assembly halls, so that each person may be supplied with 30 or more cubic feet of fresh air per minute. In the colonial days, the _open fire place_ afforded the ordinary means for heating rooms. This heated the room mainly by _radiation_. It was wasteful as most of the heat passed up the chimney. This mode of heating secured ample _ventilation_. Fire places are sometimes built in modern homes as an aid to ventilation.
Benjamin Franklin seeing the waste of heat in the open fire places devised an iron box to contain the fire. This was placed in the room and provided heat by conduction, convection, and radiation. It was called _Franklin's stove_ and in many forms is still commonly used. It saves a large part of the heat produced by burning the fuel and some ventilation is provided by its draft.
=159. Heating by Hot Air.=--The presence of stoves in living rooms of homes is accompanied by the annoyance of scattered fuel, dust, ashes, smoke, etc. One attempt to remove this inconvenience led to placing a large stove or fire box in the basement or cellar, surrounding this with a jacket to provide a space for heating air which is then conducted by pipes to the rooms above. This device is called the hot-air furnace. (See Fig. 141.) The heated air rises because it is pushed up by colder, denser air which enters through the cold-air pipes. The _hot-air furnace_ provides a good circulation of warm air and also ventilation, provided some cold air is admitted to the furnace from the outside. One objection to its use is that it may not heat a building evenly, one part being very hot while another may be cool. To provide even and sufficient heat throughout a large building, use is made of _hot water_ or _steam heating_.
=160. Hot-water Heating.=--In hot-water heating a furnace arranged for heating water is placed in the basement. (See Fig. 142.) Attached to the top of the heater are pipes leading to the radiators in the various rooms; other pipes connect the radiators to the bottom of the boiler. The heater, pipes, and radiators are all filled with water before the fire is started. When the water is warmed, it expands and is pushed up through the pipes by the colder water in the return pipe. The circulation continuing brings hot water to the radiator while the cooled water returns to the heater, the hot radiators heating the several rooms.
=161. Steam Heating.=--In _steam heating_ a steam boiler is connected to radiators by pipes. (See Fig. 143.) The steam drives the air out of the pipes and radiators and serves as an efficient source of heat. Heating by steam is _quicker_ than heating with hot water. It is therefore preferred where quick, efficient heating is required. Hot water is less intense and more economical in mild weather and is often used in private homes.
=162. Direct and Indirect Heating.=--In heating by _direct radiation_ (Figs. 142, 143), the steam or hot-water radiators are placed in the rooms to be heated. With direct radiation, ventilation must be provided by special means, such as opening windows, doors, and ventilators. Sometimes radiators are placed in a box or room in the basement. Air from out of doors is then driven by a fan over and about the hot radiators. The air thus heated is conducted by pipes to the several rooms. This arrangement is called _indirect heating_. (See Fig. 144.) The latter method, it may be observed, provides both heat and ventilation, and hence is often used in schools, churches, court houses, and stores. Since heated air, especially in cold weather, has a low _relative humidity_ some means of moistening the air of living rooms should be provided. Air when too dry is injurious to the health and also to furniture and wood work. The excessive drying of wood and glue in a piece of furniture often causes it to fall apart.
=163. Vacuum Steam Heating.=--In steam heating, air valves (Fig. 145) are placed on the radiators to allow the air they contain to escape when the steam is turned on. When all the air is driven out the valve closes. Automatic vacuum valves (Fig. 146) are sometimes used. When the fire is low and there is no steam pressure in the radiators the pressure of the air closes the valve, making a partial vacuum inside. The boiling point of water falls as the pressure upon it is reduced. As water will not boil under ordinary atmospheric pressure until its temperature is 100°C. (212°F.), it follows that by the use of vacuum systems, often called vapor systems, of steam heating, water will be giving off hot vapor even after the fire has been banked for hours. This results in a considerable saving of fuel.
=164. The Plenum System of Heating.=--In the plenum system of heating (see Fig. 147) fresh air is drawn through a window from outdoors and goes first through tempering coils where the temperature is raised to about 70°. The fan then forces some of the air through heating coils, where it is reheated and raised to a much higher temperature, depending upon the weather conditions. Both the hot and tempered air are kept under pressure by the fan in the plenum room and are forced from this room through galvanized iron ducts to the various rooms to be heated. The foul air is forced out of the room through vent ducts which lead to the attic where it escapes through ventilators in the roof.
A thermostat is placed in the tempered-air part of the plenum room to maintain the proper temperature of the tempered air. This thermostat operates the by-pass damper under the tempering coils, and sometimes the valves on the coils. The mixing dampers at the base of the galvanized-iron ducts are controlled by their respective room thermostats. Attic-vent, fresh-air, and return-air dampers are under pneumatic switch control. A humidifier can be provided readily for this system. This system of heating is designed particularly for school houses where adequate ventilation is a necessity.
=165. The Thermostat.=--One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals and of unequal rates of expansion, as brass and iron, are securely fastened together.
The thermostatic strip _T_ moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port _C_. When the port _C_ is completely closed (Fig. 148_a_) the full air pressure collects on the diaphragm _B_ which forces down the main valve, letting the compressed air from the main pass through the chamber _D_ into chamber _E_ as the valve is forced off its seat. The air from chamber _E_ then passes into the branch to operate the damper.
When port _C_ is fully open (Fig. 148_b_) the air pressure on diaphragm _B_ is relieved, the back pressure in _E_ lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when _C_ is closed.
Important Topics
1. Transmission of heat in fluids.
2. Convection. Drafts of a chimney. Land and sea breezes.
3. Heating and ventilation of buildings.
(a) By hot air. (b) Hot-water heating. (c) Steam heating. (d) Direct and indirect heating. (e) Vacuum steam heating. (f) The plenum system. (g) The thermostat.
Exercises
1. Is a room heated mainly by conduction, convection, or radiation, from (a) a stove, (b) a hot-air furnace, (c) a steam radiator?
2. Name three natural convection currents.
3. Explain the _draft_ of a chimney. _What_ is it? _Why_ does it occur?
4. Make a _cross-section_ sketch of your living room and indicate the convection currents by which the room is heated. _Explain_ the heating of the room.
5. Make a sketch showing how the water in the hot-water tank in the kitchen or laundry is heated. Explain your sketch, indicating convection currents.
6. Is it economical to keep stoves and radiators highly polished? Explain.
7. If you open the door between a warm and a cool room what will be the direction of the air currents at the top and at the bottom of the door? Explain.
8. If a hot-water heating system contains 100 cu. ft. of water how much heat will be required to raise its temperature 150°F.?
9. Why does a tall chimney give a better draft than a short one?
10. Explain how your school room is heated and ventilated.
11. Should a steam or hot-water radiator be placed near the floor or near the ceiling of a room? Why?
12. In a hot-water heating system an open tank connected with the pipes is placed in the attic or above the highest radiator. Explain its use.
(6) THE MOISTURE IN THE ATMOSPHERE, HYGROMETRY
=166. Water Vapor in the Air.=--The amount of water vapor present in the air has a marked effect upon the weather and the climate of a locality. The study of the moisture conditions of the atmosphere, or hygrometry, is therefore a matter of general interest and importance. The water vapor in the atmosphere is entirely due to evaporation from bodies of water, or snow, or ice. In the discussion of evaporation, it is described as due to the gradual escape of molecules into the air from the surface of a liquid. This description fits exactly the conditions found by all careful observers. Since the air molecules are continually striking the surface of the liquid, many of them penetrate it and become absorbed. In the same manner many vapor molecules reenter the liquid, and if enough vapor molecules are present in the air so that as many vapor molecules reenter the liquid each second as leave it, the space above the liquid is said to be _saturated_ as previously described. (See Art. 18.)
=167. Conditions for Saturation.=--If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the retarding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated.
Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.
=168. The Formation of Dew.=--If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the cold surface as _dew_. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of grass and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, _frost_ forms instead of dew.
=169. Formation of Fog.=--If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This constitutes a _fog_. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, _e.g._, moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a _cloud_ is formed. If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and _rain_ results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that _hail_ is said to be formed.
=170. The Dew Point.=--The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the _dew point_. This is often determined in the laboratory by partly filling a polished metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point.
=171. The Humidity of the Atmosphere.=--After the dew point has been obtained, one may compute the _relative humidity_ or _degree of saturation of the atmosphere_, from the table given below. This is defined as the _ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature_.
For example, if the dew point is 5°C. and the temperature of the air is 22°C., we find the densities of the water vapor at the two temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly. Determinations of humidity may give indication of rain or frost and are regularly made at weather bureau stations. They are also made in buildings such as greenhouses, hospitals, and schoolhouses to see if the air is moist enough. For the most healthful conditions the relative humidity should be from 40 per cent. to 50 per cent.
WEIGHT OF WATER (_w_) IN GRAMS CONTAINED IN 1 CUBIC METER OF SATURATED AIR AT VARIOUS TEMPERATURES (_t_°)C.
--------+------ _t_°C. | _w_ --------+------ -10 | 2.1 - 9 | 2.4 - 8 | 2.7 - 7 | 3.0 - 6 | 3.2 - 5 | 3.5 - 4 | 3.8 - 3 | 4.1 - 2 | 4.4 - 1 | 4.6 0 | 4.9 1 | 5.2 2 | 5.6 3 | 6.0 4 | 6.4 5 | 6.8 6 | 7.3 7 | 7.7 8 | 8.1 9 | 8.8 10 | 9.4 11 | 10.0 12 | 10.6 13 | 11.3 14 | 12.0 15 | 12.8 16 | 13.6 17 | 14.5 18 | 15.1 19 | 16.2 20 | 17.2 21 | 18.2 22 | 19.3 23 | 20.4 24 | 21.5 25 | 22.9 26 | 24.2 27 | 25.6 28 | 27.0 29 | 28.6 30 | 30.1 --------+------
=172. Wet and Dry Bulb Hygrometer.=--A device for indicating the relative humidity of the air is called an _hygrometer_. There are various forms. The _wet_ and _dry bulb hygrometer_ is shown in Fig. 149. This device consists of two thermometers, one with its bulb dry and exposed to the air, the other bulb being kept continually moist by a wick dipping into a vessel of water. An application of the principle of cooling by evaporation is made in this instrument. Unless the air is saturated so that evaporation is prevented, the wet-bulb thermometer shows a lower temperature, the difference depending upon the amount of moisture in the air, or upon the relative humidity. Most determinations of relative humidity are made with this kind of instrument. It is necessary in order to make an accurate determination, to fan or set the air in motion about the thermometers for some time before reading them. The relative humidity is then found by using tables giving the relative humidity that corresponds to any reading of the thermometers.
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 20°C. if its dew point is at 10°C.
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 glass with ether and place it over a thin slip of glass 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, -456°F., or -271.3°C.
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 illustrated 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 space 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. Spaces 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 balls 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 illustration for each of the factors affecting evaporation.
3. Give an illustration for each of the three evidences of molecular motions in solids.
4. Since three-quarters of the earth's surface is covered with water, why is not the air constantly saturated?
5. If the air has the temperature of the body, will fanning the perfectly dry face cool one? Explain. Will the effect be the same if the face is moist? Explain.
6. What is the cause of "Cloud Capped" mountains?
7. Why does the exhaust steam from an engine appear to have so much greater volume on a cold day in winter than on a warm one in summer?
8. What causes an unfrozen pond or lake to "steam" on a very cold day in winter, or on a very cool morning in summer?
9. As the air on a mountain top settles down the sides to places of greater pressure, how will its temperature be affected? its relative humidity? Explain.
10. On our Pacific coast, moist winds blow from the west over the mountains. Where will it rain? Where be dry? Explain.