Since no reliable data can be obtained with regard to the values and variations of specific heats at extremely low temperatures, they are a.s.sumed for the purpose of our calculation to be in each case that of the gas, and to be constant under all conditions. Latent heats are utilised in every case when available.
With these reservations, the total energy, referred to absolute zero, of one pound of oxygen gas at normal temperature of 50 F. or 511 F.
(Abs.) will be approximately
(511 02175) + 100 = 211 Thermal Units Fahrenheit.
This in work units is roughly equivalent to
211 778 = 164,000 ft. lbs.
Adopting the same method with nitrogen gas, its energy at the same initial temperature will be, per unit ma.s.s,
174,600 ft. lbs.
There is thus a somewhat close resemblance, not only in the general temperature conditions but also in the energy conditions, of the two gases oxygen and nitrogen.
It will be readily seen, however, that under the same conditions the energy state of aqueous vapour differs very considerably from either, for by the same method as before the energy per pound of aqueous vapour is equal to
{(511 04) + 1080 + 144} 778 = 1,111,000 ft. lbs.
Under ordinary terrestrial atmospheric conditions, the energy of aqueous vapour per unit ma.s.s is thus nearly seven times as great as that of either oxygen or nitrogen gas. It is to be observed, also, that three-fourths of this energy of the vapour under the given conditions is present in the form of latent energy of the gas, or what we have already termed work energy.
The values of the various temperatures and other physical features, which we have included in the Table of Properties above, and which will be utilised throughout this discussion, are merely those in everyday use in scientific work. They form simply the accessible information on the respective materials. They are the records of phenomena, and on these phenomena are based our energy calculations. Further research may reveal the true values of other factors which up to the present we have been forced to a.s.sume, and so lead to more accurate computation of the energy in each case. Such investigation, however, is unlikely to affect in any way the general object of this part of the work, which is simply to portray in an approximate manner the relative energy properties of the three gaseous substances under certain a.s.sumed conditions.
36. _Comparative Alt.i.tudes of Planetary Atmospheres_
The total energy of equal ma.s.ses of the gases oxygen, nitrogen, and aqueous vapour, as estimated by the method above, are respectively in the ratios
1 : 106 : 68
Referring back once more to the phenomena described with reference to the gravitational equilibrium of a gas, let it be a.s.sumed that the gaseous substance liberated on the surface of the planetary body is oxygen, and that the planetary body itself is of approximately the same const.i.tution and dimensions as the earth. The oxygen gas thus liberated will expand against gravity, and envelop the planet in the manner already described (-- 34). Now the total energy of a ma.s.s of one pound of oxygen has been estimated under certain a.s.sumptions (-- 35) to be 164,000 ft. lbs. The value of the gravitative attraction of the planet on this ma.s.s is the same as under ordinary terrestrial conditions, so that if the entire energy of one pound of the gas were utilised in raising itself against gravity, the height through which this ma.s.s would be raised, and at which the material would attain the level of absolute zero of temperature, a.s.suming gravity constant with increasing alt.i.tude, would be simply 164,000 ft. or approximately 31 miles. The whole energy would not, of course, be expended in the expansive movement; only the outermost surface material of the planetary gaseous envelope attains to absolute zero of temperature. In estimating the alt.i.tude of this surface, however, the precise ma.s.s of gaseous substance a.s.sumed for the purpose of calculation is of little or no importance. Whatever may be the value of the ma.s.s a.s.sumed, its total energy and the gravitative attraction of the planetary body on it are both alike entirely and directly dependent on that ma.s.s value. It is therefore clear that no matter how the ma.s.s under consideration be diminished, the height at which its energy would be completely worked down, and at which its temperature would be absolute zero, is the same, namely 31 miles. At the planet"s surface, the total energy of an infinitesimally small portion of the gaseous ma.s.s is proportional to that ma.s.s. This amount of energy is, however, all that is available for transformation against gravitation in the ascent. But at the same time, the gravitative force on the particle, that force which resists its upward movement, is proportionately small corresponding to the small ma.s.s, so that the particle will in reality require to rise to the same alt.i.tude of 31 miles in order to completely transform its energy and attain absolute zero of temperature. When the expansive process is completed, the outer surface of the spherical gaseous envelope surrounding the planet is then formed of matter in this condition of absolute zero; this height of 31 miles is then the alt.i.tude or depth of the statical atmospheric column at a point on the planetary surface where the temperature is 50 F.
It is to be particularly noted that this height is entirely dependent on the gravitation, temperature, and energy conditions a.s.sumed.
With respect, also, to the a.s.sumption made above, of constant gravitation with increasing alt.i.tude, the variation in the value of gravity within the height limits in which the gas operates is so slight, that the energy of the expanding substance is completely worked down long before the variation appreciably affects the estimated alt.i.tude of absolute zero. In any case, bearing in mind the approximate nature of the estimate of the energy of the gases themselves, the variation of gravity is evidently a factor of little moment in our scheme of comparison.
Knowing the maximum height to be 31 miles, a uniform temperature gradient from the planetary surface to the outermost surface of the atmospheric material may be readily calculated. In the case of oxygen, the decrease of temperature with alt.i.tude will be at the rate of 16 F.
per mile, or 1 F. per 330 ft.
If the planetary atmosphere were composed of nitrogen instead of oxygen, the height of the statical atmospheric column under the given conditions would then be approximately
31 106 = 33 miles,
and the gradient of temperature 155 F. per mile.
In the case of aqueous vapour, which is possessed of much more powerful energy properties than either oxygen or nitrogen, the height of the statical column, to correspond to the energy of the material, is no less than 210 miles and the temperature gradient only 24 F. per mile.
Each of the gases, then, if separately a.s.sociated with the planetary body, would form an atmosphere around it depending in height on the peculiar energy properties of the gas. A point to be observed is that the actual or total ma.s.s of any gas thus liberated at the planet"s surface has no bearing on the ultimate height of the atmosphere which it would const.i.tute. When the expansive motion is completed, the density properties of the atmosphere would of course depend on the initial ma.s.s of gas liberated, but for any given value of gravity it is the energy properties of the gas per unit ma.s.s, or what might be termed its specific energy properties, which really determine the height of its atmosphere.
37. _Reactions of Composite Atmosphere_
It is now possible to deal with the case in which not only one gas but several gases are initially liberated on the planetary surface. Since the gases are different, then at the given surface temperature of the planet they possess different amounts of heat energy, and for each gas considered statically, the temperature-alt.i.tude gradient will be different from any of the others. The limiting height of the gaseous column for each gas, considered separately, will also depend on the total energy of that gas per unit ma.s.s, at the surface temperature. But it is evident that in a composite atmosphere, the separate statical conditions of several gases could not be maintained. In such a mixture, separate temperature-alt.i.tude gradients would be impossible. Absolute zero of temperature could clearly not be attained at more than one alt.i.tude, and it is evident that the temperature-alt.i.tude gradient of the mixture must, in some way, settle down to a definite value, and absolute zero of temperature must occur at some determinate height. This can only be brought about by energy exchanges and reactions between the atmospheric const.i.tuents. When these reactions have taken place, the atmosphere as a whole will have attained a condition a.n.a.logous to that of statical equilibrium (-- 34). Each of its const.i.tuents, however, will have decidedly departed from this latter condition. In the course of the mutual energy reactions, some will lose a portion of their energy.
Others will gain at their expense. All are in equilibrium as const.i.tuents of the composite atmosphere, but none approach the condition of statical equilibrium peculiar to an atmosphere composed of one gas only (-- 35). The precise energy operations which would thus take place in any composite atmosphere would of course depend in nature and extent on the physical properties of the reacting const.i.tuents. If the latter were closely alike in general properties, the energy changes are likely to be small. A strong divergence in energy properties will give rise to more powerful reactions. A concrete instance will perhaps make this more clear. Let it be a.s.sumed in the first place that the planetary atmosphere is composed of the two gases oxygen and nitrogen. From previous considerations, it will be clear that the natural decrease of temperature of nitrogen gas with increase of alt.i.tude is, in virtue of its slightly superior energy qualities, correspondingly slower than that of oxygen. The approximate rates are 155 F. and 16 F. per mile respectively. The tendency of the nitrogen is therefore to transmit a portion of its energy to the oxygen. Such a transmission, however, would increase the height of the oxygen column and correspondingly decrease the height of the nitrogen. When the balance is finally obtained, the height of the atmospheric column does not correspond to the energy properties of either gas, but to those of the combination. In the case of these two materials, oxygen and nitrogen, the energy reactions necessary to produce the condition of equilibrium are comparatively small in magnitude on account of the somewhat close resemblance in the energy properties of the two substances. On this account, therefore, the two gases might readily be a.s.sumed to behave as one gas composing the planetary atmosphere.
But what, then, will be the effect of introducing a quant.i.ty of aqueous vapour into an atmosphere this nature? The general phenomena will be of the same order as before, but of much greater magnitude. From the approximate figures obtained (---- 35, 36), the inherent energy of aqueous vapour per unit ma.s.s is seen to be, under the same conditions, enormously greater than that of the other two gases. In statical equilibrium (-- 34), the alt.i.tude of the gaseous column formed by aqueous vapour is almost seven times as great as that of the oxygen or nitrogen with which, in the composite atmosphere, it would be intermixed. In the given circ.u.mstances, then, aqueous vapour would be forced by these conditions to give up a very large portion of its energy to the other atmospheric const.i.tuents. The latter would thus be still further expanded against gravity; the aqueous vapour itself would suffer a loss of energy equivalent to the work transmitted from it. It is therefore clear that in a composite atmosphere formed in the manner described, any gas possessed of energy properties superior to the other const.i.tuents is forced of necessity to transmit energy to these const.i.tuents. This phenomenon is merely a consequence of the natural disposition of the atmospheric gaseous substances towards a condition of equilibrium with more or less uniform temperature gradation. The greater the inherent energy qualities of any one const.i.tuent relative to the others, the greater will be the quant.i.ty of energy transmitted from it in this way.
38. _Description of Terrestrial Case_
Bearing in mind the general considerations which have been advanced above with respect to planetary atmospheres, it is now possible to place before the reader a general descriptive outline of the circ.u.mstances and operation of an atmospheric machine in actual working. The machine to be described is that a.s.sociated with the earth.
In the earth is found an example of a planetary body of spheroidal form pursuing a clearly defined orbit in s.p.a.ce and at the same time rotating with absolutely uniform velocity about a central axis within itself. The structural details of its surface and the general distribution of material thereon will be more or less familiar to the reader, and it is not, therefore, proposed to dwell on these features here. Attention may be drawn, however, to the fact that a very large proportion of the surface of the earth is a liquid surface. Of all the material familiar to us from terrestrial experience there is none which enters into the composition of the earth"s crust in so large a proportion as water. In the free state, or in combination with other material, water is found everywhere. In the liquid condition it is widely distributed. Although the liquid or sea surface of the planet extends over a large part of the whole, the real water surface, that is, the _wetted_ surface, if we except perhaps a few desert regions, may be said to comprise practically the entire surface area of the planet. And water is found not only on the earth"s crust but throughout the gaseous atmospheric envelope. The researches of modern chemistry have revealed the fact that the atmosphere by which the earth is surrounded is not only a mixture of gases, but an exceedingly complex mixture. The relative proportions of the rarer gases present are, however, exceedingly small, and their properties correspondingly obscure. Taken broadly, the atmosphere may be said to be composed of air and water (in the form of aqueous vapour) in varying proportion. The former const.i.tuent exists as a mixture of oxygen and nitrogen gases of fairly constant proportion over the entire surface of the globe. The latter is present in varying amount at different points according to local conditions. This mixture of gaseous substances, forming the terrestrial atmosphere, resides on the surface of the planet and forms, as already described (-- 34), a column or envelope completely surrounding it; the quant.i.ty of gaseous material thus heaped up on the planetary surface is such that it exerts almost uniformly over that surface the ordinary atmospheric pressure of approximately 147 lb. per sq. inch. It is advisable, also, at this stage to point out and emphasise the fact that the planetary atmosphere must be regarded as essentially a material portion of the planet itself.
Although the atmosphere forms a movable sh.e.l.l or envelope, and is composed of purely gaseous material, it will still partake of the same complete orbital and rotatory axial motion as the solid core, and will also be subjected to the same external and internal influences of gravitation. Such are the general planetary conditions. Let us now turn to the particular phenomena of axial revolution.
In virtue of the unvarying rotatory movement of the planetary ma.s.s in the lines of the various incepting fields of its primary the sun, transformations of the axial or mechanical energy of the planet will be in continuous operation (---- 17-19). Although the gaseous atmospheric envelope of the planet partakes of this general rotatory motion under the influence of the incepting fields, the latter have apparently no action upon it. The sun"s influence penetrates, as it were, the atmospheric veil, and operating on the solid and liquid material below, provokes the numerous and varied transformations of planetary energy which const.i.tute planetary phenomena. At the equatorial band, where the velocity or axial energy properties of the surface material is greatest, these effects of transformation will naturally be most p.r.o.nounced. In the polar regions of low velocity they will be less evident. One of the most important of these transforming effects may be termed the heating action of the primary on the planet--a process which takes place in greater or less degree over the entire planetary surface, and which is the result of the direct transformation of axial energy into the form of heat (-- 18). In virtue of this heat transformation, or heating effect of the sun, the temperature of material on the earth"s surface is maintained in varying values from regions of high velocity to those of low--from equator to poles--according to lat.i.tude or according to the displacement of that material, in rotation, from the central axis. Owing to the irregular distribution of matter on the earth"s surface, and other causes to be referred to later, this variation in temperature is not necessarily uniform with the lat.i.tude. This heating effect of the sun on the earth will provoke on the terrestrial surface all the familiar secondary processes (-- 9) a.s.sociated with the heating of material. Most of these processes, in combination with the operations of radiation and conduction, will lead either directly or indirectly to the communication of energy to the atmospheric ma.s.ses (-- 27).
Closely a.s.sociated with the heat transformation, there is also in operation another energy process of great importance. This process is one of evaporative transformation. Reference has already been made to the vast extent of the liquid or wetted surface of the earth. This surface forms the seat of evaporation, and the action of the sun"s incepting influence on the liquid of this surface is to induce a direct transformation of the earth"s axial or mechanical energy into the elastic energy of a gas, or in other words into the form of work energy.
By this process, therefore, water is converted into aqueous vapour.
Immediately the substance attains the latter or gaseous state it becomes unaffected by, or transparent to, the incepting influence of the sun (-- 18). And the action of evaporation is not restricted in locality to the earth"s surface only. It may proceed throughout the atmosphere. Wherever condensation of aqueous vapour takes place and water particles are thereby suspended in the atmosphere, these particles are immediately susceptible to the sun"s incepting field, and if the conditions are otherwise favourable, re-evaporation will at once ensue. Like the ordinary heating action also, that of transformation will take place with greater intensity in equatorial than in polar regions. These two planetary secondary processes, of heating and evaporation, are of vital importance to the working of the atmospheric machine. But, as already pointed out elsewhere (---- 10, 32), every secondary operation is in some fashion linked to that machine. Other incepting influences, such as light, are in action on the planet, and produce transformations peculiar to themselves. These, in the meantime, will not be considered except to point out that in every case the energy active in them is the axial energy of the earth itself operating under the direct incepting influence of the sun. The general conditions of planetary revolution and transformation are thus intimately a.s.sociated with the operation of the atmospheric machine. In this machine is embodied a huge energy process, in the working of which the axial energy of the earth pa.s.ses through a series of energy changes which, in combination, form a complete cyclical operation. In their perhaps most natural sequence these processes are as follows:--
1. The direct transformation of terrestrial axial energy into the work energy of aqueous vapour.
2. The direct transmission of the work energy of aqueous vapour to the general atmospheric ma.s.ses, and the consequent elevation of these ma.s.ses from the earth"s surface against gravity.
3. The descent of the atmospheric air ma.s.ses in their movement towards regions of low velocity, and the return in the descent of the initially transformed axial energy to its original form.
The first of these processes is carried out through the medium of the aqueous material of the earth. It is simply the evaporative transformation referred to above. By that evaporative process a portion of the energy of motion or axial energy of the earth is directly communicated or pa.s.sed into the aqueous material. Its form, in that material, is that of work energy, or the elastic energy of aqueous vapour, and, as already pointed out, this process of evaporative transformation reaches its greatest intensity in equatorial or regions of highest velocity. In these regions also, in virtue of the working of the heat process already referred to above, the temperature conditions are eminently favourable to the presence of large quant.i.ties of aqueous vapour. The tension or pressure of the vapour, which really depends on the quant.i.ty of gaseous material present, is directly proportional to the temperature, so that in equatorial regions not only is the general action of transformation in the aqueous material most intense, but the surrounding temperature conditions in these regions are such as to favour the continuous presence of large quant.i.ties of the aqueous vapour which is the direct product of the action of transformation. The equatorial regions of the earth, or the regions of high velocity, are thus eminently adapted, by the natural conditions, to be the seat of the most powerful transformations of axial energy. As already pointed out, however, these same transformations take place over the entire terrestrial surface in varying degree and intensity according to the locality and the temperature or other conditions which may prevail. Now this transformation of axial energy which takes place through the medium of the evaporative process is a continuous operation. The energy involved, which pa.s.ses into the aqueous vapour, augmented by the energy of other secondary processes (-- 32), is the energy which is applied to the atmospheric air ma.s.ses in the second stage of the working of the atmospheric machine. Before proceeding to the description of this stage, however, it is absolutely necessary to point out certain very important facts with reference to the energy condition of the atmospheric const.i.tuents in the peculiar circ.u.mstances of their normal working.
39. _Relative Physical Conditions of Atmospheric Const.i.tuents_
It will be evident that no matter where the evaporation of the aqueous material takes place, it must be carried out at the temperature corresponding to that location, and since the aqueous vapour itself is not superheated in any way (being transparent to the sun"s influence), the axial energy transformed and the work energy stored in the material per unit ma.s.s, will be simply equivalent to the latent heat of aqueous vapour under the temperature conditions which prevail. In virtue of the relatively high value of this latent heat under ordinary conditions, the gas may be regarded as comparatively a very highly energised substance.
It is clear, however, that since the gas is working at its precise temperature of evaporation, the maximum amount of energy which it can possibly yield up at that temperature is simply this latent heat of evaporation, and if this energy be by any means withdrawn, either in whole or in part, then condensation corresponding to the energy withdrawal will at once ensue. The condition of the aqueous vapour is in fact that of a true vapour, or of a gaseous substance operating exactly at its evaporation temperature, and unable to sustain even the slightest abstraction of energy without an equivalent condensation. No matter in what manner the abstraction is carried out, whether by the direct transmission of heat from the substance or by the expansion of the gas against gravity, the result is the same; part of the gaseous material returns to the liquid form.
In the case of the more stable or permanent const.i.tuents of the atmosphere, namely oxygen and nitrogen, their physical conditions are entirely different from that of the aqueous vapour. Examination of the Table of Properties (p. 133) shows that the evaporation temperatures of these two substances under ordinary conditions of atmospheric pressure are as low as -296 F. and -320 F. respectively. At an ordinary atmospheric temperature of say 50 F. these two gases are therefore so far above their evaporation temperature that they are in the condition of what might be termed true gaseous substances. Although only at a temperature of 50 F., they may be truly described as highly superheated gases, and it is evident that they may be readily cooled from 50 F.
through wide ranges of temperature, without any danger of their condensation or liquefaction. Oxygen and nitrogen gases thus present in their physical condition and qualities a strong contrast to aqueous vapour, and it is this difference in properties, particularly the difference in evaporation temperatures, which is of vital importance in the working of the atmospheric machine. The two gases oxygen and nitrogen are, however, so closely alike in their general energy properties that, in the meantime, the atmospheric mixture of the two can be conveniently a.s.sumed to act simply as one gas--atmospheric air.
From these considerations of the ordinary atmospheric physical properties of air and aqueous vapour it may be readily seen how each is eminently adapted to its function in the atmospheric process. The peculiar duty of the aqueous vapour is the absorption and transmission of energy. Its relatively enormous capacity for energy, the high value of its latent heat at all ordinary atmospheric temperatures, and the fact that it must always operate precisely at its evaporation temperature makes it admirably suited for both functions. Thus, in virtue of its peculiar physical properties, it forms an admirable agent for the storage of energy and for its transmission to the surrounding air ma.s.ses. The low temperature of evaporation of these air ma.s.ses ensures their permanency in the gaseous state. They are thus perfectly adapted for expansive and other movements, for the conversion of their energy against gravity into energy of position, or for any other reactions involving temperature change without condensation.
40. _Transmission of Energy from Aqueous Vapour to Air Ma.s.ses_
The working of the second or transmission stage of the atmospheric machine involves certain energy operations in which gravitation is the incepting factor or agency. Let it be a.s.sumed that a ma.s.s of aqueous vapour liberated at its surface of evaporation by the transformation of axial energy, expands upwards against the gravitative attraction of the earth (---- 34, 38). As the gaseous particles ascend and thus gain energy of position, they do work against gravity. This work is done at the expense of their latent energy. Since the aqueous material is always working precisely at its evaporation temperature, this gain in energy of position and consequent loss of latent energy will be accompanied by an equivalent condensation and conversion of the rising vapour into the liquid form. This condensation will thus be the direct evidence and measure of work done by the aqueous material against the gravitational forces, and the energy expended or worked down in this way may now, accordingly, be regarded as stored in the condensed material or liquid particles in virtue of their new and exalted position above the earth"s surface. It is this energy which is finally transmitted to the atmospheric air ma.s.ses. The transmission process is carried out in the downward movement of the liquid particles. The latter, in their exalted positions, are at a low temperature corresponding to that position--that is, corresponding to the work done--and provided no energy were transmitted from them to the surrounding air ma.s.ses, their temperature would gradually rise during the descent by the transformation of this energy of position. In fact the phenomena of descent, supposing no transmission of energy from the aqueous material, would simply be the reverse of the phenomena of ascent. Since, however, the energy of position which the liquid particles possess is transmitted from them to the atmospheric ma.s.ses, then it follows that this natural increase in their temperature would not occur in the descent. A new order of phenomena would now appear. Since the evaporative process is a continuous one, the liquid particles in their downward movement must be in intimate contact with rising gaseous material, and these liquid particles will, accordingly, at each stage of the descent, absorb from this rising material the whole energy necessary to raise their temperature to the values corresponding to their decreasing elevation.
In virtue of this absorption of energy then, from the rising material, these liquid particles are enabled to reach the level of evaporation at the precise temperature of that level.
Now, considering the process as a whole, it will be readily seen that for any given ma.s.s of aqueous material thus elevated from and returned to a surface of evaporation, there must be a definite expenditure of energy (axial energy) at that surface. Since the material always regains the surface at the precise temperature of evaporation, this expenditure is obviously, in total, equal to the latent heat of aqueous vapour at the surface temperature. It may be divided into two parts. One portion of the axial energy--the transmitted portion--is utilised in the elevation of the material against gravity; the remainder is expended, as explained above, in the heating of the returning material. The whole operation takes place between two precise temperatures, a higher temperature, which is that of the surface of evaporation, and a lower temperature, corresponding to the work done, and so related to the higher that the whole of the energy expended by the working aqueous substance--in heating the returning material and in transmitted work--is exactly equivalent to the latent heat of aqueous vapour at the high or surface temperature. But, as will be demonstrated later, the whole energy transmitted from the aqueous material to the air ma.s.ses is finally returned in its entirety as axial energy, and is thus once more made available in the evaporative transformation process. The energy expended in raising the temperature of the working material returning to the surface of evaporation is obviously returned with that material.
Both portions of the original expenditure are thus returned to the source in different ways. The whole operation is, in fact, completely cyclical in nature; we are in reality describing "Nature"s Perfect Engine," which is completely reversible and which has the highest possible efficiency.[1] Although the higher temperature at the evaporation surface may vary with different locations of that surface, in every case the lower temperature is so related to it as to make the total expenditure precisely equal to the latent heat at that evaporation temperature.[2] It must be borne in mind also, that all the condensed material in the upper strata of the atmosphere must not of necessity return to the planetary liquid surface. On the contrary, immediately condensation of the aqueous vapour takes place and the material leaves the gaseous state, no matter where that material is situated, it is once more susceptible to the incepting influences of the sun. Re-evaporation may thus readily take place even at high alt.i.tudes, and complete cyclical operations may be carried out there. These operations will, however, be carried out in every case between precise temperature limits as explained above.