143 length, and the precise temperature of the vapour is not ture, as given by these eminent experimenters, is the follow- in which E, denotes the elasticity of the steam in atmospheres; in which e denotes the elasticity of the steam in atmospheres; and t the temperature above 212° Fahrenheit. The following table is derived from the table given by MM. Dulong and Arago, in their Report, and it is extended from twenty-four to fifty-three atmospheres, by calculation, according to the preceding formula: 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 178 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 the surrounding temperature. 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 one communicating with a manometer of free air o, near the elastic force of the vapour of water below 100° Centigrade. quently that in the vessel c. Then, by heating this vessel slowly, the water which it contains enters into a state of ebullition 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. 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 The elastic force of the vapours of attention of philosophers. 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. of this liquid be the same in both tubes. The globe and its stop-cock are now removed, and in their place is put the funnel c, furnished with a stop-cock a, which differs in its construction from the ordinary stop-cocks. It is not pierced through and through, but has only a small cavity as seen at on the right of the figure. Having poured into the funnel c the liquid which is to be vaporised, having marked the level I of the mercury and opened the stop-cock b, the stop-cock a is then turned in such a manner that the cavity is filled with the liquid; it is then turned again, so that the liquid may enter the space A, and there be vaporised. Thus the liquid is made to enter, drop by drop, until the air in the tube is saturated with vapour, which is ascertained by the level 1 ceasing to descend. As the tension of the vapour produced in the space A is added to that of the air already there, the volume is increased; but it is easily reduced to its original volume, by pouring an additional quantity of mercury into the tube B. When the mercury is by this means made to rise in the large tube to the level 1 which it had at first, there is observed in the tubes B and A a difference of level Bo, which evidently represents the tension of the vapour which has been produced; for the air having resumed its original volume, its tension is not changed. Now if some drops of the same liquid which was introduced into the space A be passed into the barometric vacuum, a depression exactly equal to Bo is observed; and this proves clearly that, at the same temperature, the tension of a vapour is the same in a gas as in a vacuum. 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 at different temperatures, has compared the tension of the vapour of water in air and in a vacuum. He found that it was always feebler in the former than in the latter case; but the differences were so trifling, that the law of Gay-Lussac is not diminished in its generality and value. Fig. 191. 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 eurious work, entitled Reflexions sur la puissance mécanique du feu, viz. that the same It is composed of a glass tube A, to the extremities of which quantity of heat can only are cemented two iron stop-cocks b and d. The lower stop- produce the same amount of labour, whatever may be the cock is furnished with a short tube, which puts the tube A in nature of the gas on which it acts, provided that no loss is communication with a second tube в of smaller diameter. A occasioned by improper or defective arrangements. Experience scale is placed between these two tubes, in order to measure has fully confirmed the theoretical considerations on which the height of the column of mercury contained in each. The this principle was founded by M. Carnot. In fig. 191, is repretube A is then filled with dry mercury, and the stop-cocks bsented a sort of jack or turnspit, which is very common in 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 Its mechanism is so several parts of the continent, and which is put in motion by Reflections on the Mechanical Power of Fire. |