hydrants with straw and other non-conducting substances to protect them from the action of frost; and, as running water does not readily freeze, taps are often kept open (in spite of municipal regulations !) as long as the frost lasts.

EXAMPLES ON CHAPTER VII

Read Arts. 43, 44 again; also the note at end of Chap. VI. Whenever weights are expressed in pounds (instead of grammes) use as your unit of heat the 'pound-degree' or amount of heat required to raise the temperature of 1 lb. of water through 1o.

The latent heat of water may be taken as 80.

1. What will be the result of mixing 10 lbs. of snow at o° with 4 lbs. of water at 60°?

First find whether all or only part of the snow will be melted. The amount of heat required to melt 10 lbs. of snow would be 10 x 80 800 pound-degree units of heat. The temperature of the mixture cannot fall below o°, and 4 lbs. of water in cooling from 60° to o° only give out 4 x 60=240 pound-degrees, or enough

240

to melt =3 lbs. of snow. Thus the result will be a mixture of

80

7 lbs. of snow and 7 lbs. of water, all at o°.

2. How much ice at o° will be melted by 1000 gm. of boiling water? 3. When equal weights of boiling water and melting ice are mixed, the ice all melts, and the resulting temperature is 10°: find from this the latent heat of fusion.

4. How much hot water at 75° will just melt 30 lbs. of ice?

5. 300 grammes of melting ice are mixed with 700 grammes of boiling water, and the resulting temperature is 46°: what is the latent heat of fusion?

6. A brass cylinder weighing 80 gm. was heated to 100° and dropped into an ice-calorimeter. The amount of ice melted was 9 gm. : find the

specific heat of the brass.

7. Two copper balls of the same weight and raised to the same temperature are laid, the one on a cake of slowly-melting ice, and the other on a cake of wax. The latter sinks in the more deeply. What inference would you draw from this?

To a

8. Explain the statement that the latent heat of water is 80. pound of ice at o° are communicated 100 units of heat (pound-degrees Centigrade). What change of temperature does the ice undergo, and in what way is its volume altered?

9. 143 grammes of water were obtained when a kilogramme of iron at 100° was introduced into an ice-calorimeter what was the specific heat of the iron ?

10. A quantity of ice is thrown into a basin containing 4 lbs. of water

at 30°, and after all the ice has melted the temperature is found to have fallen to 8° how much ice was thrown in?

II. What is meant by a unit of heat? Taking the specific heat of lead as 0.031, and its latent heat as 5.07, find the amount of heat necessary to raise 15 lbs. of lead from a temperature of 115° C. to its melting-point, 325° C., and to melt it.

CHAPTER VIII

CHANGE OF STATE-VAPORISATION AND

CONDENSATION

47. Evaporation and Boiling.—There are two ways in which a liquid may be changed into a vapour.

The first of these is the slow process which is called evaporation. You have doubtless noticed that, if water is left exposed in a shallow dish or saucer, it gradually dries up or evaporates. This evaporation of water goes on at all ordinary temperatures, but most rapidly in warm dry weather. Alcohol (methylated spirit) evaporates more rapidly than water; and ether evaporates so quickly that, if you pour a few drops on the palm of your hand, the ether will disappear in a few seconds, and produce an intense sensation of cold.

The second process is the familiar one of boiling. You should begin your study of this by boiling some water in a glass flask. When the cold water is heated, the first thing you notice is the expulsion of the air which it always contains in solution. This makes its appearance in the form of minute bubbles, which gradually rise through the water and are expelled. Larger bubbles, of steam or water-vapour, soon make their appearance, but as they rise into the upper and cooler water, they condense and collapse with a peculiar rattling noise. This is what causes the 'singing' of a kettle -always a sign that boiling is not far off. Finally bubbles of steam begin to rise rapidly and freely from all parts of the water, and the actual boiling sets in as soon as the whole of the liquid has been heated to a certain temperature, called its boiling-point. Observe that the steam itself is, like air,

invisible; the cloud that forms above the mouth of the flask consists of condensed water-particles, not water-vapour.

The distinction between the two processes (evaporation and boiling) is this. Evaporation goes on at all temperatures, but only from the surface of the liquid. Boiling consists in the rapid production of bubbles of vapour throughout the mass of the liquid, and only takes place at a definite temperature (the boiling-point of the liquid).

The

EXPT. 28. To find the boiling-point of a liquid. All that is needed for a rough determination is a small long-necked flask or test-tube (Fig. 26), fitted with a double-bored cork, in which are inserted the thermometer and a bent tube for the escape of the vapour. thermometer should be pushed down until the bulb nearly touches the liquid, but it should not dip into it. Alcohol or methylated spirit will do well for the experiment. If you wish to recover the liquid, you can do so by connecting the bent tube to a condenser (see Art. 52). If the liquid boils by fits and starts, or 'bumps,' you should put into the testtube a few bits of crumpled tin-foil or platinum-foil. This will make the boiling more regular.

Fig. 26.

48. Vapour - Pressure. - When water is introduced into a closed space containing air (e.g. a corked flask), it goes on evaporating slowly until the air contains as much water-vapour as it can hold in suspension; the evaporation then stops, and the air is said to be saturated with water-vapour.

If the water is introduced into a vacuous space (e.g. a flask out of which all the air has been pumped), the evaporation goes on more rapidly; practically it is instantaneous. You already know that the vapour of boiling water (steam) exerts a pressure it is by this pressure of steam that all steam-engines work. You have now to learn that even at ordinary temperatures water-vapour exerts a small but measurable

pressure.

This pressure can be observed and measured by introducing water into a Torricellian vacuum (Art. 28).

EXPT. 29. Fill a barometer-tube with mercury and invert it as in Expt. 15. Make a bent pipette, fill it with the liquid to be introduced, and push the curved point of it under the mercury and into the tube (Fig. 27). Ether is the best liquid to start with. Blow cautiously into the upper end of the pipette, so as to make the ether ascend drop by drop, and watch the effect. The first drop will evaporate almost before it reaches the vacuum, and the column of mercury

will fall through a few centimetres. This fall is due to the pressure exerted by the ether-vapour, for there is nothing else above the mercury column to exert any pressure. As the ether ascends drop by drop, the mercury falls lower and lower until a point is reached, after which the introduction of more ether does not cause any further depression : the evaporation stops, and a layer of liquid ether is seen resting on the mercury. Before this, the space above the mercury was only partially saturated with ethervapour, or contained unsaturated vapour; it is now saturated, and the vapour exerts its maximum pressure, i.e. the greatest pressure that it can exert at the temperature of the room.

Fig. 28.