Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/558

This page needs to be proofread.


508

CALORIMETRY

living o* rate of Ireat-supply about ^ to 6 watts. The water equi- electric current simultaneously, in such a manner as to secure valent of the calorimeter was about 85 grammes, and was deter- approximately the same rise of temperature for two or more mined by varying the quantity of water from 140 to 260 or 280 widely different values of the flow of liquid. An example taken grammes, so that the final results depended on a difference in the from the Electrician, September 1897, of one of the earliest weight of water of 120 to 140 grammes. The range of temperature experiments by this method on the specific heat of mercury will in each experiment was 14° to 26° C. The rate of rise was observed make the method clearer. The flow-tube was about 1 metre long with a mercury thermometer standardized by comparison with a and 1 millim. in diameter, coiled in a short spiral inside the platinum thermometer under the conditions of the experiment. vacuum jacket. The outside of the vacuum jacket was immersed The time of passing each division was recorded on an electric in a water jacket at a steady temperature equal to that of the chronograph. The duration of an experiment varied from about inflowing mercury. 30 to 70 minutes. Special observations were made to determine Specific Eeat of Mercury by continuous Electric Method. the corrections for the heat supplied by stirring, and that lost Specific Heat. by radiation, each of which amounted to about 10 per cent, of Plow of Hg. Rise of Temp. Watts. Heat-loss. the heat-supply. The calorimeter was gilded, and completely Per gm. deg. hdO d0 gm./sec. surrounded by a nickel-plated steel enclosure, forming the bulb •13780 joples 0-655 14*862 118-753 of a mercury thermo-regulator, immersed in a large water-bath, "03297 cals. 0-685 7-912 124-594 maintained at a constant temperature. In spite of the large corrections the results were extremely consistent, and the value It is assumed as a first approximation that the heat-loss is proof the temperature-coefficient of the diminution of the specific portional to the rise of temperature <70, provided that dO is nearly heat of water, deduced from the observed variation in the rate of the same in both cases, and that the distribution of temperature rise at different points of the range 15° to 25°, agreed with the in the apparatus is the same for the same rise of temperature value subsequently deduced from Rowland’s experiments over the whatever the flow of liquid. The result calculated on these same range, when his thermometers were reduced to the same scale. assumptions is given in the last column in joules, and also in Griffiths’ final result for the average value of the calorie over this calories at 20° C. The heat-loss in this example is large, nearly range was 4 "192 joules, taking the E.M.F. of the Clark cell at per cent, of the total supply, owing to the small flow and the 15° G. to be l-4342 volts. The difference from Rowland’s value, 4-5 rise of temperature, but this correction was greatly reduced 4-181, could be explained by supposing the E.M.F. of the Clark large subsequent observations on the specific heat of water by the cells to have in reality been 1-4323 volts, or about 2 millivolts in method. In the case of mercury the liquid itself can be less than the value assumed. Griffiths has since applied_ the same same to conduct the electric current. In the case of water or method to the measurement of the specific heat of aniline, and utilized other liquids it is necessary to employ a platinum wire stretched the latent heat of vaporization of benzene and water. as heating conductor. This introduces additional § 13. Schuster mid Ganwon.—The method employed by Schuster along the tube of construction, but does not otherwise affect the and Gannon for the determination of the specific heat of water in difficulties The absolute value of the specific heat deduced terms of the international electric units (Phil. Trans. A, 1895, p. method. depends on the absolute values of the electrical 415) corresponded to the expression EOT, and differed in many necessarily employed in the investigation. But for the determinaessential details from that of Griffiths. The current through a standards of relative values of specific heats in terms of a standard platinoid resistance of about 31 ohms in a calorimeter containing tion or of the variations of specific heat of a liquid, the method 1500 grammes of water was regulated so that the potential liquid, only on the constancy of the standards, which can be difference on its terminals was equal to that of twenty B.O.T. depends and accurately tested. The result found by this method Clark cells in series. The duration of an experiment was about readily the specific heat of water at 20° C. would agree with that of ten minutes, and the product of the mean current and the time, for within the probable limits of error, if the ohm were namely CT, was measured by the weight of silver deposited in a Rowland as correct, and the Clark cell at 15° C. were supposed to voltameter, which amounted to about 0'56 gramme. The uncer- taken an E.M.F. of 1-4333 volts. These values appear to be tainty due to the correction for the water equivalent was minimized have probable in the light of recent researches, but the more by making it small (about 27 grammes) in comparison with the extremely important of electrical methods to calorimetry is to water weight. The correction for external loss was reduced .by the relativeapplication measurements for which it is peculiarly adapted. employing a small rise of temperature (only 2-22°), and making § 15. Variation of Specific Heat of Water—The question of the the rate of heat-supply relatively rapid, nearly 24 watts. The variation specific heat of water has a peculiar interest and platinoid coil was insulated from the water by shellac varnish. importanceof the connexion with the choice of a thermal unit. The wire had a length of 760 cms., and the P.D. on its terminals Many of theinuncertainties the reduction of older experiments, was nearly 30 volts. The rate of stirring adopted was so slow such as those of Regnault,inarise uncertainty in regard to that the heat generated by it could be neglected. The result the unit in terms of which theyfrom are expressed, which again found was 4-191 joules per calorie at 19° C. This agrees very well depends on the scale of the particular thermometer employed in with Griffiths considering the difficulty of measuring so small a investigation. The first experiments of any value were those rise of temperature as 2° with a mercury thermometer. Admitting the Regnault in 1847 on the specific heat of water between 110 C. that the electro-chemical equivalent of silver increases with the of 192° C. They were conducted on a very large scale by the age of the solution, a fact which has since been discovered, and and of mixture, but showed discrepancies of the order of 0-5 that the E.M.F. of the Clark cell is probably less than 1;4340 method per cent., and the calculated results in many cases do not agree volts (the value assumed by Schuster and Gannon), there is no with the data. This may be due merely to deficient explanation difficulty in reconciling the result with that of Rowland. details of tabulation. We may probably take the tabulated § 14. Callendar and Barnes (Brit. Assoc. Reports, 1897 and 1899) of as showing correctly the rate of variation between 110 and adopted an entirely different method of calorimetry, as well as a values 190° C., but the values in terms of any particular thermal unit different method of electrical measurement. A steady current of must remain uncertain to at least 0 per cent, owing to tho liquid, Q grammes per second, of specific heat, Js joules per uncertainties of the thermometry. Regnault himself adopted the degree, flowing through a fine tube, 1 or 2 millims. in diameter, formula, . is heated by a steady electric current during its passage through =l + 0-000)04i + 0-000,000,9<2 (Regnault), (3) S the tube, and the difference of temperature dQ between the inflowing and the outflowing liquid is measured by a single read- for the specific heat s at any temperature t C. in terms of the ing with a delicate pair of differential platinum thermometers. specific heat at 0°C. taken as the standard. This formulaJias The difference of potential E between the ends of the tube, and since been very generally applied over the whole range 0 to the electric current C through it, are measured on a very 200° C., but the experiments could not in reality give any accurately calibrated potentiometer, in terms of a Clark cell and information with regard to the specific heat at temperatures a standard resistance. If M0 is the radiation loss in watts we below 100° C. The linear formula proposed by Bosscha from an independent reduction of Regnault’s experiments is probably have the equation, EC=JsQdO + hdO . . . (2). within the limits of accuracy between 100 and 200 C., so far as the mean rate of variation is concerned, but the absolute values The advantage of this method is that all the conditions are require reduction. It may be written— steady, so that the observations can be pushed to the limit of s=s100 +-00023 (£-100) (Bosscha-Regnault) (4). accuracy and sensitiveness Of the apparatus. The water The work of Pfaundler and Platter, of Hirn, of Jamin and equivalent of the calorimeter is immaterial, since there is no appreciable change of temperature. The heat-loss can be reduced Amaury, and of many other experimentalists who succeeded to a minimum by enclosing the flow-tube in a hermetically sealed Regnault, appeared to indicate much larger rates of increase than glass vacuum jacket. The insulation is the most perfect he had found, but there can be little doubt that the discrepancies possible. No stirring is required, if the tube, is sufficiently fine of their results, which often exceeded 5 per cent., were due to and the velocity of flow adequate. The conditions can be very lack of appreciation of the difficulties of calorimetric measureeasily varied through a wide range. The heat-loss MO is ments. The work of Rowland by the mechanical method was tne determined and eliminated by varying the flow of liquid and the first in which due attention was paid to the thermometry and to