be replaced by a shallow capsule, such as may be made of sheet copper (Fig. 34).

EXPT. 37.-Water may be made to evaporate so rapidly as to freeze by its own evaporation. This is most simply done by means of Wollaston's cryophorus (or ice-carrier'), a good form of which is shown in Fig. 35. It consists of a glass tube with a bulb at each end. One of these is half-filled with water; the rest of the space contains nothing but water-vapour, the air having been driven out by boiling the water and sealing up the instrument while full of steam.

If the bulb A is surrounded by a freezing mixture (Art. 45), the water-vapour in it condenses; more vapour flows over from B to take its place and is continuously condensed in A. So much heat is taken from B by this process, that in about half an hour its contents will have frozen.

If the water in the cryophorus more than half-fills the bulb B, some of it should be transferred to A: otherwise the expansion in freezing may burst the glass. The freezing mixture should be stirred from time to time. The bulb B should be covered with flannel or with a woollen sock, and it is well to shake it slightly if the freezing does not start in due time.

Fig. 35.

The cryophorus may be regarded as a distillation-apparatus, of which the freezing mixture and bulb A form the condenser. There is no outside source of heat: the water-vapour in passing over transfers heat from B to A, thus cooling the water in B to the freezingpoint, and then converting it into ice.

58. Applications and Illustrations.-Machines for making ice artificially have been constructed on the principles explained above. In Carré's machine a bottle of water is placed in connection with a powerful air-pump and a reservoir containing strong sulphuric acid. After working the pump for a few minutes, the water begins to boil, the vapour is rapidly

absorbed by the acid, and the water soon freezes to a solid mass of ice.

The refreshing effect produced by watering a dusty road on a hot day is not due to the laying of the dust alone: the water by its evaporation produces a pleasant coolness. The simplest way of cooling a bottle of wine in hot dry weather is to wrap round it a wet towel; if it is then put to stand in a draught the rapid evaporation of the water will cool it several degrees below the temperature of the air (see Expt. 35).

Our own bodies are cooled by evaporation, which when too rapid produces chills and colds, such as are often caught by exposure to draughts or after taking a warm bath. The air expired from our lungs is always charged with water-vapour, and from the surface of our bodies water is continually removed by perspiration and evaporation. You have probably noticed that a dog when hot after running quickly hangs out his tongue, thus exposing a larger surface for evaporation and increasing the cooling effect.

The lowest temperatures hitherto attained have been produced by the rapid evaporation of liquids with very low boiling-points (liquefied gases).

EXAMPLES ON CHAPTER VIII

1. Explain exactly the nature of boiling.

Is it possible to make lukewarm water boil without heating it, and, if so, how?

2. How would you measure the maximum pressure of alcohol-vapour at a temperature of 25°?

3. How would you distinguish between vaporisation and ebullition? Does the boiling-point of a liquid depend upon the pressure on its surface? Illustrate your answer with an experiment.

4. How many heat-units are required to convert 50 grammes of water at 12° into steam at 100°?

5. A vessel containing 30 gm. of ice is placed over a spirit-lamp: how much heat will be required to melt it and vaporise the water completely? 6. The calorific power of Welsh steam coal is 8240. How many pounds of water at 100° could be converted into steam by the combustion of 1 lb. of this coal?

The most convenient heat-unit to employ here is the 'pound-degree,' or amount of heat required to raise 1 lb. of water through 1o. 536 of these units are required to convert 1 lb. of water at 100° into steam. In the combustion of 1 lb. of coal 8240 units are evolved, or an amount sufficient to convert into steam 8240

53615.4 lbs. of water at 100°.

7. 1 lb. of a certain sample of coal is found to be sufficient to evaporate 15 lbs. of water at 100°: how much heat does it give out in burning? 8. How many pounds of steam at 100° will just melt 50 lbs. of ice at 0°?

9. 10 gm. of steam at 100° is condensed in a kilogramme (1000 gm.) of water at o°, and the temperature of the water is thereby raised to 6.3° : what value does this give for the latent heat of steam?

10. How many grammes of steam at 100° must be passed into 200 gm. of ice-cold water in order to raise it to the boiling-point? What will happen if more steam than this is passed in?

11. Describe an experiment showing that water can be frozen by its own evaporation. What weight of vapour must evaporate in order to freeze a gramme of water at freezing-point?

CHAPTER IX

HYGROMETRY

59. Water-vapour in our Atmosphere.—The presence of water-vapour in our atmosphere may be shown as follows.

EXPT. 38. Place some calcium chloride [CaCl2] in a saucer and leave it exposed to the air. The salt becomes wet on the surface and gradually dissolves in the water which it attracts from the air.

Substances like calcium chloride and sulphuric acid, which combine eagerly with water, are said to be hygroscopic, and are used for drying air and other gases (see Art. 60).

EXPT. 39.-Dry the outside of a glass tumbler or beaker and pour cold water (or iced water) into it; take it into a warm room. The surface of the glass becomes dimmed. As the air in contact with it is gradually cooled the vapour present is deposited upon it in the form of minute drops of water or dew.

You must not conclude from such experiments that the air is fully charged or saturated with water-vapour. This seldom happens; and when it does, the air is incapable of taking up any more water-vapour. Now common observation shows us that, except on very damp days, water exposed to the air rapidly evaporates, as, for example, when wet clothes are hung out to dry. We conclude, then, that our atmosphere generally contains a certain proportion of water-vapour, but seldom sufficient to saturate it. A good 'drying' day is one on which the amount of water-vapour present is far from being sufficient to saturate the air; but the rate of evaporation also depends

upon the presence or absence of wind, for when the air in contact with a moist surface is frequently renewed the moisture is more rapidly removed.

The branch of Physics that deals with the moisture of the atmosphere is called Hygrometry.

60. Absolute amount of Water-vapour.-The amount of water-vapour contained in a given volume of air can be measured by slowly aspirating the air through a series of tubes (Fig. 36) containing a hygroscopic substance which absorbs

T

B

T

Fig. 36. ASPIRATOR AND DRYING-TUBES.

and retains the water. Any large bottle or jar with a stopcock at the bottom will serve as an aspirator (A). This is filled with water which, as it runs out, draws a current of air through the U-tubes C, D, and E. These contain pumicestone soaked with sulphuric acid, and are carefully weighed before and after the experiment; the increase of weight tells us the total amount of water absorbed. The volume of air which has passed through the apparatus can be found by measuring the water which runs out from the aspirator. The results are expressed by saying that the air contains so many grains of water-vapour per cubic foot, or a certain fraction of a gramme