Dalton next increased the temperature by sur rounding the tube containing the mercurial column with a larger tube filled in succession with water of different temperatures; this pro duced for each temperature a difference in the depressiou of the height of the column; and when the water was at the temperature of 100', the depression, instead of being half an inch,was almost precisely three times as much. The cut represents the apparatus employed by Dr. Dalton, in which a, is the barom eter tube filled with mer cury to the height of f, and its lower end plunged into the basin of mercury c.
The graduated scale for measuring the height of the column is denoted by b. The larger tube around the barometer tube to con tain the water of different temperatures is denoted by d. A thermometer, e, is inserted at its upper end by which to ascertain the temperature of the enclosed water and, con sequently, that of the va yor within the barometer.
With this simple contriv ance, Dr. Dalton made a series of experiments to determine the repulsion of the atoms of steam; or, in other words, the elastic force of aqueous vapor, corresponding to the dif ferent degrees of Fahren heit's scale from zero up to the boiling point. To facilitate, the operations and to allow for any changes that might take place in the pressure of the atmosphere during the continuance of the experiment, another tube was placed beside the first, in the same basin, and the descent of the mercurial column of the first tube estimated from the top of that in the second, which, to render the measure more gradate, may be effected by means of a small telescope, sliding on an accu ated rod, and movable in a horizontal plane. By placing water of a given temperature within the outer tube and then gradually cooling it after each observation, and finally filling the same tube with freezing mixtures, a table similar to the following was constructed. Dalton's experiments, however, have been repeated with additional precautions by other scientists and particularly by M. Regnault, from whose work the table on page 997 has been compiled, giving the elastic force of aqueous vapor, in English inches of mercuiy, temperature of Fahrenheit's scale's. The first column of the table gives the temperature of the water and vapor in the Torri cellian vacuum for every ten degrees; the sec ond, the depression of the mercury or the elastic force of the vapor, corresponding to the several degrees of temperature of the first column. The remaining cdlumns give the depression of the mercury for the intermediate degrees, this arrangement being adopted to save space. For example, if we wish to know the elastic pressure of vapor at the temperature of 70°, by looking opposite to 70°, in the second column, we find 0.733 or nearly seven-tenths and a third inches of mercury. Again, if we wish the amount of repul sive force of the atoms of vapor at the tempera ture of 86°, we cast our eye along the line of 80°. until it comes under the 6°, which is at the top. of the table, aud find 1.242 or very nearly an. inch and a quarter as the height of a column of mercury which vapor of water will sustain with out being condensed into a liquid, at the temper ature of 86°. By looking along the table it will be seen that there are equal increments of elastic pressure. Thus, while the elastic force of vapor at 20° is sufficient to depress the mer curial column a little more than one-tenth of an inch, at 40° it supports nearly two and a half times as much, at 60° five times, at 80° ten times, and at 100° nineteen times. The reason of this. is not difficult to understand, since it is evident. that the elastic pressure of the vapor must be increased by the action of two causes: First, by increasing the temperature, the vapor tends to. expand just as air would do under the same cir cumstances; and second, by the same increase of temperature, a new portion of water is con verted into vapor, which, being forced into the same space, increases the density, and conse quently, the elasticity of the vapor which existed there before. Dr. Dalton also showed that there is a remarkable difference between vapor which exists over water, and vapor separated from the liquid from which it is produced. In the first case, as 1,ve have seen, eveiy increase of tempera ture causes the formation of a new quantity of vapor, which serves to increase the density aud consequently the repulsive energy of the vapor previously existing. Hence, as we have shown before, the expansive power of vapor or steam increases in a geometrical ratio, while the tem perature increases in an arithmetrical ratio, that is, an addition of a few degrees of heat produces more than a proportional degree of elastic force. The case, however, is very different with vapor separated from the water from which it is pro duced: it then obeys the same law as atmos pheric air, and increases in elasticity with equal additions of temperature. The atmosphere increases its elastic force by one four hundredth and ninetieth part for every degree of Fahren heit above the freezing point; the vapor of water follows the same law. The table as given is limited to 100°, and is sufficient for resolving problems relative to the hygrometrical condition of the atmosphere. It is, however, important for the use of the steam engineer that it should be extended to a much higher degree, and accordingly experiments have been made for this purpose by a number of persons, and par ticularly by M. Regnault at the expense of the French government. From that table we may see that, at the temperature of 212°, the elastic force of vapor balances thirty inches of mercury and is then just equal to the pressure of the atmosphere. This fact gives the explanation of the phenomenon of boiling, since the vapor formed at the temperature of 212° has just suffi cient repulsive power to expand beneath the pressure of the atmosphere, and to pass up in volumes through the water, giving it the peculiar agitation known as boiling. It is further evi dent from the same table that vapor is given off from ice, even at zero or 32° below freezing point; if, therefore, a lump of this substance in a cold day be placed under the receiver of an air pump, even when the apparatus is cooled down to zero, a portion of it will immediately spring into vapor, sufficient to fill the whole capacity of the cylinder, when the air is withdrawn; and if this vapor in its turn be reffioved by working the pump, another portion of the ice will pass into the state of vapor, and if the pressure of this be removed, another quantity of ice will be evapora ted ; and if the pumping be continued sufficiently long all the ice will be dissipated in vapor with out passing through the intermediate condition of water. Instead of continuing to work the pump,in order to evaporate the ice, we may pro duce the same effect by placing within the receiver a broad dish containing sulphuric acid, -which will absorb the vapor as fast as it is formed. We may, however, convince ourselves Inamediately of the evaporation of ice by expos ing a given weight of it during a cold day in the shade while the temperature is below freezing. It will be found sensibly, though slowly, to diminish in quantity. The same effect, how was furnished by Dr. Dalton, and has since been corrected by more refined experiments, is of great value in various branches of science. The very simplicity of the method employed is an evidence of scientific genius of the highest character, and is well calculated to excite our admiration, as well as to call forth our gratitude, on account of the important truths it reveals. Dr. Dalton, although thoroughly imbued with a love of science for its own sake, and a profound thinker, was eminently a practi cal man, in the proper sense of the term. He had not only the sagacity to frame significant questions to be propounded to Nature, but also the ingenuity to devise simple means by which the answers to these questions would be given in terms the most precise and accurate. Again, there is another circumstance in regard to vapor which is of essential importance in understand ing the part which it plays in producing the diversified changes of the weather, namely, the great amount of heat which it contains at dif ferent temperatures. It is well known that the quantity of heat that a body contains is not ever, is exhibited in the process of drying clothes in cold weather, which though they maybe stiff ened by the frozen water with which they have been wetted, soon become dry and pliable by the -evaporation of the ice. The apparatus of Dr. Dalton enables us to make the following experi ment, which has an important bearing on some of the phenomena of meteorology: If, while the column of mercury is at the temperature, for example, of 60', and a small quantity of water is resting on its upper end, the space above being -filled with vapor due to this temperature, we place under the lower end of the tube beneath the surface of the mercury a small crystal of -common salt it will rise through the mercury by its specific levity, and be dissolved in part or whole by the stratum of water at the top. Now, as soon as this solution begins to take -place, we shall see the column of mercury a,scend; a portion of the vapor will be absorbed, and the tension of the remainder be dimin ished. In this case, the attraction of the salt for the particles of water neutralizes a part of their repulsive force and thus dimin ishe,s the weight of mercury the vapor eau sup port. For the same reason, salt water boils at a temperature several degrees higher than 212°, though the vapor produced in this case has only the elastic force of that due to pure water. From the foregoing we conclude that the quan tity of vapor from the surface of the ocean is less and has less tension and density, than that from the surface of fresh water lakes, at the same temperature. The table which actually measured by the thermometer or the temperature which it exhibits; for example, if a cubic foot of air at 60° be expanded without receiving or losing heat, its temperature will be much diminished, because the same amount of heat which was before contained in a given space is now distributed through a larger space. If an ounce of steam from boiling water, which indicates a temperature of 212°, be condensed in water at 60°, it will give out to the latter enough heat to elevate six times the quantity of water to the boiling temperature: that is, six times as much water through 152°, or the same amount of water 912'; or, in other words, after having given out more than 900' of heat in the act of being converted from a vapor to a liquid, it still retains a temperature of 212°. The heat which is thus set free, and has not been recognized by the thermometer, is called latent heat. In thus condensing a given quantity of vapor, from water at different temperatures, in a given quantity of cold water, and noting the elevation of temperature of the latter, it has been shown hy Dr. Dalton and others that an ounce of vapor at all temperatures contains very nearly the same amount of heat, adding the latent and sensible heat together. This constancy of the amount of heat arises from the fact, that as we increase the thermometric heat a new portion of vapor is forced into the same space, its density increases, and the amount of latent heat is diminished; hence if the attenuated vapor from ice were received in a syringe, and suddenly condensed until its density became equal to that of boiling water, its temperature would be 212. On account of the great amount of latent heat of vapor, heat must be absorbed front) all sur rounding bodies during the process of evapora tion; and in all cases of the reverse process, that is, of the conversion of vapor into water, an equal amount of heat must be given out. This absorption of heat by vapor at the place of its formation, and the evolution of an equal amount at the place where it is condensed into water, is one of the most efficient means of varying the temperature of different portions of the earth from that which they would naturally acquire under the regular periodical variation due to the changes of declination of the sun. In the evaporation of a cubic foot of water it is known from experiment that an amount of heat is absorbed equal to that evolved from the combustion of twenty pounds of dry pine wood, and consequently every cubic foot of rain water which falls from the clouds leaves in the air above an equal amount of extraneous heat, which tends to abnormally raise the temperature due to the elevation, and to produce powerful upward currents above, and horizontal motions of the air below. We may also recall in this
place thg fact that water, in passing from the state of ice to that of a liquid, absorbs 140° of heat, which is again evolved in the act of freezing, and that this also is an efficient means by which colder portions of the earth are mollified in temperature. We are also indebted to Dr. Dalton for another important series of experiments, which relate to the mingling of air and vapor. In the experiments before given the vapor was weighed and its temperature and tension determined in a separate state and unnaingled with the air. To ascertain the effect which would be produced on the tension of vapor when suffered to be exerted in a space already occupied with air of different densities, Dr. Dalton employed the same method of experi menting previously described. A barometer tube was filled and inverted in a basin of mer cury, a quantity of air was then admitted, which, rising into the Torricellian vacuum, pressed by its elasticity on the surface of the mercury and caused it to descend a given num ber of divisions of the scale, which were accu rately noted: a small quantity of water was next admitted, which, rising to the top of the mer curial column, was, after a few moments, in part converted into vapor, while the mercury was observed to be depressed. When the expen ment was repeated with different quantities of air above the mercurial column and at different temperatures, produced by varying the heat of the water in the external tube, or, which would amount to the same thing, by varying the tem perature of the room, the remarkable fact was discovered that the depression of the mercurial column, due to the introduction of the water, was precisely the same at the same temperature as when the experiment was made with a vacuum; for example, at the temperature of 60°, whatever might be the elasticity of the air within the tube, the introduction of the water always gave an additional depression of half an inch. From this result the important fact is deduced, that the tension or elastic force of vapor in air is the same as that of vapor in a vacuum; from which we might also infer that the quantity of vapor which can exist in a given space already occupied with air is the same as that which cart exist in a vacuum at the same temperature. Now, this fact may be directly proved. How? A determinate result may be obtained by the fol lowing method, which also gives us an inde pendent means of determining directly the amount of vapor which exists in the atmosphere at a given time, and which may be employed for verifying the results obtained by other means: Let a tight cask, furnished with a stop cock near its lower part, be entirely tilled with water, and let the small end of a tube, which has been drawn out in a spirit lamp, be cemented into the vent hole above, so that no air can enter the cask ex cept through the tube. Let this tube be filled with coarsely powdered dry chloride of calcium— a substance which has a great affinity for moist ture—and the upper end put in connection with an open vessel containing air entirely saturated with moisture, which can readily be effected by agitating a quantity of the liquid in the vessel from which the air is drawn; let the stop-cock be now opened, and exactly a cubic foot of water be drawn into a measured vessel, it is evident that precisely a foot of air will enter the top of the cask through the tube and between the in terstices of the pieces of chloride of calcium, the moistaire will be absorbed and its weight can be accurately ascertained from the increase of weight of the tube and its contents, which had previously been weighed for that purpose. B3r this simple experiment, as well as by the one we. have previously given, we are enabled to con clusively prove that the weight of vapor con tained in the air, in a given space, is the same as. that which would exist at the same temperature in a vacuum. To render, however, the result of this experiment absolutely perfect, a slight cor rection must be made on account of the expan sion of the air and the vapor due to the increased repulsive energy of the compound over that of the air itself. This will be evident from a due consideration of what follows. If into an ex tensible vessel, such as an India-rubber bag filled with air, a little water be injected, the bag will be suddenly expanded by the additional repulsive force of the atoms of vapor. Previous to the introduction of the water the bag will be pressed equally on the outside and on the inside on the former by the weight of the external at mosphere, and on the latter by the repulsive or elastic force of' the atoms of the enclosed air;. when the water is introduced and a portion of it springs into vapor, the elastic force of the aqueous, atoms must be added to that of the atoms of the air, and the interior will then be pressed outward with. a force equal to the sum of the two repulsions. For example, if the experiment he made at sixty degrees and the air at its normal weight, the out ward pressure within the bag previous to the introduction of the water will be equal to thirty inches of inercmy, but after the water is injected_ it will be thirty inches and a half ; hence expan sion will take place and the bag vvill be distended_ until, by the separation of the interior atoms, the repulsion is so much weakened that the pressure without and within will again be equalized. The amount of the increase in bulk will be given by the following proportion: as the pressure of thirty inches of mercury is to the pressure of thirty and one-half inches, so is the original bulk of the India-rubber bag to its bulk after the intro duction of the vapor. From experiments and_ observations it is evident that in free air the vapor exists as an independent atmosphere, being the same in weight and in tension as it would be in a vacuum of the same extent and the same tem perature. That the same amount of vapor can exist in a space filled with air as in a vacuum, at first sight appears paradoxical, but when we consider that a cubic inch of water expanded into steam at 212° occupies nearly 1,700 times the bulk which it does in the form of water, also that air may be compressed into a space many hundred times less than that of its ordinary bulk, it is evident that the extent of the void spaces is incomparably greater than the atoms themselves, and, consequently, it is not difficult to conceive that the atoms of the vapor have abundance of space in which to exist between the atoms of air and the atoms of air between those of vapor. Dr. Dalton announces this important truth by stating that air and vapor and almost all gases are vacuums to each other. This enunciation is a true expression of the state of diffusion which gases and vapors attain after the lapse of a given time, but it does not truly express the phenomena of the act of diffusion. In at perfect vacuum 24 given space is filled with vapor almost instanta neously, or with a rapidity which has not yet been estimated, but this is not the same in a space already filled with air. In this case, though the vapor ultimately diffuses itself through the air as it would do in a vacuum, yet time is required to produce this effect; the result is as if there were a mechanical or some other ob struction to the free passage of vapor through the different strata of air, and, indeed, it would appear front the following experiments that a definite force similar to that produced by a slight attraction or repulsion, is offered in the resistance of a given thickness of this medium: In the lab oratory of the Smithsonian Institution, a glass tube of about three feet in length, closed at its lower end, suspended vertically, and containing about an inch of water, has remained for several years undisturbed in this condition, without the least perceptible diminution in the amount of the liquid. In another experiment, a pane of glass was removed front an external window of a room, and the place of the glass supplied by a board, through the middle of which a hole of about an inch in diameter was made, and in this opening a tube was placed horizontally, one end being in the rtoom• and the other in the outer air. To each end of this tube a glass bulb was attached, air tight, the one within the room containing about an ounce of water, while the tube and the bulb on the outside were occupied with air. The temperature of the air within the room was, on an average, about seventy degrees, while that of the air without was, on an average, nearly thirty two degrees, and although the experiment was continued for several months during the winter, not one drop of water was distilled over into the outer bulb. When, however, the latter was sur rounded by a freezing mixture, a small quantity of vapor did pass over and was condensed into water and aLso when the vapor as contained in the outer bulb was absorbed by introducing a quantity of strong sulphuric acid into this bulb, the water in the other bulb gradually diminished in weight. From these experiments it would ap pear that there is more than a mechanical ob struction to the transfusion ot vapor through air, and that if the difference of tension of vapor in two vessels only amounts to a certain quanti ty, no transfusion from one will take place to the other, or, in other words, for each inch or foot of thickness of a stratum of air, a certain amount of unbalanced repulsive energy is required for transfusion. The rapid mingling of vapor with air is due, in a considerable degree, to the cur rents produced by the mixture itself, and by variations of temperature. It is not upon the actual amount of vapor which the air contains at a given time or place that its humidity de pends; but upon its greater or less degree of sat uration. That air is said to be dry in which evaporation takes place rapidly from a surface of water or moistened substance. In an atmos phere entirely saturated with vapor, that is, in one which is filled with as much vapor as the space which it occupies can contain, the vapor already in the air by its elastic force presses on the surface of the moist body and neutralizes the repulsive action of the water; if, however, the temperature be raised, the elastic force will be increased and a new portion will be forced. into the same space; the further, therefore, the con dition of any portion of air is from saturation the more rapid will be the evaporation from the moist bodies which it surrounds. For example, a portion of air at a temperature of 100° would contain vapor of an elastic force, were it entirely saturated, equal to a pressure of two and a half inches of mercury. If the same air, however, only contained vapor of the elastic force of 60°, or, in other words, if the dewpoint was at 60°, the elastic force would be half an inch, and con sequently there would be a force unbalanced by the pressure of vapor equal to the pressure of a column of two inches of mercury. The dryness, therefore, of the air is estimated by the difference of the elastic force of the vapor due to the tem perature of the air, and of the elastic force due to the tension of the dewpoint. In meteorologi cal works generally, when a portion of the atmosphere contains vapor equal in tension to that of the temperature of the air, it is said to be, as we have before observed, fully saturated, and its humidity is marked 100; but if the elastic force of the air as determined by the dewpoint is only one-fourth of that necessary to produce complete saturation, the relative humidity is marked 25. To find, then, the relative humidity at any time, we seek from the tables the tension of vapor due to the temperature of the air, and again due to that temperature to which it must next be cooled down in order to produce precipi tation, or full saturation, which temperature, as we have seen, is that of the dew point. We then say, as the tension of the first temperature is to 100, so is the tension of the other temperature to the per centage of saturation. In this way com parative tables of relative humidity for different places are calculated from actual observation.