being freed from pressure, freezes again and finally the wire will cut entirely through the ice, which will still remain as a single block.

This thawing and freezing of ice under great changes of pressure is seen in a large scale in glaciers, their constant flow down a slope being dependent on

this action.

291. A Freezing Mixture can be made by mixing 1 part of salt with 3 parts of snow or cracked ice. The ice in contact with the salt is melted, the heat neces

sary for the melting

FIG. 250

being withdrawn from the objects near by. The salt is dissolved, and the temperature falls to the freezing point of the salt solution, which is lower than that of water. Other salts mixed with ice or snow will give lower temperatures, as calcium chloride, for example. When this salt is mixed with snow in the proportion of 3 parts of the salt to 2 parts of snow, it will give a temperature low enough to freeze mercury.

292. Vaporization. As the molecules of water (or ice) in their vibration (§ 243) strike against the surface, many of them force their way through it and pass into the air, where they exist as molecules of water vapor. When heat is applied, the temperature is raised, which means that the velocity of the molecules is increased; and the number of molecules that pass into the air increases with their velocity. As long as the temperature is below the boiling point, the process is called evaporation.

293. Laws of Evaporation.

I. Evaporation increases with the temperature. Evaporation takes place even at very low temperatures. A block of ice left for a few days in a place where the temperature is below zero will lose a considerable amount by evaporation. Wet clothes hung out on a cold winter day will freeze at once, but will soon become dry.

II. Evaporation increases if the surface of the liquid is increased. Recent experiments show that evaporation is not directly proportional to the extent of the surface. Evaporation takes place more rapidly near the boundaries of a surface than at the center, and in the case of two circular surfaces the evaporation is nearly proportional to the respective circumferences.

III. Evaporation is inversely proportional to the pressure upon the liquid.

IV. Evaporation decreases as the air becomes saturated.

Air is said to be saturated with moisture when it will hold no more at the given temperature. If the temperature is raised, more evaporation can take place, but if it is lowered, condensation will take place; that is, some of the vapor will be changed back to tiny water drops or ice crystals. At any given temperature the amount of vapor necessary for saturation is always the same per cubic meter of space, no matter how much air is also in that space.

If the space into which evaporation takes place is inclosed and the air is removed, the evaporation takes place rapidly, and saturation is quickly reached. If the air is not removed, the evaporation takes place much more slowly, because the air molecules, striking upon the surface of the water, cause a pressure upon the surface and oppose the molecules that

NUMBER OF GRAMS OF MOISTURE NEEDED FOR SATURATION PER CUBIC METER AT VARIOUS TEMPERATURES C.

are coming from the liquid. In the first case the final pressure is that of the saturated vapor alone. In the second case it is that of the air plus that of the saturated vapor. The effect of atmospheric pressure upon the rate of evaporation is so great that the moisture present in the air is generally much less than that required for saturation.

294. The Dew-point. - Demonstration. — Pour ether into a test tube until it is half full, and put a thermometer into it. Bend a tube at right angles and place in the test tube as in Fig. 251. Connect the short end with a long rubber tube and blow gently through the ether. The ether will evaporate and the temperature will rapidly fall. Watch the surface of the lower end

FIG. 251

of the test tube and take the reading of the thermometer when moisture first appears on the outside. Now stop blowing through the ether, and its temperature will rise. Take a second reading of the thermometer when the moisture disappears. The average of the two readings will be the dew-point.

The dew-point is the temperature to which air must be lowered so that the vapor present will be enough for saturation.

Fogs, clouds, rain, and snow result from a lowering of the temperature of the air below the dew-point. The most oppressive days in summer are those in which the air is nearly saturated with water vapor.

295. Humidity of the Air. The relative humidity of the air is the ratio between the amount of moisture present in the air and the amount that would be present if the air were saturated.

If the temperature of the air is taken at the same time that the dew-point is determined, the relative humidity can be found by the help of the table in § 293. Suppose, for example, that at a time when the temperature of the air is 23° C. the dew-point is 17° C. From the table the amount of moisture present when the air is saturated at 17° C. is 14.339 g. per cubic meter. But at its temperature of 23° C. it could contain 20.355 g.; hence the relative humidity is 14.339: 20.355, or about 70 per cent.

Instruments used in determining relative humidity are called hygrometers. The wet-and-dry-bulb hygrometer consists of two similar thermometers, the bulb of one being covered with a wick, the end of which dips into water. This keeps the covering of the bulb wet, and the rate of evaporation affects the temperature of the bulb. If there is little moisture in the air, the evaporation takes place rapidly, and the wet-bulb thermometer will read considerably lower than the other. Tables are provided, by the use of which the relative humidity can be determined from the readings.

296. Boiling. If a quantity of fresh water is placed over a source of heat, the first effect will be the gathering of air bubbles on the sides of the dish. This comes from the air dissolved in the water. After a time these bubbles break away from the sides and bottom and rise to the surface. All this time the temperature of the water has been rising, which means that the velocity of the molecules within the liquid has been increasing. As this velocity increases the number of impacts of the molecules up against the surface of the water increases and there is a more rapid escape of water molecules into the air. A point is soon reached when the escape of these molecules in the form of steam is very rapid; the temperature now stops liquid is said to boil. called ebullition.

Demonstrations.

rising and the

Boiling is also

Heat water in a beaker

to about 50° and remove the flame. Fill a test tube half full of ether, insert a thermometer, and put them into the hot water, as in Fig. 252. Stir the water in the beaker with the test tube, and take the lowest temperature at which the ether boils, as its boiling point.

FIG. 252

NOTE.A flame must not be brought near the ether, as its vapor is very inflammable.

Make a similar experiment with alcohol and find its boiling point.

Fill a round-bottomed flask half full of water. Boil this over a Bunsen burner, and when steam is coming freely from the neck, remove the burner and put a stopper in the mouth of the flask. Invert the flask in a ring support, as shown in Fig. 253, and pour cold water over it. This will condense the vapor above the water and reduce the pressure upon its surface, As a result the water will