MRS. B.
It is because the circulation of particles has nearly produced an equilibrium of temperature between the liquid in the glass and that in the phial.
CAROLINE.
But these communicate laterally, and I thought that heat in liquids could be propagated only upwards.
MRS. B.
You do not take notice that the heat is imparted from one liquid to the other, through the medium of the phial itself, the external surface of which receives the heat from the water in the glass, whilst its internal surface transmits it to the liquid it contains. Now take the phial out of the hot water, and observe the effect of its cooling.
EMILY.
The currents are reversed; the external current now descends, and the internal one rises.—I guess the reason of this change:—the phial being in contact with cold air instead of hot water, the external particles are cooled instead of being heated; they therefore descend and force up the central particles, which, being warmer, are consequently lighter.
MRS. B.
It is just so. Count Rumford hence infers that no alteration of temperature can take place in a fluid, without an internal motion of its particles, and as this motion is produced only by the comparative levity of the heated particles, heat cannot be propagated downwards.
But though I believe that Count Rumford’s theory as to heat being incapable of pervading fluids is not strictly correct, yet there is, no doubt, much truth in his observation, that the communication is materially promoted by a motion of the parts; and this accounts for the cold that is found to prevail at the bottom of the lakes in Switzerland, which are fed by rivers issuing from the snowy Alps. The water of these rivers being colder, and therefore more dense than that of the lakes, subsides to the bottom, where it cannot be affected by the warmer temperature of the surface; the motion of the waves may communicate this temperature to some little depth, but it can descend no further than the agitation extends.
EMILY.
But when the atmosphere is colder than the lake, the colder surface of the water will descend, for the very reason that the warmer will not.
MRS. B.
Certainly: and it is on this account that neither a lake, nor any body of water whatever, can be frozen until every particle of the water has risen to the surface to give off its caloric to the colder atmosphere; therefore the deeper a body of water is, the longer will be the time it requires to be frozen.
EMILY.
But if the temperature of the whole body of water be brought down to the freezing point, why is only the surface frozen?
MRS. B.
The temperature of the whole body is lowered, but not to the freezing point. The diminution of heat, as you know, produces a contraction in the bulk of fluids, as well as of solids. This effect, however, does not take place in water below the temperature of 40 degrees, which is 8 degrees above the freezing point. At that temperature, therefore, the internal motion, occasioned by the increased specific gravity of the condensed particles, ceases; for when the water at the surface no longer condenses, it will no longer descend, and leave a fresh surface exposed to the atmosphere: this surface alone, therefore, will be further exposed to its severity, and will soon be brought down to the freezing point, when it becomes ice, which being a bad conductor of heat, preserves the water beneath a long time from being affected by the external cold.
CAROLINE.
And the sea does not freeze, I suppose, because its depth is so great, that a frost never lasts long enough to bring down the temperature of such a great body of water to 40 degrees?
MRS. B.
That is one reason why the sea, as a large mass of water, does not freeze. But, independently of this, salt water does not freeze till it is cooled much below 32 degrees, and with respect to the law of condensation, salt water is an exception, as it condenses even many degrees below the freezing point. When the caloric of fresh water, therefore, is imprisoned by the ice on its surface, the ocean still continues throwing off heat into the atmosphere, which is a most signal dispensation of Providence to moderate the intensity of the cold in winter.
CAROLINE.
This theory of the non-conducting power of liquids, does not, I suppose, hold good with respect to air, otherwise the atmosphere would not be heated by the rays of the sun passing through it?
MRS. B.
Nor is it heated in that way. The pure atmosphere is a perfectly transparent medium, which neither radiates, absorbs, nor conducts caloric, but transmits the rays of the sun to us without in any way diminishing their intensity. The air is therefore not more heated, by the sun’s rays passing through it, than diamond, glass, water, or any other transparent medium.
CAROLINE.
That is very extraordinary! Are glass windows not heated then by the sun shining on them?
MRS. B.
No; not if the glass be perfectly transparent. A most convincing proof that glass transmits the rays of the sun without being heated by them is afforded by the burning lens, which by converging the rays to a focus will set combustible bodies on fire, without its own temperature being raised.
EMILY.
Yet, Mrs. B., if I hold a piece of glass near the fire it is almost immediately warmed by it; the glass therefore must retain some of the caloric radiated by the fire? Is it that the solar rays alone pass freely through glass without paying tribute? It seems unaccountable that the radiation of a common fire should have power to do what the sun’s rays cannot accomplish.
MRS. B.
It is not because the rays from the fire have more power, but rather because they have less, that they heat glass and other transparent bodies. It is true, however, that as you approach the source of heat the rays being nearer each other, the heat is more condensed, and can produce effects of which the solar rays, from the great distance of their source, are incapable. Thus we should find it impossible to roast a joint of meat by the sun’s rays, though it is so easily done by culinary heat. Yet caloric emanated from burning bodies, which is commonly called culinary heat, has neither the intensity nor the velocity of solar rays. All caloric, we have said, is supposed to proceed originally from the sun; but after having been incorporated with terrestrial bodies, and again given out by them, though its nature is not essentially altered, it retains neither the intensity nor the velocity with which it first emanated from that luminary; it has therefore not the power of passing through transparent mediums, such as glass and water, without being partially retained by those bodies.
EMILY.
I recollect that in the experiment on the reflection of heat, the glass skreen which you interposed between the burning taper and the mirror, arrested the rays of caloric, and suffered only those of light to pass through it.
CAROLINE.
Glass windows, then, though they cannot be heated by the sun shining on them, may be heated internally by a fire in the room? But, Mrs. B., since the atmosphere is not warmed by the solar rays passing through it, how does it obtain heat; for all the fires that are burning on the surface of the earth would contribute very little towards warming it?
EMILY.
The radiation of heat is not confined to burning bodies: for all bodies, you know, have that property; therefore, not only every thing upon the surface of the earth, but the earth itself, must radiate heat; and this terrestrial caloric, not having, I suppose, sufficient power to traverse the atmosphere, communicates heat to it.
MRS. B.
Your inference is extremely well drawn, Emily; but the foundation on which it rests is not sound; for the fact is, that terrestrial or culinary heat, though it cannot pass through the denser transparent mediums, such as glass or water, without loss, traverses the atmosphere completely: so that all the heat which the earth radiates, unless it meet with clouds or any foreign body to intercept its passage, passes into the distant regions of the universe.
CAROLINE.
What a pity that so much heat should be wasted!
MRS. B.
Before you are tempted to object to any law of nature, reflect whether it may not prove to be one of the numberless dispensations of Providence for our good. If all the heat which the earth has received from the sun, since the creation had been accumulated in it, its temperature by this time would, no doubt, have been more elevated than any human being could have borne.
CAROLINE.
I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at such a high temperature, might have scorched our feet, we should always have had a cool refreshing air to breathe, since the radiation of the earth does not heat the atmosphere.
EMILY.
The cool air would have afforded but very insufficient refreshment, whilst our bodies were exposed to the burning radiation of the earth.
MRS. B.
Nor should we have breathed a cool air; for though it is true that heat is not communicated to the atmosphere by radiation, yet the air is warmed by contact with heated bodies, in the same manner as solids or liquids. The stratum of air which is immediately in contact with the earth is heated by it; it becomes specifically lighter and rises, making way for another stratum of air which is in its turn heated and carried upwards; and thus each successive stratum of air is warmed by coming in contact with the earth. You may perceive this effect in a sultry day, if you attentively observe the strata of air near the surface of the earth; they appear in constant agitation, for though it is true the air is itself invisible, yet the sun shining on the vapours floating in it, render them visible, like the amber dust in the water. The temperature of the surface of the earth is therefore the source from whence the atmosphere derives its heat, though it is communicated neither by radiation, nor transmitted from one particle of it to another by the conducting power; but every particle of air must come in contact with the earth in order to receive heat from it.
EMILY.
Wind then by agitating the air should contribute to cool the earth and warm the atmosphere, by bringing a more rapid succession of fresh strata of air in contact with the earth, and yet in general wind feels cooler than still air?
MRS. B.
Because the agitation of the air carries off heat from the surface of our bodies more rapidly than still air, by occasioning a greater number of points of contact in a given time.
EMILY.
Since it is from the earth and not the sun that the atmosphere receives its heat, I no longer wonder that elevated regions should be colder than plains and valleys; it was always a subject of astonishment to me, that in ascending a mountain and approaching the sun, the air became colder instead of being more heated.
MRS. B.
At the distance of about a hundred million of miles, which we are from the sun, the approach of a few thousand feet makes no sensible difference, whilst it produces a very considerable effect with regard to the warming the atmosphere at the surface of the earth.
CAROLINE.
Yet as the warm air rises from the earth and the cold air descends to it, I should have supposed that heat would have accumulated in the upper regions of the atmosphere, and that we should have felt the air warmer as we ascended?
MRS. B.
The atmosphere, you know, diminishes in density, and consequently in weight, as it is more distant from the earth; the warm air, therefore, rises only till it meets with a stratum of air of its own density; and it will not ascend into the upper regions of the atmosphere until all the parts beneath have been previously heated. The length of summer even in warm climates does not heat the air sufficiently to melt the snow which has accumulated during the winter on very high mountains, although they are almost constantly exposed to the heat of the sun’s rays, being too much elevated to be often enveloped in clouds.
EMILY.
These explanations are very satisfactory; but allow me to ask you one more question respecting the increased levity of heated liquids. You said that when water was heated over the fire, the particles at the bottom of the vessel ascended as soon as heated, in consequence of their specific levity: why does not the same effect continue when the water boils, and is converted into steam? and why does the steam rise from the surface, instead of the bottom of the liquid?
MRS. B.
The steam or vapour does ascend from the bottom, though it seems to arise from the surface of the liquid. We shall boil some water in this Florence flask, (Plate IV. Fig. 1.) in order that you may be well acquainted with the process of ebullition;—you will then see, through the glass, that the vapour rises in bubbles from the bottom. We shall make it boil by means of a lamp, which is more convenient for this purpose than the chimney fire.
Vol. I. p. 84.
see text and caption
Fig. 2. Boiling water in a flask over a Patent lamp.
Larger view (complete Plate)
EMILY.
I see some small bubbles ascend, and a great many appear all over the inside of the flask; does the water begin to boil already?
MRS. B.
No; what you now see are bubbles of air, which were either dissolved in the water, or attached to the inner surface of the flask, and which, being rarefied by the heat, ascend in the water.
EMILY.
But the heat which rarefies the air inclosed in the water must rarefy the water at the same time; therefore, if it could remain stationary in the water when both were cold, I do not understand why it should not when both are equally heated?
MRS. B.
Air being much less dense than water, is more easily rarefied; the former, therefore, expands to a great extent, whilst the latter continues to occupy nearly the same space; for water dilates comparatively but very little without changing its state and becoming vapour. Now that the water in the flask begins to boil, observe what large bubbles rise from the bottom of it.
EMILY.
I see them perfectly; but I wonder that they have sufficient power to force themselves through the water.
CAROLINE.
They must rise, you know, from their specific levity.
MRS. B.
You are right, Caroline; but vapour has not in all liquids (when brought to the degree of vaporization) the power of overcoming the pressure of the less heated surface. Metals, for instance, mercury excepted, evaporate only from the surface; therefore no vapour will ascend from them till the degree of heat which is necessary to form it has reached the surface; that is to say, till the whole of the liquid is brought to a state of ebullition.
EMILY.
I have observed that steam, immediately issuing from the spout of a teakettle, is less visible than at a further distance from it; yet it must be more dense when it first evaporates, than when it begins to diffuse itself in the air.
MRS. B.
When the steam is first formed, it is so perfectly dissolved by caloric, as to be invisible. In order however to understand this, it will be necessary for me to enter into some explanation respecting the nature of SOLUTION. Solution takes place whenever a body is melted in a fluid. In this operation the body is reduced to such a minute state of division by the fluid, as to become invisible in it, and to partake of its fluidity; but in common solutions this happens without any decomposition, the body being only divided into its integrant particles by the fluid in which it is melted.
CAROLINE.
It is then a mode of destroying the attraction of aggregation.
MRS. B.
Undoubtedly.—The two principal solvent fluids are water, and caloric. You may have observed that if you melt salt in water, it totally disappears, and the water remains clear, and transparent as before; yet though the union of these two bodies appears so perfect, it is not produced by any chemical combination; both the salt and the water remain unchanged; and if you were to separate them by evaporating the latter, you would find the salt in the same state as before.
EMILY.
I suppose that water is a solvent for solid bodies, and caloric for liquids?
MRS. B.
Liquids of course can only be converted into vapour by caloric. But the solvent power of this agent is not at all confined to that class of bodies; a great variety of solid substances are dissolved by heat: thus metals, which are insoluble in water, can be dissolved by intense heat, being first fused or converted into a liquid, and then rarefied into an invisible vapour. Many other bodies, such as salt, gums, &c. yield to either of these solvents.
CAROLINE.
And that, no doubt, is the reason why hot water will melt them so much better than cold water?
MRS. B.
It is so. Caloric may, indeed, be considered as having, in every instance, some share in the solution of a body by water, since water, however low its temperature may be, always contains more or less caloric.
EMILY.
Then, perhaps, water owes its solvent power merely to the caloric contained in it?
MRS. B.
That, probably, would be carrying the speculation too far; I should rather think that water and caloric unite their efforts to dissolve a body, and that the difficulty or facility of effecting this, depend both on the degree of attraction of aggregation to be overcome, and on the arrangement of the particles which are more or less disposed to be divided and penetrated by the solvent.
EMILY.
But have not all liquids the same solvent power as water?
MRS. B.
The solvent power of other liquids varies according to their nature, and that of the substances submitted to their action. Most of these solvents, indeed, differ essentially from water, as they do not merely separate the integrant particles of the bodies which they dissolve, but attack their constituent principles by the power of chemical attraction, thus producing a true decomposition. These more complicated operations we must consider in another place, and confine our attention at present to the solutions by water and caloric.
CAROLINE.
But there are a variety of substances which, when dissolved in water, make it thick and muddy, and destroy its transparency.
MRS. B.
In this case it is not a solution, but simply a mixture. I shall show you the difference between a solution and a mixture, by putting some common salt into one glass of water, and some powder of chalk into another; both these substances are white, but their effect on the water will be very different.
CAROLINE.
Very different indeed! The salt entirely disappears and leaves the water transparent, whilst the chalk changes it into an opaque liquid like milk.
EMILY.
And would lumps of chalk and salt produce similar effects on water?
MRS. B.
Yes, but not so rapidly; salt is, indeed, soon melted though in a lump; but chalk, which does not mix so readily with water, would require a much greater length of time; I therefore preferred showing you the experiment with both substances reduced to powder, which does not in any respect alter their nature, but facilitates the operation merely by presenting a greater quantity of surface to the water.
I must not forget to mention a very curious circumstance respecting solutions, which is, that a fluid is not nearly so much increased in bulk by holding a body in solution, as it would by mere mixture with the body.
CAROLINE.
That seems impossible; for two bodies cannot exist together in the same space.
MRS. B.
Two bodies may, by condensation, occupy less space when in union than when separate, and this I can show you by an easy experiment.
This phial, which contains some salt, I shall fill with water, pouring it in quickly, so as not to dissolve much of the salt; and when it is quite full I cork it.—If I now shake the phial till the salt is dissolved, you will observe that it is no longer full.
CAROLINE.
I shall try to add a little more salt.—But now, you see, Mrs. B., the water runs over.
MRS. B.
Yes; but observe that the last quantity of salt you put in remains solid at the bottom, and displaces the water; for it has already melted all the salt it is capable of holding in solution. This is called the point of saturation; and the water in this case is said to be saturated with salt.
EMILY.
I think I now understand the solution of a solid body by water perfectly: but I have not so clear an idea of the solution of a liquid by caloric.
MRS. B.
It is probably of a similar nature; but as caloric is an invisible fluid, its action as a solvent is not so obvious as that of water. Caloric, we may conceive, dissolves water, and converts it into vapour by the same process as water dissolves salt; that is to say, the particles of water are so minutely divided by the caloric as to become invisible. Thus, you are now enabled to understand why the vapour of boiling water, when it first issues from the spout of a kettle, is invisible; it is so, because it is then completely dissolved by caloric. But the air with which it comes in contact, being much colder than the vapour, the latter yields to it a quantity of its caloric. The particles of vapour being thus in a great measure deprived of their solvent, gradually collect, and become visible in the form of steam, which is water in a state of imperfect solution; and if you were further to deprive it of its caloric, it would return to its original liquid state.
CAROLINE.
That I understand very well. If you hold a cold plate over a tea-urn, the steam issuing from it will be immediately converted into drops of water by parting with its caloric to the plate; but in what state is the steam, when it becomes invisible by being diffused in the air?
MRS. B.
It is not merely diffused, but is again dissolved by the air.
EMILY.
The air, then, has a solvent power, like water and caloric?
MRS. B.
This was formerly believed to be the case. But it appears from more recent enquiries that the solvent power of the atmosphere depends solely upon the caloric contained in it. Sometimes the watery vapour diffused in the atmosphere is but imperfectly dissolved, as is the case in the formation of clouds and fogs; but if it gets into a region sufficiently warm, it becomes perfectly invisible.
EMILY.
Can any water dissolve in the atmosphere without its being previously converted into vapour by boiling?
MRS. B.
Unquestionably; and this constitutes the difference between vaporization and evaporation. Water, when heated to the boiling point, can no longer exist in the form of water, and must necessarily be converted into vapour or steam, whatever may be the state and temperature of the surrounding medium; this is called vaporization. But the atmosphere, by means of the caloric it contains, can take up a certain portion of water at any temperature, and hold it in a state of solution. This is simply evaporation. Thus the atmosphere is continually carrying off moisture from the surface of the earth, until it is saturated with it.
CAROLINE.
That is the case, no doubt, when we feel the atmosphere damp.
MRS. B.
On the contrary, when the moisture is well dissolved it occasions no humidity: it is only when in a state of imperfect solution and floating in the atmosphere, in the form of watery vapour, that it produces dampness. This happens more frequently in winter than in summer; for the lower the temperature of the atmosphere, the less water it can dissolve; and in reality it never contains so much moisture as in a dry hot summer’s day.
CAROLINE.
You astonish me! But why, then, is the air so dry in frosty weather, when its temperature is at the lowest?
EMILY.
This, I conjecture, proceeds not so much from the moisture being dissolved, as from its being frozen; is not that the case?
MRS. B.
It is; and the freezing of the watery vapour which the atmospheric heat could not dissolve, produces what is called a hoar frost; for the particles descend in freezing, and attach themselves to whatever they meet with on the surface of the earth.
The tendency of free caloric to an equilibrium, together with its solvent power, are likewise connected with the phenomena of rain, of dew, &c. When moist air of a certain temperature happens to pass through a colder region of the atmosphere, it parts with a portion of its heat to the surrounding air; the quantity of caloric, therefore, which served to keep the water in a state of vapour, being diminished, the watery particles approach each other, and form themselves into drops of water, which being heavier than the atmosphere, descend to the earth. There are also other circumstances, and particularly the variation in the weight of the atmosphere, which may contribute to the formation of rain. This, however, is an intricate subject, into which we cannot more fully enter at present.
EMILY.
In what manner do you account for the formation of dew?
MRS. B.
Dew is a deposition of watery particles or minute drops from the atmosphere, precipitated by the coolness of the evening.
CAROLINE.
This precipitation is owing, I suppose, to the cooling of the atmosphere, which prevents its retaining so great a quantity of watery vapour in solution as during the heat of the day.
MRS. B.
Such was, from time immemorial, the generally received opinion respecting the cause of dew; but it has been very recently proved by a course of ingenious experiments of Dr. Wells, that the deposition of dew is produced by the cooling of the surface of the earth, which he has shown to take place previously to the cooling of the atmosphere; for on examining the temperature of a plot of grass just before the dew-fall, he found that it was considerably colder than the air a few feet above it, from which the dew was shortly after precipitated.
EMILY.
But why should the earth cool in the evening sooner than the atmosphere?
MRS. B.
Because it parts with its heat more readily than the air; the earth is an excellent radiator of caloric, whilst the atmosphere does not possess that property, at least in any sensible degree. Towards evening, therefore, when the solar heat declines, and when after sunset it entirely ceases, the earth rapidly cools by radiating heat towards the skies; whilst the air has no means of parting with its heat but by coming into contact with the cooled surface of the earth, to which it communicates its caloric. Its solvent power being thus reduced, it is unable to retain so large a portion of watery vapour, and deposits those pearly drops which we call dew.
EMILY.
If this be the cause of dew, we need not be apprehensive of receiving any injury from it; for it can be deposited only on surfaces that are colder than the atmosphere, which is never the case with our bodies.
MRS. B.
Very true; yet I would not advise you for this reason to be too confident of escaping all the ill effects which may arise from exposure to the dew; for it may be deposited on your clothes, and chill you afterwards by its evaporation from them. Besides, whenever the dew is copious, there is a chill in the atmosphere which it is not always safe to encounter.
CAROLINE.
Wind, then, must promote the deposition of dew, by bringing a more rapid succession of particles of air in contact with the earth, just as it promotes the cooling of the earth and warming of the atmosphere during the heat of the day?
MRS. B.
Yes; provided the wind be unattended with clouds, for these accumulations of moisture not only prevent the free radiation of the earth towards the upper regions, but themselves radiate towards the earth; under these circumstances much less dew is formed than on fine clear nights, when the radiation of the earth passes without obstacle through the atmosphere to the distant regions of space, whence it receives no caloric in exchange. The dew continues to be deposited during the night, and is generally most abundant towards morning, when the contrast between the temperature of the earth and that of the air is greatest. After sunrise the equilibrium of temperature between these two bodies is gradually restored by the solar rays passing freely through the atmosphere to the earth; and later in the morning the temperature of the earth gains the ascendency, and gives out caloric to the air by contact, in the same manner as it receives it from the air during the night.—Can you tell me, now, why a bottle of wine taken fresh from the cellar (in summer particularly), will soon be covered with dew; and even the glasses into which the wine is poured will be moistened with a similar vapour?
EMILY.
The bottle being colder than the surrounding air, must absorb caloric from it; the moisture therefore which that air contained becomes visible, and forms the dew which is deposited on the bottle.
MRS. B.
Very well, Emily. Now, Caroline, can you inform me why, in a warm room, or close carriage, the contrary effect takes place; that is to say, that the inside of the windows is covered with vapour?
CAROLINE.
I have heard that it proceeds from the breath of those within the room or the carriage; and I suppose it is occasioned by the windows which, being colder than the breath, deprive it of part of its caloric, and by this means convert it into watery vapour.
MRS. B.
You have both explained it extremely well. Bodies attract dew in proportion as they are good radiators of caloric, as it is this quality which reduces their temperature below that of the atmosphere; hence we find that little or no dew is deposited on rocks, sand, water; while grass and living vegetables, to which it is so highly beneficial, attract it in abundance—another remarkable instance of the wise and bountiful dispensations of Providence.
EMILY.
And we may again observe it in the abundance of dew in summer, and in hot climates, when its cooling effects are so much required; but I do not understand what natural cause increases the dew in hot weather?
MRS. B.
The more caloric the earth receives during the day, the more it will radiate afterwards, and consequently the more rapidly its temperature will be reduced in the evening, in comparison to that of the atmosphere. In the West-Indies especially, where the intense heat of the day is strongly contrasted with the coolness of the evening, the dew is prodigiously abundant. During a drought, the dew is less plentiful, as the earth is not sufficiently supplied with moisture to be able to saturate the atmosphere.
CAROLINE.
I have often observed, Mrs. B., that when I walk out in frosty weather, with a veil over my face, my breath freezes upon it. Pray what is the reason of that?
MRS. B.
It is because the cold air immediately seizes on the caloric of your breath, and, by robbing it of its solvent, reduces it to a denser fluid, which is the watery vapour that settles on your veil, and there it continues parting with its caloric till it is brought down to the temperature of the atmosphere, and assumes the form of ice.
You may, perhaps, have observed that the breath of animals, or rather the moisture contained in it, is visible in damp weather, or during a frost. In the former case, the atmosphere being over-saturated with moisture, can dissolve no more. In the latter, the cold condenses it into visible vapour; and for the same reason, the steam arising from water that is warmer than the atmosphere, becomes visible. Have you never taken notice of the vapour rising from your hands after having dipped them into warm water?
CAROLINE.
Frequently, especially in frosty weather.
MRS. B.
We have already observed that pressure is an obstacle to evaporation: there are liquids that contain so great a quantity of caloric, and whose particles consequently adhere so slightly together, that they may be rapidly converted into vapour without any elevation of temperature, merely by taking off the weight of the atmosphere. In such liquids, you perceive, it is the pressure of the atmosphere alone that connects their particles, and keeps them in a liquid state.
CAROLINE.
I do not well understand why the particles of such fluids should be disunited and converted into vapour, without any elevation of temperature, in spite of the attraction of cohesion.
MRS. B.
It is because the degree of heat at which we usually observe these fluids is sufficient to overcome their attraction of cohesion. Ether is of this description; it will boil and be converted into vapour, at the common temperature of the air, if the pressure of the atmosphere be taken off.
EMILY.
I thought that ether would evaporate without either the pressure of the atmosphere being taken away, or heat applied; and that it was for that reason so necessary to keep it carefully corked up?
MRS. B.
It is true it will evaporate, but without ebullition; what I am now speaking of is the vaporization of ether, or its conversion into vapour by boiling. I am going to show you how suddenly the ether in this phial will be converted into vapour, by means of the air-pump.—Observe with what rapidity the bubbles ascend, as I take off the pressure of the atmosphere.
CAROLINE.
It positively boils: how singular to see a liquid boil without heat!
MRS. B.
Now I shall place the phial of ether in this glass, which it nearly fits, so as to leave only a small space, which I fill with water; and in this state I put it again under the receiver. (Plate IV. Fig. 1.)* You will observe, as I exhaust the air from it, that whilst the ether boils, the water freezes.
Vol. I. p. 84.
see text and caption
Fig. 1. Ether evaporated & water frozen in the air pump. A Phial of Ether. B Glass vessel containing water. C.C Thermometers, one in the Ether, the other in the water.
Larger view (complete Plate)
CAROLINE.
It is indeed wonderful to see water freeze in contact with a boiling fluid!
EMILY.
I am at a loss to conceive how the ether can pass to the state of vapour without an addition of caloric. Does it not contain more caloric in a state of vapour, than in a state of liquidity?
MRS. B.
It certainly does; for though it is the pressure of the atmosphere which condenses it into a liquid, it is by forcing out the caloric that belongs to it when in an aëriform state.
EMILY.
You have, therefore, two difficulties to explain, Mrs. B.—First, from whence the ether obtains the caloric necessary to convert it into vapour when it is relieved from the pressure of the atmosphere; and, secondly, what is the reason that the water, in which the bottle of ether stands, is frozen?
CAROLINE.
Now, I think, I can answer both these questions. The ether obtains the addition of caloric required, from the water in the glass; and the loss of caloric, which the latter sustains, is the occasion of its freezing.
MRS. B.
You are perfectly right; and if you look at the thermometer which I have placed in the water, whilst I am working the pump, you will see that every time bubbles of vapour are produced, the mercury descends; which proves that the heat of the water diminishes in proportion as the ether boils.
EMILY.
This I understand now very well; but if the water freezes in consequence of yielding its caloric to the ether, the equilibrium of heat must, in this case, be totally destroyed. Yet you have told us, that the exchange of caloric between two bodies of equal temperature, was always equal; how, then, is it that the water, which was originally of the same temperature as the ether, gives out caloric to it, till the water is frozen, and the ether made to boil?
MRS. B.
I suspected that you would make these objections; and, in order to remove them, I enclosed two thermometers in the air-pump; one which stands in the glass of water, the other in the phial of ether; and you may see that the equilibrium of temperature is not destroyed; for as the thermometer descends in the water, that in the ether sinks in the same manner; so that both thermometers indicate the same temperature, though one of them is in a boiling, the other in a freezing liquid.
EMILY.
The ether, then, becomes colder as it boils? This is so contrary to common experience, that I confess it astonishes me exceedingly.
CAROLINE.
It is, indeed, a most extraordinary circumstance. But pray, how do you account for it?
MRS. B.
I cannot satisfy your curiosity at present; for before we can attempt to explain this apparent paradox, it is necessary to become acquainted with the subject of LATENT HEAT: and that, I think, we must defer till our next interview.
CAROLINE.
I believe, Mrs. B., that you are glad to put off the explanation; for it must be a very difficult point to account for.
MRS. B.
I hope, however, that I shall do it to your complete satisfaction.
EMILY.
But before we part, give me leave to ask you one question. Would not water, as well as ether, boil with less heat, if deprived of the pressure of the atmosphere?
MRS. B.
Undoubtedly. You must always recollect that there are two forces to overcome, in order to make a liquid boil or evaporate; the attraction of aggregation, and the weight of the atmosphere. On the summit of a high mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat is required to make water boil, than in the plain, where the weight of the atmosphere is greater.* Indeed if the weight of the atmosphere be entirely removed by means of a good air-pump, and if water be placed in the exhausted receiver, it will evaporate so fast, however cold it maybe, as to give it the appearance of boiling from the surface. But without the assistance of the air-pump, I can show you a very pretty experiment, which proves the effect of the pressure of the atmosphere in this respect.
Observe, that this Florence flask is about half full of water, and the upper half of invisible vapour, the water being in the act of boiling.—I take it from the lamp, and cork it carefully—the water, you see, immediately ceases boiling.—I shall now dip the flask into a bason of cold water.†
CAROLINE.
But look, Mrs. B., the hot water begins to boil again, although the cold water must rob it more and more of its caloric! What can be the reason of that?
MRS. B.
Let us examine its temperature. You see the thermometer immersed in it remains stationary at 180 degrees, which is about 30 degrees below the boiling point. When I took the flask from the lamp, I observed to you that the upper part of it was filled with vapour; this being compelled to yield its caloric to the cold water, was again condensed into water—What, then, filled the upper part of the flask?
EMILY.
Nothing; for it was too well corked for the air to gain admittance, and therefore the upper part of the flask must be a vacuum.
MRS. B.
The water below, therefore, no longer sustains the pressure of the atmosphere, and will consequently boil at a much lower temperature. Thus, you see, though it had lost many degrees of heat, it began boiling again the instant the vacuum was formed above it. The boiling has now ceased, the temperature of the water being still farther reduced; if it had been ether, instead of water, it would have continued boiling much longer, for ether boils, under the usual atmospheric pressure, at a temperature as low as 100 degrees; and in a vacuum it boils at almost any temperature; but water being a more dense fluid, requires a more considerable quantity of caloric to make it evaporate quickly, even when the pressure of the atmosphere is removed.
EMILY.
What proportion of vapour can the atmosphere contain in a state of solution?
MRS. B.
I do not know whether it has been exactly ascertained by experiment; but at any rate this proportion must vary, both according to the temperature and the weight of the atmosphere; for the lower the temperature, and the greater the pressure, the smaller must be the proportion of vapour that the atmosphere can contain.
To conclude the subject of free caloric, I should mention Ignition, by which is meant that emission of light which is produced in bodies at a very high temperature, and which is the effect of accumulated caloric.
EMILY.
You mean, I suppose, that light which is produced by a burning body?
MRS. B.
No: ignition is quite independent of combustion. Clay, chalk, and indeed all incombustible substances, may be made red hot. When a body burns, the light emitted is the effect of a chemical change which takes place, whilst ignition is the effect of caloric alone, and no other change than that of temperature is produced in the ignited body.
All solid bodies, and most liquids, are susceptible of ignition, or, in other words, of being heated so as to become luminous; and it is remarkable that this takes place pretty nearly at the same temperature in all bodies, that is, at about 800 degrees of Fahrenheit’s scale.
EMILY.
But how can liquids attain so high a temperature, without being converted into vapour?
MRS. B.
By means of confinement and pressure. Water confined in a strong iron vessel (called Papin’s digester) can have its temperature raised to upwards of 400 degrees. Sir James Hall has made some very curious experiments on the effects of heat assisted by pressure; by means of strong gun-barrels, he succeeded in melting a variety of substances which were considered as infusible: and it is not unlikely that, by similar methods, water itself might be heated to redness.
EMILY.
I am surprised at that: for I thought that the force of steam was such as to destroy almost all mechanical resistance.
MRS. B.
The expansive force of steam is prodigious; but in order to subject water to such high temperatures, it is prevented by confinement from being converted into steam, and the expansion of heated water is comparatively trifling.—But we have dwelt so long on the subject of free caloric, that we must reserve the other modifications of that agent to our next meeting, when we shall endeavour to proceed more rapidly.