tops of the tubes, and the temperatures were observed at different degrees from 0° to 50° Centigrade. By meaus of an agitator, the different strata of the liquid were constantly mixed with each other, in order to preserve a uniform temperature in all the parts of the bath in which the two barometric tubes were placed. Also a plate of glass fixed in the sides of the iron vessel enabled the experimenter to observe the difference of the level of the mercury in the two tubes. By this apparatus, M. Regnault accurately measured the elastic force of vapour from 0° to 50° Centigrade, but he could not employ it for higher temperatures, on account of the limited extent of the bath. Tension of Steam above the Boiling Point.-Two methods have been employed in measuring the elastic force of steam at higher temperatures than 100° Centigrade, the one by MM. Dulong and Arago in 1830; the other by M. Regnault in 1844. The apparatus of the former experimenters consisted of a boiler of very thick iron-plate, capable of holding 80 litres, or about 17% imperial gallons. Two gun barrels, closed at their lower extremity, were immersed in the water of the boiler, to the sides of which they were firmly fastened. Each barrel was filled with mercury, and contained a thermometer intended to show the temperature of the water and of the steam in the interior of the boiler. In order to measure the tension of the steam, the boiler was put in communication with a manometer of compressed air, which had been experimentally graduated. By noting degree after degree the temperatures indicated by the thermometers, and observing at the same time the indications of the manometer, these experimenters actually measured the tension of steam up to twenty-four atmospheres. They then determined by means of the following formula, temperatures and the pressures of steam as far as fifty atmospheres. These researches having been made at the instance of the Royal Academy of Sciences of Paris, a report of them was published in the Memoirs of the Academy," vol. x. 1831. The formula which connects the elasticity of steam with the tempera tion of the elasticity of steam above the boiling point, which The apparatus adopted by M. Regnault for the determinatension of the vapour of water either above or below 100° we now proceed to describe, admits of the measurement of the Centigrade. It consists in boiling water in a close vessel, under ebullition is effected. Then on the principle that at the instant a known pressure, and in measuring the temperature at which of ebullition the elastic force of the vapour or steam disengaged is exactly equal to the pressure which the liquid supports, we ascertain the tension of the steam or vapour and its corresponding temperature, which resolves the problem. The apparatus is composed of a brass vessel c, fig. 189, hermetically closed and filled with water to about one-third of its capacity. Four thermometers are inserted in the cover; two immersed in the upper strata of the liquid, and the other two in the lower strata. From the reservoir c, proceeds a tube A B, which is adapted to the orifice of a glass globe, having the capacity of twenty-four litres, or 5-28 imperial gallons, and filled with air. The tube AB is surrounded with a jacket D, in which circulates a current of cold water, which flows from a reservoir E. From the upper part of the globe м proceed two tubes, the one communicating with a manometer of free air o, near the apparatus, and the other HH', made of lead, communicating with an air-pump, or with a forcing-pump, according as the air in the globe is to be rarefied or compressed; and the reservoir K, which contains the globe, is filled with water at the surrounding temperature. Suppose now that the first experiments are to measure the elastic force of the vapour of water below 100° Centigrade. The extremity H' of the leaden pipe is fixed to the platen of the air-pump, and the air in the globe м is rarefied, and conse quently that in the vessel c. Then, by heating this vessel FROM 100° TO 230°-9 CENT., ACCORDING TO M. REGNAULT. TEMPERATURE. TEMPERATURE. slowly, the water which it contains enters into a state of ebulli- TABLE OF THE ELASTICITY OF STEAM IN ATMOSPHERES, tion at a temperature lower than 100° Centigrade, in proportion to the degree of rarefaction to which the air has been carried; that is, the pressure which acts upon the liquid is proportionably less. Moreover, the vapour condensing in the tube A B, which is constantly kept cooled to the same degree, the pressure originally indicated by the manometer is not increased; a fact which proves that the tension of the vapour, during the ebullition, remains equal to the pressure which acts upon the liquid. Then by consulting on the one side the manometer, and on the other the thermometers, the tension of the vapour at a known temperature is determined. Again, allowing a little air to enter into the tubes and into the boiler, in order to increase the pressure, a new observation is made, and so on, until the temperature of 100° Centigrade is attained. In order to measure the elastic force of the vapour of water above 100° Centigrade, the orifice H is put in communication with a forcing-pump, by means of which the air of the globe and of the boiler are subjected to successive pressures greater than that of the atmosphere. Thus the boiling of the water is retarded, and the simultaneous observation of the manometer and the thermometers shows the tension of vapour at temperatures higher than 100° Centigrade. The experiments of M. Regnault being among the latest that have been published, it will be useful to add here a table of the results at which he has arrived. 234) Pressure Pressure Atmos. in Atmos. Cent. 100° 0 Fahr. 2120.00 Cent. Fahr. 1 1980.8 389.84 15 120 6 249 08 273 .02 291 20 These tables show that the elastic force or pressure of the vapour of water and steam increases according to a certain law more rapidly than the temperature; but this law is not yet clearly ascertained. The table of M. Regnault differs from that of MM. Dulong and Arago; and, of course, the empirical formula given by these philosophers does not quite apply to the former; by calculation, this formula gives 27.22 atmospheres instead of 28, for the temperature of 230° 9 Centigrade or 447° 62 Fahrenheit. Water is the only liquid whose vapour, from its important applications, has engaged the attention of philosophers. The elastic force of the vapours of other liquids has not been determined with accuracy or to any extent. It is known, however, that substances in solution, as salts and acids, at the the same temperature as that of water, diminish the elastic force of the vapour of such mixtures, and this diminution increases in proportion as the solution becomes more concentrated; for ebullition then takes place only at a higher temperature. The following table will show the boiling points of water mixed with salt in certain proportions up to the point of saturation. LESSONS IN PHYSICS. Mixture of Gases and Vapours -The following are the laws which regulate the mixture of gases and vapours: 1st. The tension and consequently the quantity of the vapour which saturates a given space are the same, at the same temperature, whether this space contains a gas or is a vacuum. 2nd. The elastic force of the mixture is equal to the sum of the elastic forces of the gas and the vapour mixed together, the gas being referred to its primitive volume. In order to prove these laws, M. Gay-Lussac employed the apparatus represented in fig. 190. Fig. 190. A It is composed of a glass tube A, to the extremities of which are cemented two iron stop-cocks b and d. The lower stopcock is furnished with a short tube, which puts the tube A in communication with a second tube в of smaller diameter. scale is placed between these two tubes, in order to measure the height of the column of mercury contained in each. The tube A is then filled with dry mercury, and the stop-cocks and d being shut, we first screw on the stop-cock b, at the place of the funnel c, a glass globe filled with dry air or any other gas, and furnished also with a stop-cock which is closed. Next, opening the three stop-cocks, a part of the mercury is allowed to flow from the tube A, which is replaced by dry air from the globe. The stop-cocks are then closed, and as the air in the space a expands on issuing from the globe, and is under a pressure less than that of the atmosphere, it is forced back by pouring some mercury into the tube B, until the level of this liquid be the same in both tubes. The globe and its As to the second law, it is proved by the preceding experiment, because when the mercury has resumed its level 1, the mixture supports the atmospheric pressure which acts at the top of the tube F, plus the weight of the column of mercury BO. Now these two pressures exactly represent, the one the tension of the dry air, and the other the tension of the vapour. Whence, the second law may be considered as a consequence of the first. The apparatus which we have described only admits of experiments at the ordinary temperature; but M. Regnault, by means of an apparatus capable of being employed He found that it at different temperatures, has compared the tension of the vapour of water in air and in a vacuum. was always feebler in the former than in the latter case; but Applications. In addition to the well-known application of steam, it has often been proposed to employ vapours and gases of various kinds, as a moving power; as, for instance, by the expansion of heated air, or by the alternate vaporisation and liquefaction of different substances, such as ether, carbonic acid, etc. It may be useful, however, to mention here an important principle announced for the first time in 1824, by M. S. Carnot, in a small but curious work, entitled Reflexions sur la puissance mécanique du feu, viz. that the same quantity of heat can only nature of the gas on which it acts, provided that no loss is occasioned by improper or defective arrangements. Experience produce the same amount of labour, whatever may be the sented a sort of jack or turnspit, which is very common in has fully confirmed the theoretical considerations on which this principle was founded by M. Carnot. In fig. 191, is represeveral parts of the continent, and which is put in motion by the current of the air which ascends the chimney in consequence of the continual rarefaction of the lower strata which it contains, by the heat of the fire. evident as to require no particular description. This apparatus is described in the MSS. of Leonardo Da Vinci, who died in 1519, and its invention and use probably originated in a much remoter period. Its mechanism is so Reflections on the Mechanical Power of Fire. |