in each of the experiments; and, taking the mean of the numbers so found, find the heat produced during the combination of one gramme of oxygen with marsh gas, and then apply a similar correction for the heat produced by the condensation of the vapour of water to that employed in exercise (198). 1353. Determination of the heat disengaged during the combination of olefiant gas with oxygen. 201. The olefiant gas employed in the experiments was prepared and purified by the usual processes. It was still found to contain 64 vols. of carbonic oxide in every 100 vols., in accordance with the observation first made by Dr. J. Davy. It is necessary, in reducing the results, to take into account the heat produced by the combustion of this portion of carbonic oxide. In order to insure the complete combustion of the gas, and at the same time to diminish the force of the explosion, nearly four and a half volumes of oxygen were taken for every volume of olefiant gas. 1st Experiment. M Mc 363 9 c. c. 2nd Experiment. 364.0 c. c. 363.1 c. c. 265 3 grams. V 28.1 grams. 28.1 grams. Determine from these numbers the heat evolved during the combination of one litre of dry olefiant gas, measured at 0° C., and under a pressure of 29.92 in., with oxygen, in each of the experiments; and, taking the mean of the two numbers so found, find the heat produced during the combination of one gramme of oxygen with olefiant gas; and then apply a similar correction for the heat produced by the condensation of the vapour of water to that employed in exercise (198). 1354. Combination of oxygen with solid and fluid bodies. -A considerable modification of the apparatus was required for the determination of the heat produced during the combination of solid and liquid substances with oxygen. The slowness of the combustion in most cases, made it necessary to operate upon a larger scale; and as the apparatus could no longer be inverted, it was also necessary to distribute the heat by a different method. 1355. Fig. 33 exhibits the general form of the apparatus. The combination took place in a copper vessel of about four litres capacity. The combustible was placed in a platina cup, shown in Fig. 34, which is suspended from the lid of the copper vessel by means of platina wires. A fourth wire, also of platina, but insulated by being surrounded by a glass tube, descends through an opening in the lid, and is connected below with the platina cup through the medium of a very fine platina wire, and above with a circular disc of copper, which is seen detached in Fig. 34, and in its proper position in Fig. 33. Before the commencement of an experiment, this disc was firmly fixed to the lid of the copper vessel, but it was also carefully insulated from it. Thus by bringing the disc and any other part of the copper vessel into contact with the opposite poles of a voltaic battery, the fine platina wire could be instantly ignited. 1356. In performing an experiment, the copper vessel was first filled with pure oxygen gas, the lid carrying the platina cup, &c., then introduced into its place, the copper disc attached to the lid, and its metallic connection with the insulated wire, c, carefully secured. The whole was next placed in the calorimeter, which contained the proper quantity of water, previously cooled to the required temperature, and weighed. The inner vessel was secured in its place by the vertical rod, a a. The calorimeter was covered with a lid containing apertures for the vertical rod and the thermometer, and the whole was surrounded by an outer vessel of tin plate, to prevent the effects of radiation. The details of the arrangement will be obvious from an inspection of Fig. 33. By means of the horizontal arm, c c, the inner vessel could be agitated through the water in the calorimeter. A pin, shown at b, restrained the motion of the vertical rod within such limits that the inner vessel was never permitted to rise during the agitation above the surface of the water in the calorimeter. Upon the sides and bottom of the inner vessel, small hollow knobs were placed, which maintained at all times a certain distance between the two vessels. 1357. Previous to the commencement of an experiment, the inner vessel was gently moved up and down till every part of the apparatus had acquired the same temperature. The ignition was effected by a similar method to that already described in the previous section, by bringing the vertical rod and the copper disc respectively into contact with the terminal wires of a galvanic arrangement. The same aperture in the lid served for the introduction of the thermometer, and, afterwards, of the galvanic wire. After the combination had begun, the inner vessel was gently moved up and down within the calorimeter for a sufficient period of time to allow, not only the combustion to be completed, but the heat thereby produced to be uniformly distributed through the whole of the apparatus. In every experiment, after the observation of the final temperature, the agitation was again repeated during two minutes, in order to ascertain positively that the whole of the heat had been obtained. 1358. The longer period of time occupied in these experiments rendered the corrections for the cooling and heating influence of the air of more importance than in the former observations. To determine with absolute accuracy the value of these corrections, under the varying circumstances of each experiment, would have been extremely difficult. Dr. Andrews therefore endeavoured so to arrange the experiments that the amount of correction to be applied in each case might be very small; so small, indeed, that the application of an imperfect approximation might be practically sufficient. From the effects of friction, the proximity of the person of the observer, and other causes, the rate of heating was always greater than the rate of cooling, for equal differences between the temperature of the air and of the apparatus; and for the same reasons, the latter was found to maintain a stationary temperature only when the thermometer in it indicated a temperature about 0°3 C. higher than that of the surrounding air. If we represent by a the difference between the temperature of the air and of the apparatus, the correction, V, for the gain or loss of heat sustained by the apparatus during m minutes, will be expressed by the formula, T=+mlat03)0°0025. 1359. The values of V given by this expression agree, within the ranges of temperature which occurred in these experiments, very closely with the direct results of observation. 1360. The usual time which elapsed between the observation of the initial and final temperatures was sixteen minutes; and in such cases it was assumed that the apparatus was at the minimum temperature during one-and-ahalf minute, at the maximum during eight minutes, and during the intermediate period at the temperature of the air. In other cases, where the combination took place more quickly, the corrections were made on the assumption that the apparatus was at the minimum point during one minute, and at the maximum during one-half of the whole time occupied by the experiment. EXERCISES. 1361. Determination of the heat disengaged during the combination of carbon and oxygen.-The carbon was em ployed in the form of wood-charcoal. It was purified by the method of M. Dumas from all oxidable matters, first by ebullition in strong nitro-muriatic acid, and afterwards by exposure for several hours at a strong red heat to the action of dry chlorine gas. To expel all volatile compounds, it was finally exposed to a strong white heat under a layer of charcoal. The earthy impurities, together with a certain portion of carbon (which, notwithstanding the great excess of oxygen, always escaped combustion), remained in the platina dish after each experiment. By deducting the weight of this residue from that of the carbon originally taken, the weight of the carbon consumed was immediately obtained. To obtain with accuracy the weight of the charcoal, it was introduced in the state of fine powder into the platina cup already referred to; and after being heated nearly to ignition, the latter was enclosed in a copper box, which, when covered by its lid, communicated with the external air only by a very small aperture. The whole was then allowed to cool in vacuo over sulphuric acid; and when cold, a stream of dry air was admitted into the receiver. The aperture in the lid being now closed, the weight of the entire was determined. 1362. To obtain complete combustion a very large excess of oxygen was employed; but even with this precaution, carbonic oxide was discovered in the residual gas in several of the following experiments. 202. In this and the two following exercises, M designates the weight of the substance burned; T, the temperature of the air, and the other letters the same quantities as in the previous exercises, 1st Experiment. 2nd Experiment. 1.177 grams, 10°.4 |