boiler; the tube at the bottom is connected with a condenser. Thus, on putting the top of the cylinder into connection with the boiler, a current of steam passes through the copper cylinder, raising it and the spiral inside to the temperature of 100°. If now we put the lower end of the spiral into communication with the boiler, the steam passes through the spiral, emerging through the nozzle. The spiral being kept hot at 100°, the steam inside it is freed from moisture and emerges from the nozzle in a dry state. The nozzle is connected with the spiral by means of a short piece of india-rubber tubing. This should be surrounded with cotton wool; the cylindrical heater is placed inside a wooden box, and surrounded with wool, or felt, or some other non-conducting substance. Sometimes it is more convenient to use the boiler itself to dry the steam; in this case the copper spiral is placed inside the boiler, from which one end emerges. The other end of the spiral inside the boiler is open above the level of the water. The steam, before emerging from the boiler, has to circulate through the spiral, and this dries it thoroughly. The calorimeter may conveniently take the form of a flask, or pear-shaped vessel, of thin copper, supported by silk threads inside another copper vessel. Its water equivalent must be determined in the same way as has been described in the section on specific heat (p. 276). In doing this, however, it must be remembered that the steam will probably raise the water to a temperature considerably higher than is the case in the determination of the specific heat of a metal. In like manner the temperature of the hot water used in finding the water equivalent should be considerably higher than that which was found most suitable in the previous experiments; it may with advantage be some 60° to 70°. Now water at this high temperature may cool considerably in being poured into the calorimeter, and care must be used to prevent loss of heat from this as far as possible. In allowing the steam to pass into the calorimeter the following method may be adopted: See that the steam passes freely from the nozzle, and note the temperature of the water in the calorimeter; pinch the india-rubber tube connecting the nozzle with the calorimeter for an instant, and immerse one end of the nozzle under the water, then allow the steam to flow until the temperature has risen about 20°. Raise the nozzle until its end is just above the level of the water in the calorimeter; again pinch the india-rubber tubing, stopping the flow of steam, and remove the calorimeter; note the highest point to which the temperature rises; this will be the value of 0, the common temperature. By pinching the tube as described above, the steam is prevented from blowing over the outer surface of the calorimeter. If, on the other hand, the tube be pinched and the flow stopped while the nozzle is under the water, the steam in the nozzle at the moment will be condensed, and the atmospheric pressure will drive some water up into the nozzle, and this will produce error. If the calorimeter is small there is some danger that the steam from the nozzle may flow directly on to the thermometer, and thus raise its temperature more than that of the surrounding water. This may be avoided by the use of a calorimeter of sufficient size. Another method of avoiding this error, and one which will lead to more accurate results, is the following, which has, however, the disadvantage of requiring more elaborate apparatus. The calorimeter contains a spiral tube of thin copper, ending in a closed vessel of the same material. This is completely surrounded by water, and the dry steam is passed through it instead of into the water. The water in the calorimeter is kept well stirred, and the heat given out by the steam in condensing is transmitted through the copper spiral and vessel to the water. The rise of temperature is noted as before, and when the temperature reaches its highest point, that is taken as the common temperature of the water, spiral, and calorimeter. The heat absorbed by the spiral and vessel is determined with the water equivalent; the quantity of water in the spiral at the end gives the mass of steam condensed. (See Regnault's paper on the 'Latent Heat of Steam.' Mémoires de l'Academie, T. XXI.) The calculation is proceeded with in the usual way. Experiment.-Determine the latent heat of steam. Enter the results as below: Weight of water in calorimeter 221'3 gms. Temp. of steam given by thermometer in heater 100° Common temp. of mixture. Water equivalent of cal. Latent heat of steam 41° C. 10.9 · 532.7 40. The Method of Cooling. To determine the Specific Heat of a Liquid. A known weight of the liquid is put into a copper vessel with a thermometer. This is hung by means of silk threads, like the calorimeter, inside another copper vessel which is closed by a lid with a cork in it supporting the thermometer. The exterior vessel is kept in a large bath of water at a known temperature, the bath being kept well stirred. It is intended to be maintained at the temperature of the room throughout the experiment; the bath is simply to ensure this. A small stirrer should pass through the cork which holds the thermometer, to keep the liquid well stirred. The outer surface of the inner vessel and the inner surface of the outer should be coated with lampblack. The liquid is heated up to, say, 70° or 80°, and then put into the calorimeter. Allow the liquid to cool, and note the intervals taken by it to cool, through, say, each successive degree. If the rate of cooling is too rapid to allow this to be done, note the intervals for each 5° or 10°, and calculate from these observations the mean rate of cooling for the range experimented on, say from 70° to 30o. Suppose we find that, on the average, it cools 3° in a minute. Then, if the liquid weigh 25 grammes and its specific heat be c, the quantity of heat which leaves it in one minute is 25x3xC. Now empty the liquid out from the calorimeter and perform a similar experiment with water instead. The water should fill the calorimeter to the same level, and be raised to the same temperature as the liquid previously used. Let us now suppose that there are 32 grammes of water, and that the temperature of the water falls through 19 of a degree in one minute; thus the quantity of heat which escapes from the water per minute is 32 × 9 units. The quantity of heat radiated from one surface at a given temperature to another at a constant lower temperature depends solely on the nature and material of the surfaces and the temperature of the warmer surface.1 In the two experiments described above, the surfaces are of the same nature; thus the rate at which heat escapes must be the same for the two experiments at the same tem peratures, .. 25×3×c=32 × '9, We can get the result required from the observations more quickly thus: Observe the time it takes the temperature to fall, say, from 60° to 55° in the two cases; let it be t1 minutes and tą minutes respectively. Then the fall of temperature per minute in the two cases respectively is 5/t, and 5/t2. The amount of heat which is transferred in the first case 1 See Garnett, Heat, ch. ix. Deschanel, Natural Philosophy. P. 399, &c. is 5c M1/t, and in the second it is 5M2/2, M1, M, being the masses of the liquid and the water respectively. Thus The effect of the vessel has hitherto been entirely neglected. Let k be its specific heat and m its mass, then in the first case the heat lost is Instead of calculating the quantity km, we may find by experiment the water equivalent of the vessel and thermometer and use it instead of km. Experiment.-Determine the specific heat of the given Mean specific heat (uncorrected for calorimeter) = 734 Correction for calorimeter Specific heat of liquid -'013 = *721 |