Now turn D into position A, and set the pump working. When the difference of level of the mercury columns in C is 4 cms., turn the stop-cock into position C, turn off the water at tap T1, and, after waiting two minutes, read the thermometer and manometer three times at half-minute intervals. After taking the observations, turn on the water again, place D vertical with the dot to the right, and diminish the pressure 4 cms. further. Take readings as before. Repeat the observations at intervals of 4 cms. pressure till the temperature is about 75°; then with the pressure increasing again to that of the atmosphere. Take the mean of each set of three, correct it for the errors of the thermometer, and tabulate your results as follows: Now plot a curve with pressures as abscissae, and the corrected temperatures as ordinates, representing the points obtained with the pressure decreasing by a cross, those with the pressure increasing by a circle. The two sets of observations should not differ by more than about 1° from each other. If they do, sufficient time has not been allowed between each observation for the temperature of the steam to become constant. From the observations calculate the temperatures corresponding to pressures of 760, 720, 680 mms., etc., by graphical construction or as follows: Let 101 be the observed pressure which is nearest to the pressure p for which the temperature t of boiling is to be determined. Let p2 be a second observed pressure near p. Then if t1 and to be the temperatures of boiling corresponding to P1, P2 The second term will be less than one degree, if the observations have been taken according to the above instructions. Hence this term need only be calculated to one significant figure. Tabulate the results along with Regnault's as follows: SECTION XXIII. HYGROMETRY. Apparatus required: Daniell's hygrometer, wet and dry bulb thermometers, ether. The most important determination in connection with the aqueous vapour present in the atmosphere is that of the "relative humidity" or "fractional saturation," ie. the ratio of the amount of moisture actually present at any time in a given space, to the amount which would at that temperature saturate the space. This quantity may be determined directly by passing a known volume of moist air through tubes filled with drying materials and finding the increase in weight of these materials due to the moisture absorbed. It is determined in practice however by one of two methods:-dew point observations, or wet and dry bulb observations. In the first of these methods the air is cooled down till the vapour present in it begins to condense, and the temperature at which condensation begins is observed. The instrument used is called a dew point hygrometer. The form known as Daniell's hygrometer consists of a tube with a bulb at each end containing ether and vapour of ether only. The outer surface of the lower bulb is silvered, gilded or made of black glass and encloses a thermometer; the upper bulb is covered with muslin (Fig. 43). A thermometer fixed to the stem indicates the temperature of the air. Allow the instrument to stand for 10 or 15 minutes in a room at constant temperature. Then read the thermometer within the ball and that on the stem. Fig. 43. Determine the dew point at a quiet spot outside the laboratory, in the following way. Tilt the instrument till the ether has run into the lower bulb. Pour a little ether on the muslin surrounding the upper bulb. As this ether evaporates the bulb is cooled and the ether in its interior condensed. This reduces the pressure of ether vapour within the balls, and in consequence ether in the lower bulb evaporates and the ether and bulb are cooled. Watch carefully the polished surface of the lower bulb to see when moisture is first deposited on it. Keep the muslin saturated with ether, and occasionally shake the instrument so as to stir the liquid in the lower bulb. Note the temperature of the air and of the thermometer in the bulb when moisture first appears. The instant of appearance may be best detected by watching the image of some object in the polished surface of the bulb. Cease applying ether to the muslin and watch when the last sign of moisture disappears. Note again the temperatures and take the mean of the readings in each case. The mean temperature indicated by the thermometer in the bulb is the dew point. Find from tables the pressure of aqueous vapour at the dew point, and at the temperature of the air. The ratio of the first to the second is the relative humidity, or fractional saturation. Readings of thermometers after instrument had stood 15 min. in room: Thermometer in air = 19.2° C. The second method consists in observing the difference of readings of two thermometers, one with its bulb surrounded by a wet wick and the other with its bulb free. The arrangement is known as the wet and dry bulb thermometer (Fig. 44). The indications of the wet bulb do not depend, however, exclusively on the hygrometric state of the air, for if the air near the thermometers is in motion, evaporation is more rapid and the reading of the wet bulb lowered in consequence. The readings can only be interpreted by means of tables based on experiments, but it is found by comparison with more accurate hygrometers, that if the air round the thermometers is neither absolutely still nor violently disturbed, these tables give results which are sufficiently accurate for meteorological purposes. Compare the readings of the two thermometers before the muslin on the wet bulb is moistened. Saturate the muslin surrounding the wet bulb of the hygrometer provided, and the wick hanging Fig. 44. from it, with water, and place it in a position so that the air can flow across the bulbs of the thermometers. the air is still, suspend the instrument by a string from a fixed point, and set it slowly swinging as a pendulum. If The temperature of the wet bulb will be found to decrease. When it becomes steady read both the thermometers, and use table 35, Lupton, p. 30, to find the pressure of the aqueous vapour present in the air. Then divide by the pressure of saturated vapour at the temperature of the dry bulb. The quotient is the relative humidity, or fractional saturation. Record as follows: Reading of dry bulb after standing 15 min. in room = 17.5° C. bulb with dry muslin Difference Reading of dry bulb in air دو = 17.4° .. temperature of air as measured on other therms. 17.0° = |