(48.)

(48.)On considering more attentively the operation of the machine, the following circumstances gradually unfolded themselves to him.

Let us suppose the piston at the top of the cylinder, and the space in the cylinder below it, filled with steam so as to balance the pressure of the atmosphere above the piston. Under such circumstances the steam, as will presently be explained, must have the temperature of boiling water. But that the steam should have, and should maintain, this temperature, it was evidently necessary that the inner surface of the cylinder in contact with it should have the same temperature: for if it had a lower temperature, it would take heat from the steam, and reduce the temperature of the latter. Now the cylinder being a mass of metal, has a quality in virtue of which heat passes freely through its dimensions, so that its inner surface could not be maintained at a temperature more elevated than that of its dimensions extending from the inner surface to the outer surface. Therefore, to maintain the steam contained in the cylinder at the proper temperature, it was essential that the whole of the solid metal composing the cylinder should be itself at that temperature.

Things being in this state, it was required that a vacuum should be produced under the piston to give effect to the atmospheric pressure above it, by relieving it from the pressure below. This, indeed, would appear to have been attained by introducing as much cold water within the cylinder as would be sufficient to reconvert the steam contained in it into water; but Watt found, in his experiments on the atmospheric model, that the piston would not descend with the proper force, unless a vastly greater quantity of water were introduced into the cylinder than the quantity which he had ascertained to be[Pg088]necessary for the reconversion of the steam into water. The cause of this he perceived and fully explained.

If we suppose as much, and no more, cold water introduced into the cylinder as would reconvert the steam contained in it into water, then we should have in the bottom of the cylinder a quantity of warm water with a vacuum above it: but the entire mass of metal composing the cylinder itself, which was previously at the temperature of boiling water, would still be at the same temperature. The warm water, resting in contact with this metal in the bottom of the cylinder, would be immediately heated by it, and would rise in its temperature, while the metal of the cylinder itself would be somewhat lowered in temperature by the heat which it would thus impart to the warm water contained in it. Under these circumstances, as we shall presently explain, steam would be produced from the water, which would fill the cylinder; and although such steam would not have a mechanical pressure equal in amount to the atmosphere, and therefore would not altogether prevent the piston from descending if it had no load to move, yet it would deprive the engine of so great a portion of its legitimate power as to render it altogether inefficient. But this defect would be removed by throwing into the cylinder a sufficient quantity of cold water, not only to destroy the steam contained in it, but also to cool the entire mass of metal composing the cylinder itself, until it would be reduced to such a temperature that the vapour proceeding from the water contained in it would have so small a pressure that it would not seriously or injuriously obstruct the descent of the piston.

The piston being made to descend with such force as to render the machine practically efficient, it would then be necessary again to make it ascend; and to accomplish this, Watt found that the boiler should supply a quantity of steam many times greater than was necessary to fill the cylinder. Mature reflection on the circumstances which have been just explained, enabled him to discover how this undue quantity of steam was rendered necessary.

Let it be recollected, that when the piston has reached the bottom of the cylinder, the whole mass of the cylinder, and[Pg089]the piston itself, are reduced to so low a temperature that the vapour of water, having the same temperature, has no pressure sufficiently great to obstruct the action of the machine. When, in order to make the piston ascend, steam is introduced from the boiler into the cylinder under the piston, this steam encounters, in the first instance, the cold surfaces of the metal forming the bottom of the cylinder and the bottom of the piston. The first effect of this is to convert the steam which comes from the boiler into water, an effect which is produced by that steam imparting its heat to the metal with which it comes into contact. This destruction of steam continues until the metal exposed to contact with it has been heated up to the temperature of boiling water. Then, and not till then, the steam below the piston will have a pressure equal to that of the atmosphere above it, and the piston will begin to ascend. As it ascends, however, the sides of the cylinder which it exposes to the contact of the steam are cold, and partially destroy the steam. Steam, therefore, must be supplied from the boiler to replace the steam thus destroyed; nor can the piston reach the top of the cylinder until such a quantity of steam shall have flowed from the boiler into the cylinder, as shall be sufficient not only to fill the cylinder under the piston, but likewise, by its condensation, to raise the whole mass of the cylinder and piston to the temperature of boiling water.

Such were the circumstances which forced themselves upon the attention of Watt, in the course of repairing, and subsequently trying, the model of the atmospheric engine, at Glasgow. Being informed generally of the uses of the engine in the drainage of mines, and of the vast expense attending its operation, by reason of the quantity of fuel which it consumed, he saw how important any improvement would be by which the extensive sources of waste which had thus presented themselves could be removed. He saw also, that all that portion of steam which was expended, not in filling the cylinder under the piston, but in heating the great mass of metal composing the cylinder and piston, from a low temperature to that of boiling water, upon each stroke of the piston, was so much heat lost, and that the proportion of the fuel expended in evaporating the steam thus wasted would be saved, if by any[Pg090]expedienthe could make the piston descend without cooling the cylinder. But in order to estimate the full amount of this waste, and to discover the most effectual means of preventing it, it was necessary to investigate the quantity of heat necessary for the evaporation of a given quantity of water; also, the quantity of steam which a given quantity of water would produce, as well as other circumstances connected with the temperature and pressure of steam. He, therefore, applied himself to make experiments with a view to elucidate these questions; and succeeded in obtaining results which led to the discovery of some of the most important of those physical phenomena, on the due application of which, the efficacy of the steam engine, which he afterwards invented, depended, and which also form striking facts in the general physics of heat.

(49.)The first question to which he directed his experiments, was the determination of the extent to which water enlarged its volume, or magnitude, when it passed into steam. To ascertain this, he filled a thin Florence flask with steam, of a pressure equal to the atmosphere, and weighed it accurately. The same flask was then filled with water, and weighed again. Finally, the weight of the flask itself was ascertained. It is evident, that by such means, the exact weight of the steam which filled the flask, and of the same bulk of water, would be obtained. He found that the water weighed about eighteen hundred times more than the steam; from whence he inferred that the steam which filled the flask contained about eighteen hundred times less water than the flask would contain.[17]

[Pg091]Having once ascertained this point, he was able, by observing the quantity of water evaporated in the boiler of the atmospheric model, to compute the volume of steam which was supplied to the cylinder. It was evident, that for every cubic inch of water evaporated in the boiler, eighteen hundred cubic inches of steam were supplied to the cylinder. Having accurately observed the evaporation of the boiler for a short time, and the number of strokes made by the piston in the same time, he found that the quantity of water evaporated in the boiler would supply about four times as much steam as the cylinder would require. He consequently inferred, that about three-fourths of the steam produced was wasted.

The next question to which he directed his experiments, was to ascertain the quantity of cold water necessary to be injected into the cylinder, in order to condense the steam contained in it. To ascertain this, he attached a pipe to a boiler, by which he was enabled to conduct the steam from the boiler into a glass jar containing cold water at fifty-two degrees of temperature. The steam, as it passed from the boiler through the pipe, was condensed by the cold water, and continued to be so condensed, until, by the heat which it imparted to the water, the latter began to boil, and would then condense no more steam. On comparing the water in the glass jar, when boiling, with the water originally contained in it at fifty-two[Pg092]degrees, the quantity was found to be increased in the proportion of six to seven, very nearly; from which he inferred, that to reduce one ounce of steam to water, it was necessary to mix about six ounces of cold water with it.

He was further led to the conclusion, that steam contains a vast quantity of heat, by the following experiment. He heated, in a close digester, a quantity of water several degrees above the common boiling point. When thus heated, by opening a stop-cock, he allowed the compressed steam to escape into a cold vessel; in three or four seconds, he found that the heat of the water in the digester was reduced from a very high temperature to the common boiling point; yet, that all the steam which escaped from it, and which carried off with it the superabundant heat, formed only a few drops of water when condensed; from which he inferred, that this small quantity of water, in the form of steam, contained as much heat as was sufficient to raise all the water in the digester from the boiling point to the temperature at which it was before the steam was allowed to escape.

Having thus ascertained the exact quantity of cold water which ought to be injected into the cylinder in order to condense the steam which filled the cylinder, he found, on comparing the quantity necessary to be injected in order to enable the piston to descend, that this quantity was about four times as great as that which was necessary to condense the steam. This led him to the conclusion, that about four times as much heat was destroyed in the cylinder as needed to be destroyed, if the object were the mere condensation of the steam. This result fully corroborated the other conclusion, deduced, from the proportion which he found between the quantity of steam supplied by the boiler and the actual contents of the cylinder.

(50.)Watt was forcibly struck with these circumstances, not only on account of their importance in an economical point of view, when their relation to steam power was considered, but still more so, as indicating phenomena in the physics of heat altogether novel to him.

He, therefore, eagerly sought his friend Dr. Black, to whom he communicated these results. Then, for the first time, he[Pg093]was informed, by Black, of the theory ofLATENT HEAT, which had recently been discovered by him, and of which these very phenomena formed the basis.

Some passages in the works of Dr. Robison produced an erroneous impression, that a large share of the merit of the discoveries of Watt which have been just explained was due to Dr. Black, to whose instructions on the subject of latent heat Watt was represented to have owed the knowledge of those facts which led to his principal inventions and improvements. We shall here give, in the words of Watt himself, his explanation of the circumstances which led to this error. This explanation is given in a letter addressed by Watt to Dr. Brewster, in May 1814, and prefixed to the third volume of Brewster's edition ofRobison's Mechanical Philosophy:—

"The representations of friends whose opinions I highly value induce me to avail myself of this opportunity of noticing an error into which not only Dr. Robison, but apparently also Dr. Black, has fallen, in relation to theoriginof my improvements upon the steam engine, and which not having been publicly controverted by me, has, I am informed, been adopted by almost every subsequent writer upon the subject of latent heat."Dr. Robison, in the article Steam Engine, after passing an encomium upon me, dictated by the partiality of friendship, qualifies me as the 'pupiland intimate friend of Dr. Black,'—a description which not being there accompanied with any inference, did not particularly strike me at the time of its first perusal. He afterwards, in the dedication to me of his edition of Dr. Black's lectures upon chemistry, goes the length of supposing me to have professed to owe my improvements upon the steam engine to the instructions and information I had received from that gentleman, which certainly was a misapprehension; as, though I have always felt and acknowledged my obligations to him for the information I had received from his conversation, and particularly for the knowledge of the doctrine of latent heat, I never did norcouldconsider my improvements as originating in those communications. He is also mistaken in his assertion (p. 8. of the preface to the above work), that 'I had attended two courses[Pg094]of the doctor's lectures;' for, unfortunately for me, the necessary avocations of my business prevented me from attending his or any other lectures at college; and as Dr. Robison was himself absent from Scotland for four years at the period referred to, he must have been misled by erroneous information. In p. 184. of the lectures, Dr. Black says, 'I have the pleasure of thinking that the knowledge we have acquired concerning the nature of elastic vapours, in consequence of my fortunate observation of what happens in its formation and condensation, has contributed in no inconsiderable degree to the public good bysuggestingto my friend Mr. Watt of Birmingham, then of Glasgow, his improvement on this useful engine' (meaning the steam engine of which he is then speaking). There can be no doubt from what follows in his description of the engine, and from the very honourable mention which he has made of me in various parts of his lectures, that he did not mean to lessen any merit that might attach to me as an inventor; but, on the contrary, he was always disposed to give me fully as much praise as I deserved."And were that otherwise doubtful, it would, I think, be evident from the following quotation from a letter of his to me, dated 13th February 1783, where, speaking of an intended publication by a friend of mine, on subjects connected with the history of steam, he says, 'I think it is very proper for you to give him a short account of your discoveries and speculations;and particularly to assert clearly and fully your sole right to the honour of the improvements of the steam engine.' And in a written testimonial which he very kindly gave me, on the occasion of a trial at law against a piracy of my invention in 1796-7, after giving a short account of the invention, he adds, 'Mr. Watt was the sole inventor of the capital improvement and contrivance above mentioned.'"Under this conviction of his candour and friendship, it is very painful to me to controvert any assertion or opinion of my revered friend; yet, in the present case I find it necessary to say, that he appears to me to have fallen into an error; and I hope, in addition to my assertion, to make that appear by the short history I have given of my invention, in my[Pg095]notes upon Dr. Robison's essay, as well as by the following account of the state of my knowledge previous to my receiving any explanation of the doctrine of latent heat; and also from that of the facts which principally guided me in the invention."It was known very long before my time, that steam was condensed by coming into contact with cold bodies, and that it communicated heat to them; witness the common still, &c. &c."It was known, by some experiments of Dr. Cullen and others, that water and other liquids boiled in vacuo at very low heats; water below 100°."It was known to some philosophers that the capacity or equilibrium of heat, as we then called it, was much smaller in mercury and tin than in water."It was also known that evaporation caused the cooling of the evaporating liquid, and bodies in contact with it."I had myself made experiments to determine the following facts:—"First, the capacities of heat for iron, copper, and some sorts of wood, comparatively with water."Second, the bulk of steam compared with that of water."Third, the quantity of water evaporated in a certain boiler by a pound of coals."Fourth, the elasticities of steam at various temperatures greater than that of boiling water, and an approximation to the law which it followed at other temperatures."Fifth, how much water in the form of steam was required every stroke by a small Newcomen's engine, with a wooden cylinder six inches diameter, and twelve inches stroke."Sixth, the quantity of cold water required in every stroke to condense the steam in that cylinder, so as to give it a working power of about 7 lb. on the inch."Here I was at a loss to understand how so much cold water could be heated so much by so small a quantity of water in the form of steam; and I accordingly applied to Dr. Black, and then first understood what was called latent heat."But this theory, though useful in determining the quantity of injection necessary where the quantity of water[Pg096]evaporated by the boiler, and used by the cylinder, was known, and in determining, by the quantity and heat of the hot water emitted by Newcomen's engines, the quantity of steam required to work them did not lead to the improvements I afterwards made in the engine. These improvements proceeded upon the old established fact, that steam was condensed by the contact of cold bodies; and the later known one, that water boiled in vacuo at heats below 100°, and consequently that a vacuum could not be obtained unless the cylinder and its contents were cooled every stroke to below that heat."

"Dr. Robison, in the article Steam Engine, after passing an encomium upon me, dictated by the partiality of friendship, qualifies me as the 'pupiland intimate friend of Dr. Black,'—a description which not being there accompanied with any inference, did not particularly strike me at the time of its first perusal. He afterwards, in the dedication to me of his edition of Dr. Black's lectures upon chemistry, goes the length of supposing me to have professed to owe my improvements upon the steam engine to the instructions and information I had received from that gentleman, which certainly was a misapprehension; as, though I have always felt and acknowledged my obligations to him for the information I had received from his conversation, and particularly for the knowledge of the doctrine of latent heat, I never did norcouldconsider my improvements as originating in those communications. He is also mistaken in his assertion (p. 8. of the preface to the above work), that 'I had attended two courses[Pg094]of the doctor's lectures;' for, unfortunately for me, the necessary avocations of my business prevented me from attending his or any other lectures at college; and as Dr. Robison was himself absent from Scotland for four years at the period referred to, he must have been misled by erroneous information. In p. 184. of the lectures, Dr. Black says, 'I have the pleasure of thinking that the knowledge we have acquired concerning the nature of elastic vapours, in consequence of my fortunate observation of what happens in its formation and condensation, has contributed in no inconsiderable degree to the public good bysuggestingto my friend Mr. Watt of Birmingham, then of Glasgow, his improvement on this useful engine' (meaning the steam engine of which he is then speaking). There can be no doubt from what follows in his description of the engine, and from the very honourable mention which he has made of me in various parts of his lectures, that he did not mean to lessen any merit that might attach to me as an inventor; but, on the contrary, he was always disposed to give me fully as much praise as I deserved.

"And were that otherwise doubtful, it would, I think, be evident from the following quotation from a letter of his to me, dated 13th February 1783, where, speaking of an intended publication by a friend of mine, on subjects connected with the history of steam, he says, 'I think it is very proper for you to give him a short account of your discoveries and speculations;and particularly to assert clearly and fully your sole right to the honour of the improvements of the steam engine.' And in a written testimonial which he very kindly gave me, on the occasion of a trial at law against a piracy of my invention in 1796-7, after giving a short account of the invention, he adds, 'Mr. Watt was the sole inventor of the capital improvement and contrivance above mentioned.'

"Under this conviction of his candour and friendship, it is very painful to me to controvert any assertion or opinion of my revered friend; yet, in the present case I find it necessary to say, that he appears to me to have fallen into an error; and I hope, in addition to my assertion, to make that appear by the short history I have given of my invention, in my[Pg095]notes upon Dr. Robison's essay, as well as by the following account of the state of my knowledge previous to my receiving any explanation of the doctrine of latent heat; and also from that of the facts which principally guided me in the invention.

"It was known very long before my time, that steam was condensed by coming into contact with cold bodies, and that it communicated heat to them; witness the common still, &c. &c.

"It was known, by some experiments of Dr. Cullen and others, that water and other liquids boiled in vacuo at very low heats; water below 100°.

"It was known to some philosophers that the capacity or equilibrium of heat, as we then called it, was much smaller in mercury and tin than in water.

"It was also known that evaporation caused the cooling of the evaporating liquid, and bodies in contact with it.

"I had myself made experiments to determine the following facts:—

"First, the capacities of heat for iron, copper, and some sorts of wood, comparatively with water.

"Second, the bulk of steam compared with that of water.

"Third, the quantity of water evaporated in a certain boiler by a pound of coals.

"Fourth, the elasticities of steam at various temperatures greater than that of boiling water, and an approximation to the law which it followed at other temperatures.

"Fifth, how much water in the form of steam was required every stroke by a small Newcomen's engine, with a wooden cylinder six inches diameter, and twelve inches stroke.

"Sixth, the quantity of cold water required in every stroke to condense the steam in that cylinder, so as to give it a working power of about 7 lb. on the inch.

"Here I was at a loss to understand how so much cold water could be heated so much by so small a quantity of water in the form of steam; and I accordingly applied to Dr. Black, and then first understood what was called latent heat.

"But this theory, though useful in determining the quantity of injection necessary where the quantity of water[Pg096]evaporated by the boiler, and used by the cylinder, was known, and in determining, by the quantity and heat of the hot water emitted by Newcomen's engines, the quantity of steam required to work them did not lead to the improvements I afterwards made in the engine. These improvements proceeded upon the old established fact, that steam was condensed by the contact of cold bodies; and the later known one, that water boiled in vacuo at heats below 100°, and consequently that a vacuum could not be obtained unless the cylinder and its contents were cooled every stroke to below that heat."

LOCH LOMOND.

FOOTNOTES:[16]We are indebted for many of the anecdotes of the life of Watt to theEloge Historique, recently published by M. Arago, who was furnished with all the documents and circumstances relating to this celebrated person which were considered proper for publication, by his son, the present James Watt, Esq., of Aston Hall, near Birmingham, and to the notes added to this memoir by Mr. Muirhead, a relative of Mr. Watt.[17]The following is the account of these experiments given in Watt's own words:—"It being evident that there was a great error in Dr. Desagulier's calculations of Mr. Beighton's experiments on the bulk of steam, a Florence flask, capable of containing about a pound of water, had about one ounce of distilled water put into it; a glass tube was fitted into its mouth, and the joining made tight by lapping that part of the tube with packthread covered with glazier's putty. When the flask was set upright, the tube reached down near to the surface of the water, and in that position the whole was placed in a tin reflecting oven before a fire until the water was wholly evaporated, which happened in about an hour, and might have been done sooner, had I not wished the heat not much to exceed that of boiling water. As the air in the flask was heavier than the steam, the latter ascended to the top, and expelled the air through the tube. When the water was all evaporated, the oven and flask were removed from the fire, and a blast of cold air was directed against one side of the flask, to collect the condensed steam in one place. When all was cold, the tube was removed, the flask and its contents were weighed with care; and the flask being made hot, it was dried by blowing into it by bellows, and when weighed again was found to have lost rather more than four grains, estimated at41⁄3grains. When the flask was filled with water, it was found to contain about171⁄8ounces avoirdupois of that fluid which gave about 1800 for the expansion of water converted into steam of the heat of boiling water."This experiment was repeated with nearly the same result, and in order to ascertain whether the flask had been wholly filled with steam, a similar quantity of water was for the third time evaporated; and, while the flask was still cold, it was placed inverted with its mouth (contracted by the tube) immersed in a vessel of water, which it sucked in as it cooled, until in the temperature of the atmosphere it was filled to within half an ounce measure of water."In repetitions of this experiment at a later date, I simplified the apparatus by omitting the tube, and laying the flask upon its side in the oven, partly closing its mouth by a cork, having a notch on one side, and otherwise proceeding as has been mentioned.

[16]We are indebted for many of the anecdotes of the life of Watt to theEloge Historique, recently published by M. Arago, who was furnished with all the documents and circumstances relating to this celebrated person which were considered proper for publication, by his son, the present James Watt, Esq., of Aston Hall, near Birmingham, and to the notes added to this memoir by Mr. Muirhead, a relative of Mr. Watt.

[17]The following is the account of these experiments given in Watt's own words:—"It being evident that there was a great error in Dr. Desagulier's calculations of Mr. Beighton's experiments on the bulk of steam, a Florence flask, capable of containing about a pound of water, had about one ounce of distilled water put into it; a glass tube was fitted into its mouth, and the joining made tight by lapping that part of the tube with packthread covered with glazier's putty. When the flask was set upright, the tube reached down near to the surface of the water, and in that position the whole was placed in a tin reflecting oven before a fire until the water was wholly evaporated, which happened in about an hour, and might have been done sooner, had I not wished the heat not much to exceed that of boiling water. As the air in the flask was heavier than the steam, the latter ascended to the top, and expelled the air through the tube. When the water was all evaporated, the oven and flask were removed from the fire, and a blast of cold air was directed against one side of the flask, to collect the condensed steam in one place. When all was cold, the tube was removed, the flask and its contents were weighed with care; and the flask being made hot, it was dried by blowing into it by bellows, and when weighed again was found to have lost rather more than four grains, estimated at41⁄3grains. When the flask was filled with water, it was found to contain about171⁄8ounces avoirdupois of that fluid which gave about 1800 for the expansion of water converted into steam of the heat of boiling water."This experiment was repeated with nearly the same result, and in order to ascertain whether the flask had been wholly filled with steam, a similar quantity of water was for the third time evaporated; and, while the flask was still cold, it was placed inverted with its mouth (contracted by the tube) immersed in a vessel of water, which it sucked in as it cooled, until in the temperature of the atmosphere it was filled to within half an ounce measure of water."In repetitions of this experiment at a later date, I simplified the apparatus by omitting the tube, and laying the flask upon its side in the oven, partly closing its mouth by a cork, having a notch on one side, and otherwise proceeding as has been mentioned.

[17]The following is the account of these experiments given in Watt's own words:—

"It being evident that there was a great error in Dr. Desagulier's calculations of Mr. Beighton's experiments on the bulk of steam, a Florence flask, capable of containing about a pound of water, had about one ounce of distilled water put into it; a glass tube was fitted into its mouth, and the joining made tight by lapping that part of the tube with packthread covered with glazier's putty. When the flask was set upright, the tube reached down near to the surface of the water, and in that position the whole was placed in a tin reflecting oven before a fire until the water was wholly evaporated, which happened in about an hour, and might have been done sooner, had I not wished the heat not much to exceed that of boiling water. As the air in the flask was heavier than the steam, the latter ascended to the top, and expelled the air through the tube. When the water was all evaporated, the oven and flask were removed from the fire, and a blast of cold air was directed against one side of the flask, to collect the condensed steam in one place. When all was cold, the tube was removed, the flask and its contents were weighed with care; and the flask being made hot, it was dried by blowing into it by bellows, and when weighed again was found to have lost rather more than four grains, estimated at41⁄3grains. When the flask was filled with water, it was found to contain about171⁄8ounces avoirdupois of that fluid which gave about 1800 for the expansion of water converted into steam of the heat of boiling water.

"This experiment was repeated with nearly the same result, and in order to ascertain whether the flask had been wholly filled with steam, a similar quantity of water was for the third time evaporated; and, while the flask was still cold, it was placed inverted with its mouth (contracted by the tube) immersed in a vessel of water, which it sucked in as it cooled, until in the temperature of the atmosphere it was filled to within half an ounce measure of water.

"In repetitions of this experiment at a later date, I simplified the apparatus by omitting the tube, and laying the flask upon its side in the oven, partly closing its mouth by a cork, having a notch on one side, and otherwise proceeding as has been mentioned.

GLASGOW.

[Pg097]TOCINX

EXPOSITION OF PHYSICAL PRINCIPLES.—THERMOMETER.—METHOD OF GRADUATING IT.—FREEZING AND BOILING POINTS.—LATENT HEAT OF WATER.—QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER.—QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO ICE.—PROCESS OF BOILING.—OF RECONVERSION OF STEAM INTO WATER.—QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO STEAM.—BOILING POINT OF WATER.—DIFFERENT IN DIFFERENT PLACES.—DEPENDS ON THE BAROMETER.—VARIES WITH THE PRESSURE.—EXPERIMENTAL PROOF OF THIS.—BOILS AT LOWER TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE ATMOSPHERE.—SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS THE SAME.—THE FUEL NECESSARY TO EVAPORATE WATER THE SAME, WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS EVAPORATED.—MECHANICAL FORCE OBTAINED BY EVAPORATION.—THIS FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES.

(51.)We shall pause here to put the reader in possession of the physical and mechanical principles connected with the evaporation of water and other liquids, which are necessary to enable him to understand the full extent of the value and the merit of the discoveries of Watt, and to comprehend the[Pg098]structure and operation of the steam engine in its improved form, as it has passed to us from his hands.

As we shall frequently have occasion to refer to the indications of a thermometer, we shall first explain the principle of that instrument as it is commonly used in this country.

The thermometer is an instrument used for the purpose of measuring and indicating the temperature or sensible heat of material substances.

Heat, like all other physical agents, can only be measured by its effects. One of these effects best suited for this purpose, is the change of dimension which all bodies undergo in consequence of their change of temperature. In general, when heat is applied to a material substance, that substance undergoes an enlargement of bulk; and if heat be abstracted from it, it suffers a diminution of bulk. This variation of magnitude is not always in the same proportion as the increase or diminution of temperature; but it is so when applied to certain substances and between certain limits. One of the substances whose expansion and contraction through an extensive range of temperature has been found to be nearly uniform, and which is attended with other convenient qualities for a thermometer, is the liquid calledmercuryorquicksilver. A mercurial thermometer is constructed in the following way:—

A glass tube is made with a small and uniform bore: upon the end of this tube, a bulb is blown, having a magnitude very great compared with the bore of the tube. Let us suppose this bulb and a part of the tube to be filled with mercury. If the mercury contained in the bulb be heated, it will expand, and being more susceptible of expansion than the glass which contains it, the bulb will be too small for its augmented volume: the mercury in the bulb can only, therefore, obtain room for its increased bulk by pressing the mercury in the tube upwards, which it will accordingly do. The increase of volume which the mercury in the bulb therefore undergoes, will be exhibited by the increased length of the column in the tube. Since the bore of the tube is made so exceedingly minute compared with the magnitude of the bulb, a very small quantity of mercury forced[Pg099]from the bulb into the tube, will cause a considerable increase of the length of the column. Small degrees of expansion will therefore be rendered very apparent, and may be accurately measured. The following is the method by which the thermometer calledFahrenheit's thermometeris graduated.

The tube and bulb being prepared and supplied with mercury, as already explained, let the instrument be plunged in a vessel of melting ice. It will be found that the mercury will stand in the tube at a certain point, from which it will not vary so long as any ice remains not completely melted in the vessel. Let a mark be made on the tube, or on a scale attached to the tube, at the point corresponding to the top of the column: the point thus marked is called thefreezing point.

Now let the instrument be immersed in a vessel of boiling water, the barometer at the time having the height of thirty inches. It will be found that so long as the water is kept boiling, the column of mercury in the tube will remain stationary. Let the point corresponding with the top of the column be marked on the tube, or on the scale attached to it. This is called theboiling point. Let the space on the scale between the freezing and boiling points be now divided into 180 equal parts: each of these parts is called adegree. Let the same divisions be continued upon the scale below the freezing point, until thirty-two divisions be taken; let the lowest division be then marked 0, and let the successive divisions upwards from that be numbered 1, 2, 3, &c. In like manner, let the same divisions be continued above the boiling point, as far as the tube will admit.

It is evident that, under these circumstances, the freezing point will be marked by 32, and the boiling point by 212. It is usual to express the degrees of a thermometer in the same manner as the degrees of a circle, by placing a small ° above the number. Thus the freezing point is expressed by 32°, and the boiling point by 212°.

The reason the degrees were commenced at 32° below the freezing point was, because, when the thermometer was invented, that temperature was supposed to be the lowest degree of cold possible, being that of a certain mixture of[Pg100]snow and salt. This, however, has since been found to be an error, very much lower temperatures being obtained by various physical expedients.

The temperature of a body is, then, that elevation to which the thermometer would rise when immersed in that body. Thus, if in plunging the thermometer in water we found the mercury to rise or fall to the division marked 100, we should then say, the temperature of the water was 100°.

Let us suppose a spirit lamp, or other regular source of heat, applied to a bath of mercury, so as to maintain the mercury at a fixed temperature of 200°, and let another vessel, containing a quantity of ice at a temperature of 20° be immersed in the mercury. Let a thermometer be placed in the mercury, and another in the ice. The following effects will then ensue. The thermometer immersed in the ice will be observed gradually to rise from 20° upwards, until it indicates the temperature of 32°. It will then become stationary, and the ice which had hitherto remained in a solid state will begin to melt and be converted into water. This process of liquefaction will continue for a considerable time, during which the thermometer immersed in the ice will constantly be maintained at 32°. At the moment, however, when the last portion of ice is liquefied, the thermometer will begin again to rise. The coincidence of this ascent of the thermometer with the completion of the liquefaction of the ice, may be very easily observed, because the ice being lighter, bulk for bulk, than water, will float on the surface, and so long as a particle of it remains unmelted it will be distinctly seen.

Now it cannot be doubted that, during the whole of this process, the mercury, supposed to be maintained at 200°, constantly imparts heat to the ice; yet, from the moment the liquefaction begins, until it is completed, no increased temperature is exhibited by the thermometer immersed in the melting ice. If during this part of the process no heat were received by the ice from the mercury, the consequence would be, that the application of the lamp would cause the temperature of the mercury to rise above 200°, which may be easily demonstrated by withdrawing the vessel of ice from the mercurial bath during the process of liquefaction. The moment[Pg101]it is withdrawn, the thermometer immersed in the mercury, instead of remaining fixed at 200°, will begin to rise, although the action of the lamp remains the same as before; from which it is evident that the heat which now causes the mercury to rise above 200° was before received by the melting ice.

The heat which thus enters ice in the process of liquefaction, and which is not indicated by the thermometer, is for this reason calledlatent heat. It will be perceived that this phrase is the name of a fact, and not of an hypothesis. That heat really enters the water, and is contained in it, has been established by the experiments; and to declare that it is present there, is to declare an established fact. To call it by the namelatentheat, is to declare another established fact, viz., that it is not sensible to the thermometer.

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,sensibleorfree 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°feelequally 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 intovapour, orsteam, 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 calledebullitionorboiling. 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 infig.15.

Fig. 15.

Ais a cistern of heated mercury, in which the glass vesselB, containing water, is immersed. From the top of the vesselBproceeds a glass tubeC, inclining downwards, and entering a glass vesselD, which is immersed in a cisternEof cold water. If the process already described be continued until the water by constant ebullition has disappeared, as already mentioned,[Pg105]from the vesselB, it will be found that a quantity of water will be collected in the vesselD; 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 vesselB. 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 tubeCand the upper part of the vesselB, 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 tubeCis cold, and the inner surface dry. When the process of ebullition has continued a short time, the tubeCwill 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 vesselBto the vesselD.

(54.)These effects are easily explained. The water in the vesselBis 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 vesselB. 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 tubeC, 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 tubeC. At length, however, the whole of the tubeCis 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 inBincreases, it drives before it the steam previously collected in the tubeC, and forces it into the vesselB. 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 vesselD; 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 vesselB, the quantity will be found to be gradually increased in the vesselD; 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 inDis exactly equal to the weight which the water inBhas lost.

(55.)The demonstration is, therefore, perfect, that the gradual diminution of the boiling water in the vesselBis produced by the conversion of that water into steam by the heat. In the process first described, when the top of the vesselBwas 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 vesselBis 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 vesselBbe 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]vesselBremains immersed, it prevents the mercury from increasing in temperature. It therefore receives that heat which would otherwise raise the temperature of the quicksilver.

Fig. 16.

If a thermometer be immersed in the steam which collects in the upper part of the vesselB, 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 vesselA(fig.16.), and let another vesselB, in which water is kept constantly boiling at the temperature of 212°, communicate withAby a pipeCproceeding from the top, so that the steam may be conducted fromB, and escape from the mouth of the pipe at some depth below the surface of the water inA. 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 inAwill be gradually heated by the steam combined with it and received through the pipeC. If this process be continued until the water inAis 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 vesselBin the form of steam, and has been reconverted into water by the inferior temperature of the water inA. Now, this ounce of water received in the form of steam into the vesselAhad, 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 inB, 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.

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