Chapter I (5)

These facts show us that heat is capable of existing in bodies in two distinct states, in one of which it is sensible to the thermometer, and in the other not. Heat which is sensible to the thermometer is called, for distinction, _sensible_ or _free heat_. It may be here observed, that heat which is sensible to the thermometer is also perceptible by the senses, and heat not sensible to the thermometer is not perceptible by the senses. Thus, ice at 32° and water at 32° _feel_ equally cold, and yet we have seen that the latter contains considerably more heat than the former.

Dr. Black, who first noticed the remarkable fact to which we have now alluded, inferred that ice is converted into water by communicating to it a certain quantity or dose of heat, which enters into combination with it in a manner analogous to that which takes place when bodies combine chemically. The heat, thus combined with the solid ice, loses its property of affecting the senses or the thermometer, and the effects therefore bear a resemblance to those cases of chemical combination in which the constituent elements change their sensible properties when they form the compound.

The fact that the thermometer immersed in the ice remains stationary only as long as the process of liquefaction is going on, shows that this absorption of heat is necessarily connected with that process, and that, were it not for the conversion of [Pg102] the solid ice into liquid water, the heat which is so received would be sensible, and would cause the thermometer immersed in the ice to rise. Before the time of Black it was supposed that the slightest addition of heat would cause solid ice to be converted into water, and that the thermometer would immediately pass from the freezing temperature to higher degrees. The experiments above described, however, show the falsehood of such a supposition. If, while the mercurial bath, in which the ice is immersed, is maintained at the temperature of 200°, the length of time necessary to complete the liquefaction of the ice be observed, it would be found that that time is about twenty-eight times the length of time which it would take to raise the liquid water from 32° to 37°; and if it be assumed that the same quantity of heat is imparted to the ice, during the process of liquefaction, during each minute, as is imparted to the water, during each minute, in rising from 32° to 37°, it will follow, that to liquefy the ice requires twenty-eight times as much heat as is necessary to raise the water from 32° to 37°. It appears, therefore, that, instead of a small quantity of heat being necessary to melt the ice, a very considerable portion is absorbed in that process.

Having ascertained the remarkable fact, that heat is absorbed in a large quantity in the conversion of ice into water, without rendering the body so absorbing it warmer, let us now inquire what the exact quantity of heat so absorbed is. We have already stated that, if the quantity communicated in equal times be the same, the heat necessary to liquefy a given weight of ice would be twenty-eight times as much as would be necessary to raise the same weight of water from 32° to 37°; or, if the heat necessary to raise water through every 5° be the same, that quantity of heat would be sufficient to raise water from 32° to 172°: and hence we infer, that as much heat is absorbed in the liquefaction of a given quantity of ice as would raise the same quantity of water through 140 degrees of the thermometric scale.

(52.) Let us now examine the analogous effects produced by the continued application of heat to water in the liquid state.

Let a small quantity of water be placed in a glass flask of considerable size, and then closed so as to prevent the escape [Pg103] of any vapour. Let this vessel be now placed over the flame of a spirit lamp, so as to cause the water it contains to boil. For a considerable time the water will be observed to boil, and apparently to diminish in quantity, until at length all the water disappears, and the vessel is apparently empty. If the vessel be now removed from the lamp, and suspended in a cool atmosphere, the whole of the interior of its surface will presently appear to be covered with a dewy moisture; and at length a quantity of water will collect in the bottom of it, equal to that which had been in it at the commencement of the process. That no water has at any period of the experiment escaped from it, may be easily determined, by performing the experiment with the glass flask suspended from the arm of a balance, counterpoised by a sufficient weight suspended from the other arm. The equilibrium will be preserved throughout, and the vessel will be found to have the same weight, when to all appearance it is empty, as when it contains the liquid water. It is evident, therefore, that the water exists in the vessel in every stage of the process, but that it becomes invisible when the process of boiling has continued for a certain length of time; and it may be shown that it will continue to be invisible, provided the flask be exposed to a temperature considerably elevated. Thus, for example, if it be suspended in a vessel of boiling water, the water which it contains will continue to be invisible; but the moment it is withdrawn from the boiling water, and exposed to the cold air, the water will again become visible, as above mentioned, forming a dew on the inner surface, and finally collecting in the bottom, as in the commencement of the experiment.

In fact, the liquid has, by the process of boiling, been converted into _vapour_, or _steam_, which is a body similar in its leading properties to common air, and, like it, is invisible. It will hereafter appear that it likewise possesses the property of elasticity, and other mechanical qualities enjoyed by gases in general.

(53.) Again, let an open vessel be filled with water at 60°, and placed in a mercurial bath, which is maintained, by a fire or lamp applied to it, at the temperature of 230°. Place a thermometer in the water, and it will be observed gradually to [Pg104] rise as the temperature of the water is increased by the heat which it receives from the mercury in which it is immersed. The water will steadily rise in this manner until it attains the temperature of 212°; but here the thermometer immersed in it will become stationary. At the same time the water contained in the vessel will become agitated, and its surface will present the same appearance as if bubbles of air were rising from the bottom, and issuing at the top. A cloudy vapour will be given off in large quantities from its surface. This process is called _ebullition_ or _boiling_. If it be continued for any considerable time, the quantity of water in the vessel will be sensibly diminished; and at length every particle of it will disappear, and the vessel will remain empty. During the whole of this process, the thermometer immersed in the water will remain stationary at 212°.

Now, it will be asked, what has become of the water? It cannot be imagined that it has been annihilated. We shall be able to answer this by adopting means to prevent the escape of any particle of matter from the vessel containing the water, into the atmosphere or elsewhere. Let us suppose that the top of the vessel containing the water is closed, with the exception of a neck communicating with a tube, and let that tube be carried into another close vessel removed from the cistern of heated mercury, and plunged in another cistern of cold water. Such an apparatus is represented in _fig._ 15.

A is a cistern of heated mercury, in which the glass vessel B, containing water, is immersed. From the top of the vessel B proceeds a glass tube C, inclining downwards, and entering a glass vessel D, which is immersed in a cistern E of cold water. If the process already described be continued until the water by constant ebullition has disappeared, as already mentioned, [Pg105] from the vessel B, it will be found that a quantity of water will be collected in the vessel D; and if this water be weighed, it will be found to have exactly the same weight as the water had which was originally placed in the vessel B. It is, therefore, quite apparent that the water has passed by the process of boiling from the one vessel to the other; but, in its passage, it was not perceptible by the sight. The tube C and the upper part of the vessel B, had the same appearance, exactly, as if they had been filled with atmospheric air. That they are not merely filled with atmospheric air may, however, be easily proved. When the process of boiling first commences, it will be found that the tube C is cold, and the inner surface dry. When the process of ebullition has continued a short time, the tube C will become gradually heated, and the inner surface of it covered with moisture. After a time, however, this moisture disappears, and the tube attains the temperature 212°. In this state it continues until the whole of the water is discharged from the vessel B to the vessel D.

(54.) These effects are easily explained. The water in the vessel B is incapable of receiving any higher temperature than 212°, consistently with its retaining the liquid form. Small portions, therefore, are constantly converted into steam by the heat received from the surrounding mercury, and bubbles of steam are formed on the bottom and sides of the vessel B. These bubbles, being very much lighter, bulk for bulk, than water, rise rapidly through the water, just in the same manner as bubbles of air would, and produce that peculiar agitation at its surface which has been taken as the external indication of boiling. They escape from the surface, and collect in the upper part of the vessel. The steam thus collected, when it first enters the tube C, is cooled below the temperature of 212° by the surface of the tube; and consequently, being incapable of remaining in the state of vapour at any lower temperature than 212°, it is reconverted into water, and forms the dewy moisture which is observed in the commencement of the process on the interior of the tube C. At length, however, the whole of the tube C is heated to the temperature of 212°, and the moisture which was previously collected upon its inner [Pg106] surface is again converted into steam. As the quantity of steam evolved from the water in B increases, it drives before it the steam previously collected in the tube C, and forces it into the vessel B. Here it encounters the inner surface of this vessel, which is kept constantly cold by being surrounded with the cold water in which it is immersed; and the vapour, being thus immediately reduced below the temperature of 212°, is reconverted into water. At first it collects in a dew on the surface of the vessel D; but as this accumulates, it drops into the bottom of the vessel, and forms a more considerable quantity. As the quantity of water is observed to be gradually diminished in the vessel B, the quantity will be found to be gradually increased in the vessel D; and if the operation be suspended at any stage of the process, and the water in the two vessels weighed, it will be found that the weight of the water in D is exactly equal to the weight which the water in B has lost.

(55.) The demonstration is, therefore, perfect, that the gradual diminution of the boiling water in the vessel B is produced by the conversion of that water into steam by the heat. In the process first described, when the top of the vessel B was supposed to be open, this steam made its escape into the air, where it was first dispersed, and subsequently cooled in separate particles, and was deposited in minute globules of moisture on the ground and on surrounding objects.

(56.) In reviewing this process, we are struck by the fact, that the continued application of heat to the vessel B is incapable of raising the temperature of the water contained in it above 212°. This presents an obvious analogy to the process of liquefaction, and leads to inquiries of a similar nature, which are attended with a like result. We must either infer, that the water, having arrived at 212°, received no more heat from the mercury; or that such heat, if received, is incapable of affecting the thermometer; or, finally, that the steam which passes off carries this heat with it. That the water receive heat from the mercury, will be proved by the fact, that, if the vessel B be removed from the mercury, other things remaining as before, the temperature of the mercury will rapidly rise, and if the fire be continued, it will even boil; but so long as the [Pg107] vessel B remains immersed, it prevents the mercury from increasing in temperature. It therefore receives that heat which would otherwise raise the temperature of the quicksilver.

If a thermometer be immersed in the steam which collects in the upper part of the vessel B, it will show the same temperature (of 212°) as the water from which it is raised. The heat, therefore, received from the mercury, is clearly not imparted in a sensible form to the steam, which has the same temperature in the form of steam as it had in the form of water. What has been already explained respecting liquefaction would lead us, by analogy, to suspect that the heat imparted by the mercury to the water has become latent in the steam, and is instrumental to the conversion of water into steam, in the same manner as heat has been shown to be instrumental to the conversion of ice into water. As the fact was in that case detected by mixing ice with water, so we shall, in the present instance, try it by a like test, viz. by mixing water with steam. Let about five ounces and a half of water, at the temperature of 32°, be placed in a vessel A (_fig._ 16.), and let another vessel B, in which water is kept constantly boiling at the temperature of 212°, communicate with A by a pipe C proceeding from the top, so that the steam may be conducted from B, and escape from the mouth of the pipe at some depth below the surface of the water in A. As the steam issues from the pipe, it will be immediately reconverted into water by the cold water which it encounters; and, by continuing this process, the water in A will be gradually heated by the steam combined with it and received through the pipe C. If this process be continued until the water in A is raised to the temperature of 212°, it will boil. Let it then be weighed, and it will be found to weigh six ounces and a half: from whence we infer, that one ounce of water has been received from the vessel B in the form of steam, and has been reconverted into water by the inferior temperature of the water in A. Now, this ounce of water received in the form of steam into the vessel A had, when in that form, the temperature of 212°. It is now [Pg108] converted into the liquid form, and still retains the same temperature of 212°; but it has caused the five ounces and a half of water with which it has been mixed, to rise from the temperature of 32° to the temperature of 212°,—and this, _without losing any temperature itself_. It follows, therefore, that, in returning to the liquid state, it has parted with as much heat as is capable of raising five times and a half its own weight of water from 32° to 212°. This heat was combined with the steam, though not sensible to the thermometer; and was, therefore, _latent_. Had it been sensible in the water in B, it would have caused the water to have risen through a number of thermometric degrees, amounting to five times and a half the excess of 212° above 32°; that is, through five times and a half 180°; for it has caused five times and a half its own weight of water to receive an equal increase of temperature. But five times and a half 180° is 990°, or, to use round numbers (for minute accuracy is not here our object), 1000°. It follows, therefore, that an ounce of water, in passing from the liquid state at 212° to the state of steam at 212°, receives as much heat as would be sufficient to raise it through 1000 thermometric degrees, if that heat, instead of becoming latent, had been sensible.

(57.) In order to derive all the knowledge from these experiments which they are capable of imparting, it will be necessary to examine very carefully how water comports itself under a variety of different circumstances.

If water be boiled in an open vessel, with a thermometer immersed, on different days, it will be observed that the fixed temperature which it assumes in boiling will be subject to a variation within certain small limits. Thus, at one time, it will be found to boil at the temperature of 210°; while, at others, the thermometer immersed in it will rise to 213°; and, on different occasions, it will fix itself at different points within these limits. It will also be found, if the same experiment be performed at the same time in distant places, that the boiling points will be subject to a like variation. Now, it is natural to inquire what cause produces this variation; and we shall be led to the discovery of the cause, by examining what other physical effects undergo a simultaneous change. [Pg109]

If we observe the height of the barometer at the time of making each experiment, we shall find a very remarkable correspondence between it and the boiling temperature. Invariably, whenever the barometer stands at the same height, the boiling temperature will be the same. Thus, if the barometer stands at 30 inches, the boiling temperature will be 212°. If the barometer fall to 29-1/2 inches, the thermometer stands at a small fraction above 211°. If the barometer rise to 30-1/2 inches, the boiling temperature rises to nearly 213°. The variation in the boiling temperature is, then, accompanied by a variation in the pressure of the atmosphere indicated by the barometer; and it is constantly found that the boiling point will remain unchanged, so long as the atmospheric pressure remains unchanged, and that every increase in the one causes a corresponding increase in the other.

(58.) From these facts it must be inferred, that the pressure excited on the surface of the water has a tendency to resist its ebullition, and to make it necessary, before it can boil, that it should receive a higher temperature; and, on the contrary, that every diminution of pressure on the surface of the water will give an increased facility to the process of ebullition, or will cause that process to take place at a lower temperature. As these facts are of the utmost importance in the theory of heat, it may be useful to verify them by direct experiment.

If the variable pressure excited on the surface of the water by the atmosphere be the cause of the change in the boiling temperature, it must happen, that any change of pressure produced by artificial means on the surface of the water must likewise change the boiling point, according to the same law. Thus, if a pressure considerably greater than the atmospheric pressure be excited on a liquid, the boiling point may be expected to rise considerably above 212°; and, on the other hand, if the surface of the water be relieved from the pressure of the atmosphere, and be submitted to a considerably diminished pressure, the water would boil below 212°.

Let B (_fig._ 17.) be a strong spherical vessel of brass, supported on a stand S, under which is placed a large spirit lamp L, or other means of heating it. In the top of this vessel are three apertures, in two of which are screwed a [Pg110] thermometer T, the bulb of which enters the hollow brass sphere, and a stop-cock C, which may be closed or opened at pleasure, to confine the steam, or allow it to escape. In the third aperture at the top, is screwed a long barometer tube, open at both ends. The lower end of this tube extends nearly to the bottom of the spherical vessel B. In the bottom of this vessel is placed a quantity of mercury, the surface of which rises to some height above the lower end of the tube A. Over the mercury is poured a quantity of water, so as to half fill the vessel B. Matters being thus arranged, the screws are made tight, so as to confine the water, and the lamp is allowed to act on the vessel; the temperature of the water is raised, and steam is produced, which, being confined within the vessel, exerts its pressure on the surface of the water, and resists its ebullition. The pressure of the steam acting on the surface of the water is communicated to the surface of the mercury, and it forces a portion of the mercury into the tube A, which presently rises above the point where the tube is screwed into the top of the vessel B. As the action of the lamp continues, the thermometer T exhibits a gradually increasing temperature; while the column of mercury in A shows the force with which the steam presses on the surface of the water in B,—this column being balanced by the pressure of the steam. Thus, the temperature and pressure of the steam at the same moment may always be observed by inspecting the thermometer T and the tube A. When the column in the tube A has risen to the height of 30 inches above the level of the mercury in the vessel B, then the pressure of the steam will be equivalent to double the pressure of the atmosphere, because, the tube A being open at the top, the atmosphere presses on the [Pg111] surface of the mercury in it. The thermometer T will be observed gradually to rise until it attains the temperature of 212°; but it will not stop there, as it would do if immersed in water boiled in an open vessel. It will, on the other hand, continue to rise; and when the column of mercury in A has attained the height of 30 inches, the thermometer T will have risen to 251°,—being 39° above the ordinary boiling point.

During the whole of this process, the surface of the water being submitted to a constantly increasing pressure, its ebullition is prevented, and it continues to receive heat without boiling. That it is the increased pressure which resists its ebullition, and causes it to receive a temperature above 212°, may be easily shown. Let the stop-cock C be opened; immediately the steam in B, having a pressure considerably greater than that of the atmosphere, will rush out, and will continue to issue from C, until its pressure is balanced by the atmosphere. At the same time the column of mercury in A will be observed rapidly to fall, and to sink below the orifice by which it is inserted in the vessel B. The thermometer T will also fall until it attains the temperature of 212°. At that point, however, it will remain stationary; and the water will now be distinctly heard to be in a state of rapid ebullition. If the stop-cock C be once more closed, the thermometer will begin to rise, and the column of mercury ascending in A will be again visible.

If, instead of a stop-cock being at C, the aperture were made to communicate with a valve, like the safety-valve of a steam engine, loaded with a certain weight,—say at the rate of 15 lbs. on the square inch,—then the thermometer T, and the mercury in the tube A, would not rise indefinitely as before. The thermometer would continue to rise till it attained the temperature of 251°; and the mercury in the tube A would rise to the height of 30 inches. At this limit the resistance of the valve would be balanced by the pressure of the steam; and as fast as the water would have a tendency to produce steam of a higher pressure, the valve would be raised and the steam suffered to escape; the thermometer T and the column of mercury in A remaining stationary during this process. If the valve were loaded more heavily, the phenomena would be [Pg112] the same, only that the mercury in T and A would become stationary at certain heights. But, on the other hand, if the valve were loaded at a less pressure than 15 lbs. on the square inch, then the mercury in the two tubes would become stationary at lower points.

(59.) These experiments show that every increase of pressure above the ordinary pressure of the atmosphere causes an increase in the temperature at which water boils. We shall now inquire whether a diminution of pressure will produce a corresponding effect on the boiling point.

This may be easily accomplished by the aid of an air pump. Let water at the temperature of 200° be placed in a glass vessel under the receiver of an air pump, and let the air be gradually withdrawn. After a few strokes of the pump, the water will boil; and if the mercurial gauge of the pump be observed, it will be found that its altitude will be about 23-1/2 inches. Thus the pressure to which the water is submitted has been reduced from the ordinary pressure of the atmosphere expressed by the column of 30 inches of mercury, to a diminished pressure expressed by 23-1/2 inches; and we find that the temperature at which the water boils has been lowered from 212° to 200°. Let the same experiment be repeated with water at the temperature of 180°, and it will be found that a further rarefaction of the air is necessary, but the water will at length boil. If the gauge of the pump be now observed, it will be found to stand at about fifteen inches, showing, that at the temperature of 180° water will boil under half the ordinary pressure of the atmosphere. These experiments may be varied and repeated; and it will be always found, that, as the pressure is diminished or increased, the temperature at which the water will boil will be also diminished or increased.

(60.) The same effects may be exhibited in a striking manner without an air pump, by producing a vacuum by the condensation of steam. Let a small quantity of water be placed in a thin glass flask, and let it be boiled by holding it over a spirit lamp. When the steam is observed to issue abundantly from the mouth of the flask, let it be quickly corked and removed from the lamp. The process of boiling will then cease, and the water will become quiescent; but if the flask be plunged [Pg113] in a vessel of cold water, the water it contains will again pass into a state of violent ebullition, thus exhibiting the singular fact of water being boiled by cooling it. This effect is produced by the cold medium in which the flask is immersed, causing the steam above the surface of the water in it to be condensed, and therefore relieving the water from its pressure. The water, under these circumstances, boils at a lower temperature than when submitted to the pressure of the uncondensed vapour.

(61.) There is no limit to the temperature to which water may be raised, if it be submitted to a sufficient pressure to resist its tendency to take the vaporous form. If a strong metallic vessel be nearly filled with water, so as to prevent the liquid from escaping by any force which it can exert, the water thus inclosed may be heated to any temperature whatever without boiling; in fact, it may be made red-hot; and the temperature to which it may be raised will have no limit, except the strength of the vessel containing it, or the point at which the metal of which it is formed may begin to soften or to be fused.

(62.) The following table will show the temperature at which water will boil under different pressures of the atmosphere corresponding to the altitudes of the barometer between 26 and 31 inches.

Barometer. Boiling Point.
26 inches 204°·91
26·5 205°·79
27 206°·67
27·5 207°·55
28 208°·43
28·5 209°·31
29 210°·19
29·5 211°·07
30 212°
30·5 212°·88
31 213°·76

From this table it appears, that, for every tenth of an inch which the barometric column varies between these limits, the boiling temperature changes by the fraction of a degree expressed by the decimal ·176, or nearly by the vulgar fraction 1/6.

(63.) In the experiment already described, by which the latent [Pg114] heat of steam was determined, the water was supposed to be boiled under the ordinary pressure of the atmosphere. Having seen, however, that water may boil at different temperatures, under different pressures, the inquiry presents itself, whether the heat absorbed in vaporisation at different temperatures, and under different pressures, is subject to any variation? Experiments of the same nature as those already described, instituted upon water in a state of ebullition at different temperatures, as well below as above 212°, have led to the discovery of a very remarkable fact in the theory of vapour. It has been found that the heat absorbed by vaporisation is always less, the higher the temperature at which the ebullition takes place; and less, by the same amount as the temperature of ebullition is increased. Thus, if water boil at 312°, the heat absorbed in ebullition will be less by 100° than if it boiled at 212°; and again, if water be boiled under a diminished pressure, at 112°, the heat absorbed in vaporisation will be 100° more than the heat absorbed by water boiled at 212°. It follows, therefore, that the actual consumption of heat in the process of vaporisation must be the same, whatever be the temperature at which the vaporisation takes place; for whatever heat is saved in the sensible form, is consumed in the latent form, and _vice versâ_.

Let us suppose a given weight of water at the temperature of 32° to be exposed to any regular source by which heat may be supplied to it. If it be under the ordinary atmospheric pressure, the first 180° of heat which it receives will raise it to the boiling point, and the next 1000° will convert it into steam. Thus, in addition to the heat which it contains at 32°, the steam at 212° contains 1180° of heat. But if the same water be submitted to a pressure equal to half the atmospheric pressure, then the first 148° of heat which it receives will cause it to boil, and the next 1032° will convert it into vapour. Thus, steam at the temperature of 180° contains a quantity of heat more than the same quantity of water at 32°, by 1032° added to 148°, which gives a sum of 1180°. Steam, therefore, raised under the ordinary pressure of the atmosphere at 212°, and steam raised under half that pressure at 180°, contain the same quantity of heat,—with this difference [Pg115] only—that the one has more latent heat, and less sensible heat, than the other.

From this fact, that the sum of the latent and sensible heats of the vapour of water is constant, it follows that the same quantity of heat is necessary to convert a given weight of water into steam, at whatever temperature, or under whatever pressure, the water may be boiled. It follows, also, that, in the steam engine, equal weights of high-pressure and low-pressure steam are produced by the same consumption of fuel; and that, in general, the consumption of fuel is proportional to the quantity of water vaporised, whatever the pressure of the steam may be.[18]

(64.) Having explained the conditions under which, by supplying heat to water, it is converted into steam, and, by abstracting heat from steam, it may be reconverted into water, let us now consider the mechanical force which is developed in these phenomena.

Let A B (_fig._ 18.) be a tube, or cylinder, the base of which is equal to a square inch, and let a piston P move in it so as to be steam-tight. Let it be supposed, that under this piston there is, in the bottom of the cylinder, a cubic inch of water between the bottom of the piston and the bottom of the tube; let the piston be counterbalanced by a weight W acting over a pulley, which will be just sufficient to counterpoise the weight of the piston, so as leave no force tending to keep the piston down, except the force of the atmosphere acting above it. Under the circumstances here supposed, the piston being in contact with the water, and all air being excluded, it will be pressed down by the weight of the atmosphere, which we will suppose to be fifteen pounds, the magnitude of the piston being a square inch. [Pg116]

Now let the flame of a lamp be applied at the bottom of the tube; the water under the piston having its temperature thereby gradually raised, and being submitted to no pressure save that of the atmosphere above the piston, it will begin to be converted into steam when it has attained the temperature of 212°. According as it is converted into steam, it will cause the piston to ascend in the tube until all the water has been evaporated. If the tube were constructed of sufficient length, the piston then would be found to have risen to the height of about seventeen hundred inches, or one hundred and forty-two feet; since, as has been already explained, water passing into steam under the ordinary pressure of the atmosphere undergoes an increase of bulk in the proportion of about seventeen hundred to one.

Now in this process, the air above the piston, which presses on it with a force equal to fifteen pounds, has been raised one hundred and forty-two feet. It appears, therefore, that, by the evaporation of a cubic inch of water under a pressure equal to fifteen pounds per square inch, a mechanical force of this amount is developed.

It is evident that fifteen pounds raised one hundred and forty-two feet successively, is equivalent to one hundred and forty-two times fifteen pounds raised one foot. Now, one hundred and forty-two times fifteen is two thousand one hundred and thirty, and therefore the force thus obtained is equal to two thousand one hundred and thirty pounds raised one foot high. This being within about 110 pounds of a ton, it may be stated, in round numbers, that, by the evaporation of a cubic inch of water under these circumstances, a force is obtained equal to that which would raise a ton weight a foot high.

The augmentation of volume which water undergoes in passing into steam under the pressure here supposed, may be easily retained in the memory, from the accidental circumstance that a cubic inch of water is converted into a cubic foot of steam, very nearly. A cubic foot contains one thousand seven hundred and twenty-eight cubic inches,—which is little different from the proportion which steam bears to water, when raised under the atmospheric pressure. [Pg117]

(65.) It will, therefore, be an advantage to retain in memory the following general facts:—

1. _A cubic inch of water evaporated under the ordinary atmospheric pressure, is converted into a cubic foot of steam._

2. _A cubic inch of water evaporated under the atmospheric pressure, gives a mechanical force equal to what would raise about a ton weight a foot high._

(66.) Let us, again, suppose the piston P (_fig._ 23.) to be restored to its original position, with the liquid water beneath it; and, in addition to the weight of the atmosphere which before pressed it down, let us suppose another weight of fifteen pounds laid upon it, so that the water below shall be pressed by double the weight of the atmosphere. If the lamp were now applied, and at the same time a thermometer were immersed in the water, it would be found that the water would not begin to be converted into steam until it attained the temperature of about 250°. The piston would then begin, as before, to ascend, and the water to be gradually converted into vapour. The water being completely evaporated, it would be found that the piston would be raised to a height little more than half its former height, or 72 feet. The mechanical effect, therefore, thus obtained, will be equivalent to double the former weight raised half the former height.

In like manner, if the piston were loaded with thirty pounds in addition to the atmosphere, the whole pressure on the water being then three times the pressure first supposed, the piston would be raised to somewhat more than one third of its first height by the evaporation of the water. This would give a mechanical force equivalent to three times the original weight raised a little more than one third of the original height.

In general, as the pressure on the piston is increased, the height to which the piston would be raised by the evaporation of the water will be diminished in a proportion somewhat less than the proportion in which the pressure on the piston is increased. If the temperature at which the water is converted into steam under these different pressures were the same, then the height to which the piston would be raised by the evaporation of the water would be diminished in precisely [Pg118] the same proportion as the pressure on the piston is increased; and, in that case, the whole mechanical force developed by the evaporation of the water would remain exactly the same under whatever pressure the water might be boiled. We shall explain hereafter the extent to which the variation of temperature in the water and steam corresponding to the variation of pressure modifies this law; but, as the effect of the difference of temperatures is not considerable, it will be convenient to register in the memory the following important practical conclusion:—

(67.) _A cubic inch of water converted into steam will supply a mechanical force very nearly equal to a ton weight raised a foot high; and this force will not be subject to considerable variation, whatever be the temperature or pressure at which the water may be evaporated._

FOOTNOTES:

[18] The preceding paragraphs, and some other parts of the
present volume on the general properties of Heat, are taken
from my Treatise on Heat, in the _Cabinet Cyclopœdia_, to
which those who desire more detailed explanation and more
copious illustration should refer.

[Pg119]

CHAP. V.

WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE
WITH A DUE ECONOMY OF FUEL. — CONCEIVES THE NOTION OF
CONDENSING OUT OF THE CYLINDER. — DISCOVERS SEPARATE
CONDENSATION. — INVENTS THE AIR-PUMP. — SUBSTITUTES STEAM
PRESSURE FOR ATMOSPHERIC PRESSURE. — INVENTS THE STEAM CASE,
OR JACKET. — HIS FIRST EXPERIMENTS TO REALISE THESE
INVENTIONS. — HIS EXPERIMENTAL APPARATUS. — DIFFICULTIES OF
BRINGING THE IMPROVED ENGINES INTO USE. — WATT PRACTISES AS A
CIVIL ENGINEER. — HIS PARTNERSHIP WITH ROEBUCK. — HIS FIRST
PATENT. — DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE.

(68.) At the period to which we have now brought the history of the invention of the steam engine, Watt had obtained, chiefly by his own experiments, a sufficient knowledge of the phenomena which have been just explained, to enable him to arrive at the conclusion that a very small proportion of the whole mechanical effect attending the evaporation was really rendered available by the atmospheric engine; and that, [Pg120] therefore, extensive and injurious sources of waste existed in its machinery.

He perceived that the principal source of this wasteful expenditure of power consisted in the quantity of steam which was condensed at each stroke of the piston, in heating the cylinder previous to the ascent of the piston. Yet, as it was evident that that ascent could not be accomplished in a cold cylinder, it was apparent that this waste of power must be inevitable, unless some expedient could be devised, by which _a vacuum could be maintained in the cylinder, without cooling it_. But, to produce such a vacuum, the steam must be condensed; and, to condense the steam, its temperature must be lowered to such a point that the vapour proceeding from it shall have no injurious pressure; yet, if condensed steam be contained in a cylinder at a high temperature, it will return to the temperature of the cylinder, recover its elasticity, and resist the descent of the piston.

Having reflected on these circumstances, it became apparent to Watt, that a vice was inherent in the structure of the atmospheric engine, which rendered a large waste of power inevitable; this vice arising from the fact, that the condensation of the steam was incompatible with the condition of maintaining the elevated temperature of the cylinder in which that condensation took place. It followed, therefore, either that the steam must be imperfectly condensed, or that the condensation could not take place in the cylinder. It was in 1765, that, pondering on these circumstances, the happy idea occurred to him, that the production of a vacuum could be equally effected, though _the place_ where the condensation of the steam took place were not the cylinder itself. He saw, that if a vessel in which a vacuum was produced were put into communication with another containing an elastic fluid, the elastic fluid would rush into the vacuum, and diffuse itself through the two vessels; but if, on rushing into such vacuum, this elastic fluid, being vapour, were there condensed, or restored to the liquid form, that then the space within the two vessels would be equally rendered a vacuum;—that, under such circumstances, one of the vessels might be maintained at any temperature, however high, while [Pg121] the other might be kept at any temperature, however low. This felicitous conception formed the first step in that splendid career of invention and discovery which has conferred immortality on the name of Watt. He used to say, that the moment the idea of separate condensation occurred to him,—that is, of condensing, in one vessel kept cold, the steam coming from another vessel kept hot,—all the details of his improved engine rushed into his mind in such rapid succession, that, in the course of a day, his invention was so complete that he proceeded to submit it to experiment.

To explain the first conception of this memorable invention; let a tube or pipe, S (_fig._ 19.), be imagined to proceed from the bottom of the cylinder A B to a vessel, C, having a stop-cock, D, by which the communication between the cylinder and the vessel C may be opened or closed at pleasure. If we suppose the piston P at the top of the cylinder, and the space below it filled with steam, the cylinder and steam being at the usual temperature, while the vessel C is a vacuum, and maintained at a low temperature. Then, on opening the cock D, the steam will rush from the cylinder A B through the tube S, and, passing into the cold vessel C, will be condensed by contact with its cold sides. This process of condensation will be rendered instantaneous if a jet of cold water is allowed to play in the vessel C. When the steam thus rushing into C, has been destroyed, and the space in the cylinder A B becomes a vacuum, then the pressure of the atmosphere being unobstructed, the piston will descend with the force due to the excess of the pressure of the atmosphere above the friction. When it has descended, suppose the stop-cock D closed, and steam admitted from [Pg122] the boiler through a proper cock or valve below the piston, the cylinder and piston being still at the same temperature as before. The steam on entering the cylinder, not being exposed to contact with any surface below its own temperature, will not be condensed, and therefore will immediately cause the piston to rise, and the piston will have attained the top of the cylinder when as much steam shall have been supplied by the boiler as will fill the cylinder. When this has taken place, suppose the communication with the boiler cut off, and the cock D once more opened: the steam will again rush through the pipe S into the vessel C, where encountering the cold surface and the jet of cold water, it will be condensed, and the vacuum, as before, will be produced in the cylinder A B; that cylinder still maintaining its temperature, the piston will again descend, and so the process may be continued.

(69.) Having carried the invention to this point, Watt saw that the vessel C would gradually become heated by the steam which would be continually condensed in it. To prevent this, as well as to supply a constant jet of cold water, he proposed to keep the vessel C submerged in a cistern of cold water, from which a pipe should conduct a jet to play within the vessel, so as to condense the steam as it would pass from the cylinder.

But here a difficulty presented itself, against which it was necessary to provide. The cold water admitted through the jet to condense the steam, mixed with the condensed steam itself, would gradually collect in the vessel C, and at length choke it. To prevent this, Watt proposed to put the vessel C in communication with a pump F, which might be wrought by the engine itself, and by which the water, which would collect in the bottom of the vessel C, would be constantly drawn off. This pump would be evidently rendered the more necessary, since more or less atmospheric air, always combined with water in its common state, would enter the vessel C by the condensing jet. This air would be disengaged in the vessel C by the heat of the steam condensed therein; and it would rise through the tube S, and vitiate the vacuum in the cylinder;—an effect which would be rendered the more injurious, [Pg123] inasmuch as, unlike steam, this elastic fluid would be incapable of being condensed by cold. The pump F, therefore, by which Watt proposed to draw off the water from the vessel C, might also be made to draw off the air, or the principal part of it.

The vessel C was subsequently called a _condenser_; and, from the circumstances just adverted to, the pump F has been called the _air-pump_.

These—namely, the cylinder, the condenser, and the air-pump—were the three principal parts in the invention, as it first presented itself to the mind of Watt—and even before it was reduced to a model, or submitted to experiment. But, in addition to these, other two improvements offered themselves in the very first stage of its progress.

In the atmospheric engine, the piston was maintained steam-tight in the cylinder by supplying a stream of cold water above it, by which the small interstices between the piston and cylinder would be stopped. It is evident that the effect of this water as the piston descended would be to cool the cylinder, besides which any portion of it which might pass between the piston and cylinder and which would pass below the piston, would boil the moment it would fall into the cylinder, which itself would be maintained at the boiling temperature. This water, therefore, would produce steam, the pressure of which would resist the descent of the piston.

Watt perceived, that even though this inconvenience were removed by the use of oil or tallow upon the piston, still, that as the piston would descend in the cylinder, the cold atmosphere would follow it; and would, to a certain extent, lower the temperature of the cylinder. On the next ascent of the piston, this temperature would have to be again raised to 212° by the steam coming from the boiler, and would entail upon the machine a proportionate waste of power.

If the atmosphere of the engine-house could be kept heated to the temperature of boiling water, this inconvenience would be removed. The piston would then be pressed down by air as hot as the steam to be subsequently introduced into it. On further consideration, however, it occurred to Watt that it would be still more advantageous if the cylinder itself could be [Pg124] worked in an atmosphere of steam, having only the same pressure as the atmosphere. Such steam would press the piston down as effectually as the air would; and it would have the further advantage over air, that if any portion of it leaked through between the piston and cylinder, it would be condensed, which could not be the case with atmospheric air. He therefore determined on surrounding the cylinder by an external casing, the space between which and the cylinder he proposed to be filled with steam supplied from the boiler. The cylinder would thus be enclosed in an atmosphere of its own, independent of the external air, and the vessel so enclosing it would only require to be a little larger than the cylinder, and to have a close cover at the top, the centre of which might be perforated with a hole to admit the rod of the piston to pass through, the rod being made smooth, and so fitted to the perforation that no steam should escape between them. This method would be attended also with the advantage of keeping the cylinder and piston always heated, not only inside but outside; and Watt saw that it would be further advantageous to employ the pressure of steam to drive the piston in its descent instead of the atmosphere, as its intensity or force would be much more manageable; for, by increasing or diminishing the heat of the steam in which the cylinder was enclosed, its pressure might be regulated at pleasure, and it might be made to urge the piston with any force that might be required. The power of the engine would therefore be completely under control, and independent of all variations in the pressure of the atmosphere.

(70.) This was a step which totally changed the character of the machine, and which rendered it a STEAM ENGINE instead of an ATMOSPHERIC ENGINE. Not only was the vacuum below the piston now produced by the property of steam, in virtue of which it is reconverted into water by cold; but the pressure which urged the piston into this vacuum was due to the elasticity of steam.

The external cylinder, within which the working cylinder was enclosed, was called THE JACKET, and is still very generally used.