ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXXVI.

(Continued from page 129.)

Vapours.-Aeriform fluids which arise from liquids by the absorption of caloric are called vapours; such as those produced from ether, alcohol, water, and mercury. Those liquids which possess the power of passing into the aeriform state are called volatile; and those which give out no vapour at any temperature are called fixed, as the fat oils. There are some solids, as ice, arsenic, camphor, and generally odorific substances, which give out vapours at once without passing through the liquid state. Vapours, like gases, are usually transparent and colourless; there are only a few coloured liquids whose vapours are also coloured.

Vaporisation. The passage of a body from the liquid to the vaporous state is known under the general name of vaporisation; but by this term is particularly understood the slow production of vapour at the surface of a liquid; and by ebullition, a rapid production of vapour in the mass itself. The latter is produced, under the ordinary pressure of the atmosphere, in the same manner as fusion, at a determinate temperature for each liquid. In the case of evaporation, the effects are different; for this process goes on at various temperatures in the same liquid. Yet beyond a certain point of refrigeration, all vaporisation appears to cease. Mercury, for example, gives out no vapour below -10° Centigrade; and sulphuric acid none below 30° Centigrade.

Elastic Force of Vapours.-Like gases, vapours have an elastic force, in consequence of which they act with a certain degree of pressure on the sides of the vessels which contain them. To prove the tension of vapours, and at the same time to render them sensible to the eye, a glass tube of siphon shape inverted is half filled with mercury, fig. 186, then a drop of Fig. 186.

ether is passed into the shorter branch, which is closed, and the tube is then immersed in a water-bath about 45° Centigrade. The mercury will now sink in the smaller branch, the space A B will be filled with a gas having entirely the appearance of air, and whose elastic force evidently balances the weight of the column of mercury CD, as well as the pressure of the atmosphere which acts on D; this gas is the vapour of ether. If we cool the water in the vessel, or if we withdraw the tube from it, which will produce the same effect, the vapour which filled the space A B will rapidly disappear, and the drop of ether will reappear. If, on the contrary, the waterbath be heated more and more, the level of the mercury will sink below the point B, and thus the tension will be increased.

VOL. V.

Formation of Vapou's in a Vacuum.-In the preceding experiment, the passage of the liquid into the state of vapour takes place slowly. The same thing happens also when a volatile liquid is freely exposed to the air. In both cases the atmospheric pressure is an obstacle to the vaporisation; but it is no longer so when the liquids are placed in a vacuum. The elastic force of vapours then meeting with no resistance, their formation is instantaneous. To show this, several barometric tubes are placed in the same cistern, fig. 187. These tubes

being filled with mercury, one of them, the tube A for instance, is employed as a barometer; then drops of water, alcohol, and ether, are introduced into the tubes B, D, and E respectively. It is observed that at the moment when the liquid enters the barometric vacuum in each of the tubes, the level of the mercury sinks, as shown in the figure. Now, it is not the weight of the liquid introduced which depresses the mercury; for this weight is only a very small fraction of that of the mercury displaced. There is therefore, in the case of each liquid, an instantaneous production of vapour, of which the elastic force acts upon the mercurial column. From this experiment it is also evident that the depression of the mercury is not the same in the three tubes; it is greater in the tube where the alcohol is, than in that where the water is; and greater in the tube where the ether is, than in either of the other two. We are thus enabled to state the following laws on the formation of vapours:

1st. In a vacuum, all volatile liquids vaporise instantaneously.

2nd. At the same temperature, the vapours of different liquids do not possess the same elastic force.

As an example of the second law, the tension of the vapour of ether is nearly twenty-three times greater than that of the vapour of water.

Maximum of Tension.-When a very small quantity of a volatile liquid, such as ether, is introduced into a barometric tube, it vaporises instantaneously and completely, and the column of mercury does not experience all the depression of which it is capable; for if another small quantity of ether be introduced, the depression will increase. By continuing this operation, a moment will at last arrive when the ether introduced into the tube will cease to vaporise and will remain in the liquid state. There is therefore, for a given temperature, a limit to the quantity of vapour which can be formed in a given space. In this case, the given space is said to be saturated. Moreover, at the instant when the vaporisation of the ether ceases, the depression of the mercury ceases. There

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is, therefore, a limit to the tension of the vapour, which varies with the temperature, but which, for a given temperature, is independent of pressure.

In order to show that, in a closed space saturated with vapour and containing liquid in excess, the temperature being constant, there is a maximum of tension which the vapour cannot pass, whatever may be the pressure, we employ a barometric tube immersed in a deep cistern, fig. 81, p. 277, vol. iv. This tube is first filled with mercury, and a quantity of ether is passed into the tube sufficient to saturate the barometric chamber; there is then some liquid in excess, and the height of the mercury in the tube is ascertained by means of a scale fixed to the cistern. Now, whether the tube be immersed to a greater depth, which tends to compress the vapour; or whether it be raised, which tends to expand it, the height of the mercurial column remains constant. The tension of the vapour, therefore, remains the same in both cases, since the depression neither increases nor diminishes it. Hence, it follows, that when the vapour contained in a saturated space is compressed, a part of it returns to the liquid state; and that if, on the contrary, the pressure is diminished, a portion of the liquid remaining in excess is vaporised, and the space occupied by the vapour is saturated anew; but in both cases, the tension and the density remain constant. If the space

where the vapour is contained be not saturated, or if it do not contain liquid in excess, the vapour, when the pressure increases or diminishes, acts entirely as a gas; that is, so long as it is not brought up to the point of saturation, its tension and its density increase with the pressure. Consequently, it is evident that vapours, in a space not saturated, act according to the law of Mariotte.

Tension of the Vapour of Water below the Freezing Point.-In order to measure the elastic force of the vapour of water below 0° Centigrade, Gay-Lussac employed two barometric tubes filled with mercury and immersed in the same cistern. One of these, completely freed from air and humidity, was used to measure the pressure of the atmosphere: into the other a small quantity of water was introduced, and its barometric chamber was surrounded with a small jacket, in which was placed a frigorific mixture. By comparing the heights of the two barometers when the temperature of the frigorific mixture stood at different points of the scale, Gay-Lussac found that in the barometer which contained the water, the depression of the mercury, and consequently the tension of the vapour, were as follows:

5:00

195

21.22

Hence it is inferred that, at very low temperatures, there is still vapour of water in the air.

Tension of the Vapour of Water from the Freezing to the Boiling Point. We shall first give the process adopted by Mr. Dalton, of Manchester, who died in 1844, in order to determine the elastic force of the vapour of water from 0° to 100° Centigrade. He employed two barometric tubes A and B, fig. 188, which were immersed in an iron vessel full of mercury and placed over a furnace.

The barometer B was freed from air and humidity, and in the barometer A was put a small quantity of water. These two barometers were kept in a vessel of glass full of water, in the middle of which was immersed a thermometer T, which indicated the temperature of the liquid. By gradually heating the iron vessel, and consequently the water in the glass vessel, that which was in the tube was vaporised; and in proportion as the tension of the vapour increased, the mercury was lowered. Then, by marking degree after degree on the scale E, the depression which took place in the tube A, below the level B in the other tube, Mr. Dalton determined the elastic force of the vapour of water at every point of the thermometer between the freezing and the boiling points, and was the first to construct a table of the same, as follows:

ture, as given by these eminent experimenters, is the following :B=(1+007153 T) 5;

in which E, denotes the elasticity of the steam in atmospheres; and T, the excess of the temperature above 100° Centigrade. The same formula adapted to Fahrenheit's thermometer is as follows:

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:

TABLE OF THE ELASTICITY OF STEAM IN

Temperature. Fahrenheit.

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

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 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.