determining specific heats will be described in the next chapter. EXPT. 19.—To find the specific heat of lead shot by the method of mixtures. Apparatus required—A steam-heater: Fig. 22. this can be made by fitting a wide test-tube loosely into the neck of a corked flask ; a bent tube should also be fitted into the cork so as to allow the steam to escape (Fig. 22). A thermometer. A beaker holding about 200 C.C. to serve as a calorimeter: a cylindrical vessel made of thin sheet-brass does better as a calorimeter because it absorbs less heat; a small tin pot, such as can be obtained at a grocer's or chemist's shop does fairly well. Weigh into the calorimeter 100 grammes of water (or as much as will half fill it), and place the calorimeter on a flat cork or wrap it round with wool or flannel (to prevent loss of heat by conduction). Put 200 grammes of lead shot into the test-tube and close the mouth of the tube with a plug of cotton-wool to keep out cold air. Boil the water in the flask, and keep it boiling for quarter of an hour so that the lead may get heated to the temperature of the steam. When all is ready, observe carefully by means of the thermometer the temperature of the water in the calorimeter; suppose this to be 9°.6. Now remove the cotton-wool, lift the test-tube out and quickly pour the shot into the calorimeter. Stir the water gently by moving the bulb of the thermometer, and note the highest temperature which it reaches. Suppose this to be 14°.9; we can now calculate the specific heat of the lead shot. The lead has cooled Lets be the required specific heat. from 100 to 14°.9, i.e. through 85°.1. amount of heat it has given out in cooling is H = 200 x sx 85.1 units. This has been absorbed by the water, and has raised its temperature from 9°.6 to 14°·9, i.e. through 5°.3 The amount Make similar experiments with mercury, iron (nails), copper (short pieces of thick wire), etc. 39. Thermal Capacity.-The specific heat of a substance depends only upon the nature of the substance: it does not depend upon its size or weight, any more than its hardness or colour does. The specific heat of a brass button is the same as that of a brass cannon- -if both are made of the same kind of brass. But the quantity of heat required to warm a body through a given range of temperature depends not only upon its specific heat (s) but also upon its mass (m). The product (ms) of these two quantities is called the thermal capacity or capacity for heat of the body, and may be defined as follows: The thermal capacity of a body is measured by the amount of heat required to raise its temperature by one degree. Ex. 3.-What is the thermal capacity of a calorimeter weighing 100 gm. and made of copper of specific heat 0.095? Since 0.095 is the specific heat of copper, 0.095 of a heat-unit is 40. Results and Applications. Thus In the following table the approximate values of the specific heats of a few substances Observe that mercury has a very low specific heat. This is another reason in favour of its use in thermometers. When we wish to find the temperature of a hot liquid and dip a thermometer into it, the bulb and its contents absorb some heat, and so the liquid is slightly cooled: but the amount of heat so absorbed by a mercurial thermometer is much less than it would be, for example, if the bulb were filled with water. Water has a higher specific heat than any other liquid or solid. For this and other reasons the sea is not warmed to nearly the same extent as land by the sun's rays; nor does it cool so rapidly when they are absent. As the air above it partakes of its temperature, it follows that islands and places on or near the sea-coast enjoy a more equable climate than inland places. The differences between summer and winter temperatures in our island are only about half as great as the corresponding differences in regions of central Russia which are in the same latitude. On account of its high specific heat, water not only cools slowly but gives out a large amount of heat in the process. Among familiar applications of this we may mention hot water bottles, the foot-warmers used in railway carriages, and the systems of heating by hot-water pipes commonly used in greenhouses and public buildings. EXAMPLES ON CHAPTER VI The Read Arts. 37-39 again before you attempt to work these out. specific heats required may be taken from the table in Art. 40. You will generally find that the best and simplest method of solving a specific heat problem is the method we adopted in working out the results of Expt. 19. Find the amounts of heat given out by the hot substance and taken in by the cold substance: if no heat is lost or gained in the experiment, these must be equal and can be equated. I. A certain vessel holds 800 c. c. of water at its temperature of maximum density (4°). How much heat must be imparted to the water before it begins to boil? 2. How many units of heat are required to raise the temperature of 150 gm. of copper from 10° to 150°? 3. What is the thermal capacity of a leaden bullet weighing 100 gm. ? 4. Find the specific heat of a substance 125 gm. of which at 78°, when immersed in 250 gm. of water at 12°, gave a resulting temperature of 18°. 5. Two pounds of boiling water are poured upon 10 lbs. of mercury at 16° what will be the common temperature after mixing? 6. What is meant by saying that the specific heat' of water is thirty times as great as that of mercury? If a pound of boiling water be mixed with 3 lbs. of ice-cold mercury, what will be the temperature of the mixture? 7. If a kilogramme of mercury at 120° is poured into a vessel containing 200 gm. of ice-cold water, what will be the temperature after the whole is mixed? How would the weight and material of the vessel affect the result? 8. How is the climate of the British Isles affected by the high capacity for heat of water? 9. 30 grammes of iron nails at 100° are dropped into 60 gm. of water at 13°2, and the final temperature is found to be 18°.6: what is the specific heat of the nails? 10. What is meant by the statement that the specific heat of platinum is 0.03? In order to ascertain the temperature of a furnace, a platinum ball weighing 80 grammes is introduced into it: when this has attained the temperature of the furnace it is quickly transferred to a vessel containing 400 grammes of water at 15°. The temperature of the water rises to 20°: what was the temperature of the furnace? CHAPTER VII CHANGE OF STATE-FUSION AND SOLIDIFICATION 41. Fusion.—We have seen that the application of heat to a solid causes a rise of temperature and an increase in size: in most cases, when the temperature is raised sufficiently, the solid is converted into a liquid. This change of state is called melting or fusion. Thus ice melts at o° to form water : sulphur melts to a yellow liquid when heated somewhat above the boiling-point of water. Crystals of iodine placed in a flask and gently heated over a flame melt to an almost black liquid; this on further heating undergoes a further change of state and boils, filling the flask with a splendid purple vapour. There are some substances, such as carbon and lime, which have not yet been fused because we are not able to heat them to a sufficiently high temperature. Others, such as iron, glass, sealing-wax, and pitch, before they become liquid pass through an intermediate stage in which they are plastic and can be easily moulded into any shape. It is in this intermediate stage that the glass-blower works glass before the blowpipe and the smith works and welds wrought-iron. 42. Melting-Points. But, in general, the change from the solid to the liquid state is well marked and occurs at a definite temperature. This is called the temperature of fusion or the melting-point of the substance. The reverse changesolidification—takes place at the same temperature, provided the liquid is stirred or shaken as it is cooled; and thus we speak of o° C. either as the melting-point of ice or the freezingpoint of water. As this is taken as one of the standard points |