STEAM

THE articles STEAM and STEAM-ENGINE in the third edition of the present work were originally written by DR ROBISON, and were long the standards of reference upon these subjects. The rapid progress of science and of the mechanical arts during the present century, has now rendered it necessary to substitute for these articles those maturer results of recent research, to the attainment of which the original papers were themselves the means of very materially conducing. The value of these original researches was further enhanced by passing through the hands of the man of all others the most capable of appreciating their value, and the best qualified to increase it by his contributions. It was the early friend and companion of Professor Robison, Mr WATT himself, who, towards the close of his life, and notwithstanding the laborious nature of the undertaking, agreed to revise those articles for republication, so as to present them to the public with somewhat of that greater completeness which it is to be presumed their author would himself have conferred upon them, had he lived to see the investigations which he had begun, carried forward and completed. Mr Watt was, however, prevented by the weakness of increasing years from doing more than adding notes to these articles, and they were accordingly printed in that form; but they contain a most interesting chapter of the history of inventive genius, for they give us, in Watt's own words, the history of the progress and consummation of his own noble inventions, and display the efforts of genius working its way through the obscurities of imperfect knowledge to the discovery of pure truth and the achievement of the most exquisite combinations.

While, therefore, it was impossible to retain the articles themselves, it was highly desirable that all that had rendered them valuable should be retained, and more especially such portions as serve to record the state of the mechanics and physics of steam at that time, and the progress of the invention; and these, together with the notes of Mr Watt, it has been thought better to give in the precise words of the original, than to transpose them into language which could neither be more clear nor more appropriate than that with which their authors had invested them. Such portions of the articles on steam and the steam-engine, as have been in this manner retained, are distinguished by an appropriate mark. Paragraphs from the pen of Dr Robison have a star * placed at their commencement; those of Mr Watt, in like manner, have a cross †. To all that was interesting and valuable in the original articles, an attempt has been made to superadd whatever subsequent labour and research may have brought to light. These remarks apply to the articles STEAM and STEAM-ENGINE. STEAM NAVIGATION is entirely new.

1. * STEAM is the name given in our language to the visible, moist vapour which arises from all bodies which contain juices easily expelled from them by heats not sufficient for their combustion. Thus we say, the steam of boiling water, of malt, of a tan-bed. It is distinguished from smoke by its not having been produced by combustion, by not containing any soot, and by its being condensed

by cold into water, oil, inflammable spirits, or liquids composed of these.

2. * We see it rise in great abundance from bodies when they are heated, forming a white cloud, which diffuses itself and disappears at no very great distance from the body from which it was produced. In this case the surrounding air is found loaded with the water or moisture which seems to have produced it, and the steam seems to be completely soluble in air, composing, while thus united, a transparent elastic fluid.

3. * But, in order to its appearance in the form of an opaque white cloud, the mixture with or dissemination in the air seems necessary. If a tea-kettle boils violently, so that the steam is formed at the spout in great abundance, it may be observed, that the visible cloud is not formed at the very mouth of the spout, but at a small distance before it, and that the vapour is perfectly invisible at its first emission. This is rendered still more evident by fitting to the spout of the tea-kettle a glass pipe of any length, and of as large a diameter as we please. The steam is produced as copiously as without this pipe, but the vapour is transparent and colourless throughout the whole of the pipe. Nay, if this pipe communicate with a glass vessel terminating in another pipe, and if the vessel be kept sufficiently hot, the steam will be as abundantly produced at the mouth of this second pipe as before, and the vessel will remain quite transparent. The visibility, therefore, of the matter which constitutes the steam is an accidental circumstance, and appears to require its dissemination in the air; and we know that one perfectly transparent body, when minutely divided and diffused among the parts of another transparent body, but not dissolved in it, makes a mass which is visible. Thus oil beaten up with water makes a white opaque mass.

4. If the column of steam which ascends from a boiler that is suddenly opened, be observed in a clear dry day, when the sun is shining, the column of vapour, gradually widening as it rises, will be observed to be of a very brilliant silvery white, and will cast a strong dark shadow upon the objects which it intercepts from the direct rays of the sun: but, if the observer be placed in this shadow, the sun will appear to him to be of a strong tawny, or fiery red colour, or, if the column be very dense, the sun will be invisible. These appearances closely resemble some phenomena of the clouds, which we know are composed of watery vapour, and which sometimes appear of a fleecy white, again of a fiery red or a burnished gold colour, or again of a dappled grey, down through every degree of darkness, until the vapour become so dense and opaque as altogether to obscure the light of the sun by a thick black cloud. These appearances have been satisfactorily accounted for. Steam, in its attenuated state, is a transparent, invisible, colourless gas. When disseminated through the air, in excessive quantities, small globules are formed, of a film of water, enclosing light vapour. These globules, floating thickly in the air, form an aggregation of minute films of fluid, capable of reflecting and transmitting light. When we are so placed that the light may be reflected to us, and when the cloud is so thick as to reflect it completely, we have the same brilliant white which results from the comminution of glass, rosin, ice, and other transparent media; and, at the same time, an observer, placed on the opposite side of the cloud, sees it as a dense black opaque mass, because the light being totally

reflected, none of it can be transmitted to him. The richer colours transmitted by thinner strata of vapour are thus noticed by M. Leopold Nobili of Reggio. "The tints exhibited by the clouds in every variety of aspect are almost all comprised in Sir Isaac Newton's first ring (the white, yellow, orange, red; or the blood, tawny, copper, ochre, and fire red, and violaceous red, or No. 1-12 of Mr. Nobili's scale). Tints of this kind do not arise from refraction and diffraction, they are produced only by means of thin plates. Now the measurements of Sir Isaac Newton have shown what are the dimensions of the layers of air, of water, and of glass, which produce the colours of the several rings; and, as we know that the vesicular vapours are formed of water, and that they do not reflect or transmit any other tint, we may conclude that their external film is in no case thicker than ten millionth parts of an inch. This result appears to me to be so decidedly certain as to be entitled to a place in science." It was to the effect of thin plates on light that Newton referred the colours of all bodies; and the accounting for the rich golden hues of the clouds, and the fiery red colour of light passing through dispersed steam, by the effect of the thin plates of water enclosing the vapour spheroids of pure steam, must be regarded as one of the most satisfactory applications of his theory. (See article OPTICS.)

5. A very singular phenomenon takes place, if the flame of a candle or lamp be held below a jet of steam, as it issues from the mouth of a small pipe; the steam instantly ceases to be visible. In this case, one of two changes may be conceived to take place, either or both of which account for the permanent invisibility of the vapour: the intense heat of the flame may disperse the particles to such a distance, that there does not remain in a given space a sufficient number to form a vesicle of vapour, and it therefore remains diffused in combination with the air, which always holds a large quantity of invisible vapour, especially at high temperatures; or the vapour may be decomposed by the flame into the permanent and invisible gases, of which it consists, which may again become combined, to a certain extent, with the burning substance, and support the flame.

6. * When steam is produced, the water gradually wastes in the tea-kettle, and will soon be totally expended if we continue it on the fire. It is reasonable, therefore, to suppose that this steam is nothing but water, changed by heat into an aerial or elastic form. If so, we should expect that the privation of this heat would leave it in the form of water again. Accordingly, this is fully verified by experiment; for, if the pipe fitted to the tea-kettle be surrounded with ice, or any cold substance, no steam will issue, but water will continually trickle from it in drops; and if the process be conducted with the proper precautions, the water which we thus obtain from the pipe will be found equal in quantity to that which disappears from the tea-kettle. Steam is therefore the matter of water, converted by heat into an elastic vapour.

7. Steam, water, and ice, are three conditions of the same substance, which it assumes under different circumstances of heat and of external pressure. In each condition it obeys different laws; as a solid, ice obeys the laws of the mechanics of solid bodies; in Russia it is quarried like rock, and is used for building houses and paving ways; it is cast into moulds for domestic purposes, like iron or lead; it is painted like alabaster, and chiselled like marble; as a liquid, water is the exemplar of the hydrostatical laws of all fluids; as a vapour, it obeys the laws of aerostatics; and we now know that steam is, in all respects, similar in its constitution and phenomena to all other elastic fluids or gases. If we apply heat to a bar of extremely cold ice, it expands like other solids with heat, gradually elongating with its increased temperature, its particles receding

from one another by the repulsive action induced between the particles by the entrance of caloric between them, the cohesion of the particles becoming less and less, until at last, if the heat be continually thrown in, the cohesion of the particles is altogether overcome, they lose their aggregation, they become separable without effort, and, falling to pieces, the bar of ice loses its form and subsides into water. When thus melted, the water being placed in a vessel, and having heat applied to it, will, like other fluids, continue to expand from its point of greatest density, and will increase in bulk nearly one-twentieth by about 172°, but at last the entrance of so large a quantity of heat will produce a repulsive force between the particles so strong as to cause them suddenly to spring apart from one another, so as to recede to a distance twelve times as far asunder as in the state of water, and they have now assumed the aerial condition of gas or vapour, and constituting steam, occupy 1728 times their original space. The ice passing into the condition of water, is said to be liquefied, and the heat necessary to convert ice into water is called the caloric of liquidity of ice, or the caloric of condition of water; when water is converted into steam, the quantity of caloric necessary for this purpose is called the caloric of vaporization of water, or the caloric of elasticity of steam, and the water is then said to boil or evaporate. This process may be reversed. If the steam have been collected in a close receptacle, it may be squeezed by external compression into its original bulk, or by cooling the outside so as to withdraw the caloric of elasticity from between the particles, they may be allowed to come together by the attraction of cohesion, and resuming their original proximity to each other, appear once more in their former condition of water, and in this case the vapour is said to be condensed; and if the process of abstraction of caloric, with sufficient pressure, be continued, the liquid particles approaching each other, will gradually contract the bulk of the mass, and at a certain point will take again the original character of ice, and the liquid is then said to be congealed or frozen. The same particles of matter do "thus in turn play many parts."

Ice melts and becomes water by increment of heat.

Water evaporates into steam by increment of heat.

Steam is condensed into water by decrement of heat.

Water congeals into ice by decrement of heat.

(8.) These phenomena are not confined to one substance: many substances, apparently the most refractory, of the have been melted and again congealed, while other substances which had never been observed in any other form than that of transparent air or invisible gas, have been condensed by the expedients of modern artifice into liquids heavier than water, and have even been congealed into hard and strong solids. To so great an extent has this taken place, that we are now almost warranted in deducing, from a wide induction of facts, the following generalization; that all bodies assume the solid, liquid, or gaseous condition, according to the accidents of temperature and pressure under which they happen to be placed; and that it is merely from the circumstance of their being more ordinarily found, at the present temperature of the earth and under the weight of our present atmosphere, in one of these states rather than another, that some substances have been characterised and distinguished, and classed as permanent solids, liquids, or airs. We now speak of ice only as frozen water; but had we lived under a temperature such as that which the inhabitants of the planet Jupiter, at their distance from the sun, may be conceived to endure, we should have spoken of swallowing melted ice as we now speak of molten lead, and a separate name for melted ice would have remained unknown; or, if we conceive, in like manner, our air to be withdrawn, and the

Steam. temperature of the earth raised above $212^\{\backslash circ\}$, we should then have moved under an atmosphere of steam of the same pressure as at present, transparent and colourless, and might only have heard of water as a curious substance obtained from the compression of the air. The phenomena of steam are much simplified and more perfectly explained when we take this enlarged view of its analogy with other kinds of matter.

Phenomena of Ebullition. 9. We are most familiar with steam when in the act of rising violently from heated water in the process of ebullition. The history of steam at this crisis is highly instructive, and its phenomena may be studied with advantage by examining it in a glass vessel placed over a strong lamp. When heat is first applied, a rapid circulation of the fluid ensues. The water on the bottom, being first heated and expanded, becoming lighter than the rest, rises to the top, and is replaced by the current of colder water descending to receive in its turn a further accession of heat. By and by, small globules of steam, formed on the bottom and surrounded by a film of water, are observed adhering to the glass; as the heat increases they enlarge, in a short time several of them unite, form a bubble larger than the others, and, detaching themselves from the glass, rise upwards in the fluid. But they never reach the surface; they encounter currents of water still comparatively cold, and descending to receive from the bottom their supply of heat; and encountering them, the bubbles are robbed of their heat, shrivel up into their original bulk, and are lost among the other particles of water. In a short time the mass of the water becomes more uniformly heated, the bubbles, becoming larger and more frequent, are condensed with a loud crackling noise, and at last, when the heat of the whole mass reaches $212^\{\backslash circ\}$, the bubbles from the bottom rise without condensation through the water, swell and unite with others as they rise, and burst out upon the air in a copious volume of steam, of the same heat as the water from which they are formed, and pushing aside the air, make room for themselves. In this process, by continuing the application of heat, the whole of the water may be "boiled away" or converted into steam.

Sounds of Simmering and Ebullition. 10. The singular sounds produced from a vessel of water exposed to heat, previously to boiling, have attracted attention; the water is then vulgarly said to be simmering or singing; and, when this takes place, it is because the vessel is boiling at one place and comparatively cold at another. This noise is most distinctly heard when the fire or flame applied is small, and its heat intense, when the vessel is large and the water deep; for in that case the entrance of the caloric will take place more rapidly than the circulation can convey it to the remote particles of fluid, and so bubbles of steam will form rapidly at one place and be rapidly condensed at another; the degree of velocity with which such bubbles succeed will determine the pitch of the singing tone. We have observed this phenomenon in greatest perfection when we have attached a slender pipe to a close boiler producing steam, and carried its open mouth, of the diameter of $\backslash frac\{1\}\{2\}$ or $\backslash frac\{1\}\{3\}$ of an inch, down below the surface of cold water in a glass jar. When the mouth of the steam-pipe is held just below the surface of the water, the steam issues with great rapidity in small bubbles, producing an acute tone; and, on the other hand, when the pipe is held at a considerable depth, the concussions become more violent and louder, their intervals of succession greater, the tone is lowered, and finally, the shocks become detached, and so violent as to shake the glass and surrounding objects with much force. On this subject Professor Robison observes, "that a violent and remarkable phenomenon appears, if we suddenly plunge a lump of red-hot iron into a vessel of cold water, taking care that no red part be near the surface. If the hand be now applied to the

side of the vessel, a most violent tremor is felt, and sometimes strong thumps; these arise from the collapsing of very large bubbles. If the upper part of the iron be too hot, it warms the surrounding water so much, that the bubbles from below come up through it uncondensed, and produce ebullition without concussion. The great resemblance of this tremor to the sensation which we experience during the shock of an earthquake, has led many to suppose that the latter is produced in the same way; and their hypothesis, notwithstanding the objections which we have elsewhere stated to it, is by no means unfeasible. Any obstruction on the bottom of a boiler, on the inside, as a piece of metal or stone introduced among the water, may produce a succession of smart concussions by the sudden condensation of gas collected under it.

11. The permanence of the boiling point is one of the most remarkable of the phenomena of ebullition. When water has once been brought to boil in an open vessel, it is not possible to make the water sensibly hotter, however strongly the fire may be urged or its intensity increased. This circumstance is very striking, because we know that heat continues to be thrown in exactly as fast as before the boiling point, and that in that case the heat rose rapidly, whereas now it has altogether ceased to increase. If a thermometer of mercury, air, oil, or metal be placed among the water, the temperature will constantly increase, and expand the matter of the thermometer, until the water boils, and then, whether it boil slowly or rapidly, with a strong fire or a gentle one, the thermometer will continue to stand at the same point. This point is so well defined, as to furnish our standard for the comparison of temperatures, and is the same on all thermometers, being called the boiling point, although it is differently numbered on each, being called $212^\{\backslash circ\}$ on our common thermometer or Fahrenheit's, $80^\{\backslash circ\}$ on Reaumur's, and $100^\{\backslash circ\}$ on the centigrade thermometer.

It is also to be remarked, that the temperature of the steam issuing from boiling water is the same with the temperature of the water itself, and remains equally invariable; so that all the steam produced from water boiling at $212^\{\backslash circ\}$ is itself at $212^\{\backslash circ\}$. This remark will assist us in accounting for the disposal of the heat which the fire gives out during the time of ebullition; for it is manifest that the heat is all the while carried off by the large volumes of steam, at a temperature of $212^\{\backslash circ\}$, that are diffused through the air; and so it happens that an increase of heat in the fire, instead of increasing the heat of the water, only increases the volumes of the steam thrown off, and the quantity of heat carried away. This view of the subject is confirmed by a simple experiment. Take a strong glass flask, place water in it and a thermometer among the water, and let it be held over a lamp until the water boil, and the thermometer will be observed rising till it reach $212^\{\backslash circ\}$, when the steam will begin to escape rapidly from the neck of the flask. Let it now be corked tightly, and the heat continually applied; and it will be observed that the thermometer does not now stand at $212^\{\backslash circ\}$, but rises rapidly from that point up to $220^\{\backslash circ\}$ and $230^\{\backslash circ\}$, showing that the free escape of the steam into the open air is necessary to the permanence of the boiling point. If the heat be still applied, the experiment may be rendered still more instructive, by suddenly pulling out the cork of the flask, when the vapour will instantly rush out in a large volume, and the thermometer sink down to $212^\{\backslash circ\}$, showing that all the excess of heat has been carried off by the steam into the air.

12. We have thus seen that a large quantity of heat may be given out to the particles of a certain quantity of water, converting them into steam, and yet that the thermometer shall afford no indication of this quantity. As soon as water boils, the whole mass is heated up to $212^\{\backslash circ\}$;

and although the same heat that produced the ebullition be still continually applied, and although we know that this heat must be continually entering into the water, still it is not detected, or in any way exhibited by the thermometer. On this account, the heat given to water during ebullition is said to become latent, or lie hid from the thermometer; and, indeed, the thermometer merely indicates the intensity of heat, the calorimeter alone can measure its quantity. The quantity of heat given out to water after it has begun to boil, is more than five-fold that which is sufficient to bring it from the freezing up to the boiling point; for, if we continue the fire with the same intensity that was used in bringing it to boil, it will require more than five-fold that duration and quantity of fuel to boil all the water away, or convert it all into steam of $212^\{\backslash circ\}$ of heat. Thus the sensible heat, added from $32^\{\backslash circ\}$, will be $180^\{\backslash circ\}$, and that latent in the steam is more than five-fold; or, in other words, the insensible caloric in steam is five-fold its sensible heat; or the same quantity of matter in the condition of steam at $212^\{\backslash circ\}$, and of water at $212^\{\backslash circ\}$, will hold different quantities of caloric, in the proportion of about 6 to 1. This is called the greater capacity of steam for caloric than of water for that substance; and it is in part accounted for, by the greater distances of the particles of the matter of steam and water from each other in the former than the latter condition; for when the distances of the particles are increased 12 times, the spheres of caloric around each atom may be much larger, without increased elasticity of the caloric fluid. Dr Black was the discoverer of the admirable doctrine of latent heat.

13. Dr Dalton has thus illustrated the doctrine of latent heat, and of the increased capacity of a liquid for holding caloric, when it passes into the condition of vapour. The liquid and its vapour may be considered as two reservoirs of caloric, capable of holding different quantities of that fluid. Let figure 1. represent to us such an arrangement; the internal cylinder of smaller capacity, the external one of enlarged capacity surrounding and extending far above it, and a small open tube of glass, communicating freely at the bottom with the inside of the cylinders. Let us now conceive water to be poured into the internal cylinder, the water will manifestly flow into the slender tube till it stand on the same level in the tube as in the cylinder. If any additional quantity be now poured into the internal cylinder, the rise of water in the slender glass tube will serve as an index of the quantity of added fluid; and when it is filled to the top, the fluid will stand at the height marked $212^\{\backslash circ\}$, and will still be a correct index of the addition of fluid. But if more water be now added to it, it will not make its appearance in the slender tube, but will simply overflow from the internal cylinder over into that of enlarged capacity, so that, while a large quantity is passing into the vessel and gradually filling it up to $212^\{\backslash circ\}$, no additional rise takes place until the whole of the outer cylinder become filled to that point, after which any further addition will again become sensible, by a corresponding rise in the tube. This process is in precise analogy to the succession of circumstances in heating a liquid, and converting it into steam. The internal cylinder represents the liquid, the external one the vapour of greater capacity, and the slender glass tube at the side the thermometer placed in communication with

them. When heat flows into the liquid, it passes equally into the thermometer; and each increment of the one produces an equal increment in the other, until the liquid reaches the limit of its capacity, when it suddenly begins to enlarge its bulk and take the form of steam; but the quantity of heat required to fill up this enlarged capacity is so great as to require about $5\backslash frac\{1\}\{2\}$ times as much to fill it as was contained in the whole liquid before, so that all this time the thermometer is standing still, and it is not until the whole of the steam is thus supplied with $212^\{\backslash circ\}$ of caloric, that the thermometer will begin to show any further elevation; after which, any increment of heat thrown into the steam will make its appearance on the thermometer, and proceed as formerly, by simultaneous increments.

14. It appears, therefore, that the cause why water boiling under the open air does not reach a higher temperature than $212^\{\backslash circ\}$, is, that the steam which is raised by Dr Dalton any additional heat, carries that additional quantity of heat along with it into the air. But here a question occurs at once to the enquirer into these phenomena, viz. Why does water require to be heated up to $212^\{\backslash circ\}$ before it will throw off its increments of heat and vapour into the air? Why does not steam rise equally strongly from water at $200^\{\backslash circ\}$ or $180^\{\backslash circ\}$? The categorical reply is, that the elastic force of the heat is not sufficient to enable the steam to force its way against the pressure of the air until it reaches this point. In order to understand the means by which we arrive at this conclusion, it is necessary to know that, when the pressure of air on the surface of the water is artificially diminished, the steam does actually rise, and the water bubbles and boils with great violence, at temperatures far below $212^\{\backslash circ\}$. It is only when the surface of the water is exposed to the full pressure of the air in a common vessel that it is prevented from rising in vapour, at temperatures lower than the usual boiling point. If the surface of the hot water be protected from the pressure of the air, by being placed under a glass shade, and the air removed from the inside of it by an air-pump, the water may be made to boil at all temperatures below $212^\{\backslash circ\}$. The following table contains the results of a series of experiments made, with great care, by Dr Dalton, towards the end of last century, in order to ascertain how much of the whole pressure of the air it was necessary to remove, in order to make water boil at a given temperature. In order to understand the

way in which this table was formed, the reader must conceive a vessel of water first of all boiling at $212^\{\backslash circ\}$ in the open air, as the vessel A in figure 2, the thermometer I being placed in it. After allowing the water to cool to $200^\{\backslash circ\}$, let the vessel of water and the immersed thermometer be now placed on the plate stand P of an air-pump, and covered over with a strong glass receiver R; and let a portion of the enclosed air be now withdrawn by the pump from the inside of the receiver by the pipe F; and suppose that there are in all 30 cubical inches, or other volumes, of air in the receiver at first, then the water being at $200^\{\backslash circ\}$, when about 7

Steam. out of the 30 parts of the air have been withdrawn, leaving only about 23 parts out of 30 pressing on the water, it will be observed instantly to commence giving off steam in rapid ebullition. If next the process be repeated, only allowing the water to cool to 190°, the ebullition will not commence in this lower temperature till about 12 out of the 30 volumes of air have been withdrawn; and if, in a third experiment, the water be cooled down to 180°, the elastic force communicated by this degree of heat will not be capable of overcoming the resistance arising from the pressure of the air, until one half of the original pressure of 30 has been removed. To this process there is no limit; for as we go on lowering the temperature, we can always find a point at which the water will boil, provided the counteracting pressure be sufficiently diminished. The following is Dr Dalton's table, containing the results of his experiments, as given in his Meteorology, in 1793:

15. In vacuo, therefore, or under a rarified atmosphere the boiling point of water is lower than 212°. Now, the barometer informs us, that the pressure of our atmosphere is not constantly the same; it has normal and abnormal variations, it has horary, and menstrual, and annual variations. It frequently stands at 30 inches, sometimes at 31 inches; and on the morning of the 7th of January, 1839, it was observed at Edinburgh, by Sir John Robison, to be as low as 27 inches and six-tenths parts. Now, on that morning, water would have been found to boil in the open air at about 208°, instead of 212; and for every depression of the barometer, there is a corresponding depression of the boiling point. This variation of the boiling temperature with the variation of the barometer, and of the corresponding density of the air, is important: and the following short table shows the changes which take place within the limits of the usual variations of the weather: When the barometer stands at 31.8, water boils at 215°

But these extremes are probably greater than have ever been observed on the ordinary level of this country.

16. There is yet another variation of circumstance which affects the point of ebullition, and that is, distance from the centre of the earth and height above the level of the sea. It is well known, that, on the summit of a mountain, the pressure of the air is less than on a plain, and still less there than at the bottom of a pit or deep valley. It is now equally well known, that the cause of this is the

very limited height to which air in a dense state covers the earth, the whole atmosphere being equivalent to not more than 5 miles in depth of such air as we breathe; and it is hence obvious, that after a vertical ascent of a mile to the top of a mountain, there would be only about $\backslash frac\{1\}\{3\}$ of the atmosphere remaining above the person on its summit. One of the highest of the Andes has been ascended to such a height, that there remained only $\backslash frac\{1\}\{3\}$ of the whole atmosphere above the observer. Now, in this case, the barometer, instead of being sustained at 30 inches, its usual height, had fallen to 13 inches, because, according to the constitution of the barometer (See Arts. BAROMETER and PNEUMATICS), the height of the column of mercury in it is proportional to the quantity of air resting above it. Hence, a barometer being carried up a mountain by an observer, falling as he ascends, enables him to ascertain the height of his ascent. This he does with perfect precision, so as to determine accurately the height of any point of the mountain to which he has ascended, and where he has noticed the fall of the barometer from the point where it stood when at the bottom, by means of an allowance of nearly 100 feet of height for every tenth part of an inch that the barometer has fallen, as explained more fully under the heads BAROMETER and ATMOSPHERE.

The steam of water may be rendered the means of determining the height of a mountain, on the principle of diminished atmospheric pressure, so as to act as a substitute for the barometer. We have just seen that water gives off steam by ebullition, above or below the temperature 212°, according as the pressure of the atmosphere is greater or less than the standard pressure which sustains the barometer at 30 inches. And we have already given a table (Arts. 14 and 15), showing how much the boiling point was raised or depressed by diminishing the pressure of the atmosphere. On consulting Dr Dalton's table, we see that, when $\backslash frac\{1\}\{3\}$ of the air were removed, water boiled at so low a temperature as 180°. This, therefore, would show that, if water boiled on the top of any mountain at 180°, the barometer would stand there at a height of little more than 15 inches; and if at the bottom of the mountain water boiled at 212°, showing the barometer to be then at 30 inches, a similar allowance of height being made, viz. about 1000 feet for each inch, or 15,000 feet, would be a rude approximation to the true height. The table at the end of the third section, and the rules under the head BAROMETER in this work, will enable any one who studies this subject to form rules for closer approximation; but the following table will be of use to those who may merely wish to put it in practice.

Rule for finding heights by boiling water. — Boil pure water in an open vessel at the bottom of the elevation, and observe on the thermometer the point at which it boils. Boil it again at the top of the mountain, and observe with the thermometer the height at which it now boils: the difference of temperature, multiplied by 530 feet, will give a close approximation to the height of the upper above the lower station.

This will give an approximation; but, if greater accuracy be required, it will further be necessary to correct for the difference of the temperature of the air at the two stations, in the following manner. Add the temperatures of the air at the stations, and subtract 64 from their sum, multiply the remainder by one-thousandth part of the height found; and this will be the correction to be added to the height formerly found. The result thus found will still require a slight correction for the figure of the earth and latitude of the place; but this does not amount to more in our latitude than an addition of about two feet in a thousand, which forms a second correction. This method is, however, to be regarded only as an approximation,

for which all the corrections given under the head BAROMETER would be necessary, in order to render it equally perfect with observations by that instrument. In short, this method may be considered as a telltale on the barometer, showing where the barometer would stand if placed in its position. Thus, if water boil at 200° on the top of a mountain, that is merely to be considered as indicating that the barometer, if placed there, would stand at 22.8; after which, the process of deducing the height remains the same. To illustrate the mode of deducing heights from the boiling point, as we have given it, we take the following example.

Water boils on the top of Ben Nevis at 203.8°, while at the side of the Caledonian Canal it boils at 212°, the temperature being 30° on the summit of the mountain, and 35° below. In order to determine the height,

Calc. height, 4359.7 third approximation.

4358 true measured height—the difference being less than 2 feet.

This method, however, is seldom susceptible of so high a degree of accuracy, even with the most carefully conducted experiments.

17. This method of determining heights by the ebullition of water is not a recent invention. It was suggested originally by Mr. Fahrenheit, in the 33d volume of the Philosophical Transactions, in a paper entitled "Barometri Novi Descriptio." The subject was further matured by Cavallo, who has written concerning it in the 71st volume of the same Transactions; and the method has finally received from the Rev. F. J. H. Wollaston the highest degree of perfection of which it seems to be capable. His paper, read before the Royal Society on the 6th of March, 1817, and afterwards published in the Philosophical Transactions of that year, gives an account of the very beautiful and ingenious apparatus which he has contrived for facilitating the procedure of taking the observations with the requisite precision. Fig. 4, is a view of the whole apparatus, consisting principally of a tripod stand, surrounded by a sort of tent cover, which is quite essential for the protection of the lamp from the strong winds generally encountered at considerable altitudes. The lamp acts on a small tin vessel, which is a cylinder 5½ inches deep and 1½ in diameter, the sides of which are double, leaving an interstitial space of confined air to prevent cooling. Above this vessel is a circular plate of metal G H K, to which the thermometer is to be fixed; and the scale and neck of the thermometer are seen projecting above the stand. A (fig. 3) represents the thermometer made use of, which it is desirable to have of as strong and as compact a construction as possible, while, at the same time, its degrees should range as extensively as possible. These desiderata are attained in his construction. The bulb A, one inch in diameter, is blown thick and strong, on the end of a tube about ¼ of an inch in diameter: close above the bulb, is a cavity B, swelled out to such a size as to contain whatever mercury will

expand out of the bulb, between 32° and the lowest temperature at which the mercury is likely to boil at such altitudes as it will be used to measure. It is this which renders the instrument compact; because, if it be not taken out of the British islands, it will never, in all pro-

bility, boil at less than 200°; and thus the whole length of the stalk is left for a range of 12° or 15° of the thermometer. In the instrument figured, the scale R is 5 inches long, ¼ of an inch wide, and a length 4.15 inches is divided into 100 parts, which, by a vernier reads off to 1000 parts, being 241 parts to an inch; so that 1° Fahr. corresponds to 233 parts on the scale, or to 530 feet. Each part of the scale, as read by the vernier, will therefore correspond to 2.275 feet, being about half the degree of minuteness of the mountain barometer divided into thousandths, each of which is nearly equivalent to one foot of height. The accuracy, however, of this scale is probably greater than the degree of accuracy of which the method of observation is itself capable.

Whether an observer have or have not the means of obtaining such an instrument as this, it will be, in many cases, useful to travellers to be provided with means, more or less accurate, of making observations of this nature, on the summit of such mountains as they may have the opportunity of visiting. For this purpose, the most convenient is a small cooking apparatus, such as will supply the wants of a traveller; consisting of a round tin stand, protecting a lamp, in which a small quantity of the traveller's supply of spirituous liquid may be burnt, so as to boil some of the water of a small bottle, which he has also carried with him, or perhaps a little melted snow. An umbrella or waterproof cloak will screen the whole from the wind; and a thermometer should have been procured, with a stem as minutely divided as possible, and should be inserted, by means of a small cork, in an aperture of the lid left on purpose. The quantity of the water may be small, and it will serve a culinary purpose immediately after the operation is completed. The thermometer should be inserted only among the steam. The traveller must take great precautions for striking a light, as he will find this much more troublesome in the cold rarified air of a mountain summit than below.

18. Distillation is a method of separating a liquid from extraneous matter, by first of all converting it into steam,

and then condensing that steam so as to form the liquid. Different substances take the liquid form at various temperatures; and, therefore, the heat may be so regulated that only one substance of a mixture shall take the form of vapour, and being conveyed by a pipe through a vessel of cold water, or otherwise exposed to the cooling process, the vapour being condensed will give the pure liquid. A great improvement upon the process of separating liquids has been successfully introduced by Mr. Howard. It consists of distillation or evaporation in vacuo, and has been most usefully employed in the refining process of sugar. When sugar is dissolved in water, it requires a much higher temperature than $212^\{\backslash circ\}$ to boil the mixture, or to convert the water into steam and separate it from the solid; and as the process goes on, and the solution comes to hold less and less water, the requisite degree of heat is further augmented, until the temperature becomes so high as to injure the colour and otherwise deteriorate the article of merchandise in its crystallized state. Instead of this increased temperature, Mr. Howard places the syrup in vacuo, and thus boils it at a low and innocuous heat. This he accomplishes by pumping out the air and vaporized water from the close boiler, by means of a large air-pump driven by machinery. The process has produced a great improvement on this article of commerce, and has remunerated its inventor with an ample fortune.

Distillation in vacuo is peculiarly adapted to obtaining those delicate extracts and essential oils from vegetable substances, which are apt to suffer deterioration from the usual high temperatures.

19. * The pulse glass, an invention attributed to Dr. Franklin, is an apparatus illustrating beautifully the process of ebullition in vacuo at low temperatures. If two glass balls, A and B (fig. 5), be connected by a slender tube, and one of them, A, be filled with water, a small opening or pipe b being left at the top of the other, and this be made to boil, the vapour produced by it will drive all the air out of the other, and will at last come out itself, producing steam at the mouth of the pipe. When the ball B is observed to be occupied by transparent vapour, we may conclude that the air is completely expelled. Now, shut the pipe by sticking it into a piece of tallow or wax, the vapour in B will soon condense, and there will be a vacuum. The flame of a lamp and blow-pipe being directed to the little pipe b, will immediately cause it to close and seal hermetically. We have now a pulse glass. Grasp the ball A in the hollow of the hand; the heat of the hand will immediately expand the bubble of vapour which may be in it, and this vapour will drive the water into B, and then will blow up through it for a long while, keeping it in a state of violent ebullition, as long as there remains a drop or film of water in A. But care must be taken that B is all the while kept cold, that it may condense the vapour as fast as it rises through the water. Touching B with the hand, or breathing warm on it, will immediately stop the ebullition. When the water in A has thus been dissipated, grasp B in the hand; the water will be driven into A, and the ebullition will take place there as it did in B. Putting one of the balls into the mouth will make the ebullition more violent in the other, and the one in the mouth will feel very cold. This is a pretty illustration of the rapid absorption of the heat by the particles of water which are thus converted into elastic vapour. We have seen this little toy suspended by the middle of the tube like a balance, and thus placed in the inside of a window, having two holes, a, b, cut in the pane, in such a situation, that, when A is full of water and prepon-

Fig. 5.

derates, B is opposite to the hole b. Whenever the room became sufficiently warm, the vapour was formed in A and immediately brought the water into B, which was kept cool by the air coming into the room through the hole b. By this means B was made to preponderate in its turn, and A was then opposite to the hole a, and the process was now repeated in the opposite direction. This amusement continued as long as the room was warm enough. Instead of water, alcohol or ether may be substituted, and will act more readily.

20. The following experiment, where ebullition is produced by the application of cold, is instructive. A Florence flask F, is about $\backslash frac\{1\}\{3\}$ full of water, and is placed over a lamp E until the water boils; and when the steam has been rising for a short time violently from the

neck of the vessel, the cork S is to be applied as a stopper, and must fit with great accuracy. The flask thus closed is to be set aside for a few minutes till it have cooled considerably, and is then to be suddenly placed on a stand in the cold water W, contained in the glass reservoir R. The ebullition in the flask will recommence with a degree of violence proportioned to the coldness of the water W.

21. We have already noticed (Art. 11.) the fact that, when water is confined in a close vessel, and heat is applied to it, the water will not boil even at a temperature of $212^\{\backslash circ\}$. If heat be continually thrown into the water in this state, the particles will acquire a very high temperature; and, at the same time, the tendency of the enclosed fluid to burst the vessel will become very great. The following experiment upon this subject is one of the most interesting and the earliest of which we are in possession. It was published in 1663 by the Marquis of Worcester, and we give it in his own words. "I have taken a piece of a whole cannon, whereof the end was burst, and filled it three quarters full, stopping and screwing up the broken end, as also the touch-hole, and making a constant fire under it; within twenty-four hours it burst, and made a great crack."

It is in virtue of the great elastic force by which water, when heated, tends to expand into 1728 times its bulk, in the form of steam, that this element has become a mechanical mover, subject to the control of man. There are two great principles upon which such machines are con-

Fig. 6.

structed; the one commonly called high-pressure steam-engines, and the other low-pressure steam-engines.

In a high-pressure steam-engine, the principal source of motion is the elastic force of steam, formed by water, raised to a high temperature in confined vessels, and tending to escape from them with such force, as to impart motion and movement to solids or fluids, ingeniously arranged to receive from it velocity or direction required for the accomplishment of some end.

In a low-pressure steam-engine, the principal source of power is derived from using steam merely for the purpose of forming a vacuum. For this purpose steam is admirably calculated. It is only necessary to allow the steam of a liquid to enter any vessel filled with air; and if there be left an aperture of escape, the steam, entering in abundance, will push the air out before it. When the air has wholly escaped, it only remains necessary to close all the openings of the vessel, and allow it gradually to cool down, when the steam will be condensed, will shrivel up in the form of water into the 1728th part of its bulk, leaving the other 1727 parts vacuous. The mechanical force of a vacuum on the earth's surface is well known: it will raise water to a height of more than 30 feet, and support 15 lbs. on every square inch of surface exposed to it. Whatever, therefore, the formation of a vacuum on the earth's surface can effect, of that is the force of steam capable at low pressure, scarcely exceeding the temperature of 212°. Hence the low-pressure engine is sometimes called the condensing engine, because it acts principally by condensation of steam to form a vacuum. The high pressure of

steam, and its vacuum-forming power, are frequently used in combination.

22. There are other properties of steam, besides its mechanical force, that render its use of great practical value. Its great capacity for heat enables it to take up, at one time, and in one place, a large quantity of heat, which it may be employed as a vehicle to transfer, at a subsequent period and at a distant point, to some other substance. It is thus rendered an economizer and distributor, a reservoir of heat derived from the combustion of fuel. In this view it has great value as an agent in distributing the heat used for warming buildings, heating baths, evaporating solutions, distilling, brewing, drying, dyeing, and even for domestic cookery, and the means of extracting wholesome and nutritious food from most unpalatable materials.

In order, however, to its successful application as a mechanical power, and its profitable use in each of the various functions which it is capable of performing, it is necessary to study its various phenomena in greater detail; to obtain an intimate acquaintance with its properties; to determine its laws in the various relations of space, time, and quantity; how much heat it requires, what fuel it consumes, what force it exerts, how fast it will move, how it will condense, expand, and contract, and what relation it bears to the different fluids from which it may be derived. Each of these enquiries, and the manner in which each of these objects may be most satisfactorily attained, is the subject of one or other of the following sections of this article.

SECT. II.—EXPERIMENTAL RESEARCHES CONCERNING THE ELASTIC FORCE OF STEAM AT DIFFERENT TEMPERATURES.

23. The earliest researches we have met with into the phenomena of steam, undertaken with the philosophical purpose of obtaining experimental data for the scientific investigation of its properties and relations, are to be met with in a scarce work, printed at Basle in 1769, and entitled, "Specimen physico-chemicum de Digestore Papini; primitias experimentorum novorum circa fluidorum a calore rarefactionem et vaporem elasticitatem exhibens, &c." Auctore Jo. Henrico Ziegler." His experimental boiler consisted of a copper vessel (fig. 7) AA, silvered internally, and belted externally with massive iron hoops BB. A strong frame-work of iron, attached to the upper hoop, gives support to the circular cover B, (fig. 8.) in which there are an opening P for admitting water, another D into which an elaterometer is inserted, consisting of a bottle G, containing mercury, and a glass tube cc cased in iron, open at both ends, and immersed in the mercury at the bottom; the third or central aperture E being occupied by a copper tube F, closed below, and containing oil or other viscid liquid, to act as a bath for the bulb of the thermometer F and its protector from the pressure of the vapour. The method of using this apparatus was as follows. The digester being partly filled with water, closed and placed on the fire, the generation of the steam would raise the oil or mercury in the bath

Steam. (E) to the temperature of the water and steam within, so as to give to the thermometer F an indication of the temperature; and, at the same time, the elastic force of the steam flowing or moving by would raise it in C to a certain number of inches, so as to cause the corresponding pressure. This apparatus is both appropriate and ingenious, and indicates considerable mechanical knowledge in its inventor, a physician of Winterthur in Switzerland. Unhappily he lived too remote from the scene of the philosophical discoveries of that period, to adopt the precautions necessary to give value to his experiments. He allowed atmospheric air to mingle with the steam to such an extent as greatly to vitiate his results.

M. Betancourt's Experiments. 24. M. Betancourt visited England about the end of last century; and having been employed to select machines, models, and drawings for the Spanish government, made himself acquainted with the use of steam in Great Britain at that period. On his return, he immediately undertook a series of experiments on the force of the vapour of water, alcohol, and other liquids, at various temperatures. His apparatus is tolerably perfect; and the precautions which he adopted for the removal of atmospheric air from intermixture with the vapour, give his experiments considerable value and precision. Some of his experiments were made in vacuo; and he seems to have been one of the first philosophers who examined the production of steam at temperatures below the ordinary point of ebullition, under the pressure of the atmosphere. His experiments extend from 32° up to 279°, being 67° above the ordinary boiling point.

His apparatus (Fig. 9) consisted of a spheroidal copper boiler A, about eight inches in diameter, fifteen inches high, and a tenth of an inch in thickness; a flat cover was soldered on the top of it, and three apertures were formed into which were inserted a thermometer EC, a glass tube D, and a plug B for admit-

ting water. The glass tube being bent downwards at F, was recurved upwards at G, leaving an upright stem, ten feet high, and hermetically sealed at the top, so as to leave a perfect vacuum in that end of the tube, over a column of mercury of about 30 inches in the two

branches of the recurvation at the bottom. The boiler was provided with a stop-cock b, by which the air was extracted from the boiler previous to experiment, by means of an air-pump TV, communicating with W; and when this was accomplished so as to obtain a vacuum on both ends of the mercurial column, the mercury stood, as in the figure, on nearly the same level in both its branches. The fire was instantly applied, and the crackling noise which followed informed him that the ebullition had commenced, and the steam in the boiler pressing on that end of the mercurial column nearest to it, raised the other in the vacuum a certain quantity above its outer level, indicating its elastic force, which gradually increased until it became at the usual heat of boiling water, equal to twenty-eight French inches, the mean pressure of the atmosphere. The following table will enable us to estimate the value of these experiments; it is given in degrees of Réaumur's thermometer, of which 0° coincides with 32° of our common scale, and 80° with our boiling point 212° Fahrenheit, each degree of Réaumur being equal to 2$\backslash frac\{1\}\{2\}$ of our scale. The pressure is in French inches of mercury:—

The slight deviation of these experiments from each other indicates considerable accuracy of experiment; and the slight excess in the former of the two series is attributed to the formation of a less perfect vacuum at the commencement of the observations, arising from the smaller quantity of water in the boiler when the experiments were made.

It should, however, be noticed, that there is one omission of some importance in the experiments of M. Betancourt. He inserts the bare bulb of his thermometer into the reservoir among the water, so as to suffer all the variations imposed on it by the varying elasticity of the steam. By following the method adopted by his predecessor, M. Ziegler, of inserting a metallic tube to sustain the pressure of the steam, and forming it into a mercurial bath for containing the thermometer, and so transmitting the heat of the steam to it without exposure to variable pressure, a source of considerable error might have been avoided. This precaution is essential to a good set of experiments on steam; for a very slight pressure, even of the finger, on the bulb of a thermometer will raise it several degrees.

25. Of British philosophers, Dr Robison was one of the first to make accurate and systematic experiments on the

phenomena of the temperature and elastic force of steam. They appear to have been made in 1778. His apparatus is represented in the accompanying figure.

* ABCD (Fig. 10.) is the section of a small digester made of copper. Its lid, which was fastened to the body with screws, was pierced with three holes, each of which had a small pipe soldered into it. The first hole was furnished with a brass safety-valve V, nicely fitted to it by grinding. The area of this valve was exactly $\backslash frac\{1\}\{4\}$th of an inch. There rested on the stalk at the top of this valve the arm of a steel-yard carrying a sliding weight. This arm had a scale of equal parts, so adjusted to the weight, that the number on the scale corresponded to the inches of mercury, whose pressure on the under surface of the valve is equal to that of the steel-yard on its top; so that when the weight was at the division 10, the pressure of the steel-yard on the valve was just equal to that of a column of mercury 10 inches high, and $\backslash frac\{1\}\{4\}$th of an inch base. The middle hole contained a thermometer T firmly fixed into it, so that no vapour could escape by its sides. The ball of this thermometer was but a little way below the lid. The third hole received occasionally the end of a glass pipe SGF, whose descending leg was about 36 inches long. When this syphon was not used, the hole was properly shut with a plug.

* The vessel was half filled with distilled water which had been purged of air by boiling. The lid was then fixed on, having the third hole S plugged up. A lamp being placed under the vessel, the water boiled, and the steam issued copiously by the safety-valve. The thermometer stood at 213°, and a barometer in the room at 29.9 inches. The weight was then put on the fifth division. The thermometer immediately began to rise; and when it was at 220, the steam issued by the sides of the valve. The weight was removed to the 10th division; but, before the thermometer could be distinctly observed, the steam was issuing at the valve. The lamp was removed further from the bottom of the vessel, that the progress of heating might be more moderate; and when the steam ceased to issue from the valve, the thermometer was at 227. The weight was now shifted to 15; and, by gradually approaching the lamp, the steam again issued, and the thermometer was at 232$\backslash frac\{1\}\{2\}$. This mode of trial was continued all the way to the 75th division of the scale. The experiments were then repeated in the contrary order; that is, the weight being suspended at the 75th division, and the steam issuing strongly at the valve, the lamp was withdrawn, and the moment the steam ceased to come out, the thermometer was observed. The same was done at the 70th, 65th division, &c. These experiments were several times repeated both ways; and the means of all the results for each division are expressed in the subjoined table, where column 1st expresses the elasticity of the steam, being the sum of 29.9; and the division of the steel-yard, column 2d, expresses the temperature of the steam corresponding to this elasticity.

* A very different process was necessary for ascertaining the elasticity of the steam in lower temperatures, and consequently under smaller pressures than that of the Dr. Robison's Experiments. The glass syphon SGF was now fixed into its hole in the lid of the digester. The water was made to boil smartly for some time, and the steam issued copiously both at the valve and at the syphon. The lower end of the syphon was now immersed into a broad saucer of mercury, and the lamp instantly removed, and every thing was allowed to grow cold. By this the steam was gradually condensed, and the mercury rose in the syphon, without sensibly sinking in the saucer. The valve and all the joints were smeared with a thick clammy cement, composed of oil, tallow, and rosin, which effectually prevented all ingress of air. The weather was clear and frosty, and the barometer standing at 29.84, and the thermometer in the vessel at 42°. The mercury in the syphon stood at 29.7, or somewhat higher, thus showing a very complete condensation. The whole vessel was surrounded with pounded ice, of the temperature 32°. This made no sensible change in the height of the mercury. A mark was now made at the surface of the mercury. One observer was stationed at the thermometer, with instructions to call out as the thermometer reached the divisions 42, 47, 52, 57, and so on by every five degrees till it should attain the boiling heat. Another observer noted the corresponding descents of the mercury by a scale of inches, which had its beginning placed at 29.84 from the surface of the mercury in the saucer.

* The pounded ice was now removed, and the lamp placed at a considerable distance below the vessel, so as to warm its contents very slowly. These observations being very easily made, were several times repeated, and their mean results are set down in the following table: only observe, that it was found difficult to note down the descents for every fifth degree, because they succeeded each other so fast. Every 10th was judged sufficient for establishing the law of variation. The first column of the table contains the temperature, and the second the descent (in inches) of the mercury from the mark 29.84.

Four or five numbers at the top of the column of elasticities, are not so accurate as the others, because the mercury passed pretty quickly through these points. But the progress was extremely regular through the remaining points; so that the elasticities corresponding to temperatures above 70° may be considered as very accurately ascertained.

* Not being altogether satisfied with the method employed for measuring the elasticity in temperatures above that of boiling water, a better form of experiment was adopted. Indeed it was the want of other apparatus which made it necessary to employ the former. A glass tube was procured of the form represented in Fig. 11, having a little cistern L, from the top and bottom of which proceeded the syphons K and MN. The cistern contained mercury, and the tube MN was of a slender bore, and was about six feet two inches long. The end K was firmly fixed in the third hole of the lid, and the long leg of the syphon was furnished with a scale of inches, and firmly fastened to an upright post.

Steam. The lamp was now applied at such a distance from the vessel as to warm it slowly, and make the water boil, the steam escaping for some time through the safety-valve. A heavy weight was then suspended on the steelyard; such as it was known that the vessel would support, and, at the same time, such as would not allow the steam to force the mercury out of the long tube. The thermometer began immediately to rise, as also the mercury in the tube MN. Their correspondent stations are marked in the following table.

This form of the experiment is much more susceptible of accuracy than the other, and the measures of elasticity are more to be depended on. In repeating the experiment, they were found much more constant; whereas, in the former method, differences occurred of two inches and upwards.

We may now connect the two sets of experiments into one table, by adding to the numbers in this last table the constant height 29.9, which was the height of the mercury in the barometer during the last set of observations.

Mr Watt's 26. In the mean time, however, Mr Watt had been Experiments. led, in the course of his invention of the steam-engine, to make experiments on the elastic force of steam, of which he has given the following account, and which was annexed by himself to Dr Robison's original article in this work.

† In the winter of 1764-5, I made experiments at Glasgow on the subject, in the course of my endeavours to improve the steam-engine, and as I did not then think of any simple method of trying the elasticities of steam at temperatures less than that of boiling water, and had at hand a digester by which the elasticities at greater heats could be tried, I considered that, by establishing the ratios in which they proceeded, the elasticities at lower heats might be found nearly enough for my purpose. I therefore fitted a thermometer to the digester, with its bulb in the inside, placed a small cistern with mercury also within

the digester, fixed a small barometer tube with its end in the mercury, and left the upper end open. I then made the digester boil for some time, the steam issuing at the safety-valve, until the air contained in the digester was supposed to be expelled. The safety-valve being shut, the steam acted upon the surface of the mercury in the cistern, and made it rise in the tube. When it reached to 15 inches above the surface of the mercury in the cistern, the heat was 236°; and at 30 inches above surface, the heat was 252°. Here I was obliged to stop, as I had no tube longer than 34 inches, and there was no white glass made nearer than Newcastle-upon-Tyne. I therefore sealed the upper end of the tube hermetically, whilst it was empty, and when it was cool immersed the lower end in the mercury, which now could only rise in the tube by compressing the air it contained. The tube was somewhat conical; but, by ascertaining how much it was so, and making allowances accordingly, the following points were found, which, though not exact, were tolerably near for an aperçu. At 29½ inches (with the sealed tube) the heat was 252°, at 75½ inches the heat was 264°, and at 110½ inches 292°. (That is, after making allowances for the pillar of mercury supported, and the pillar which would be necessary to compress the air into the space which it occupied, these were the results). From these elements I laid down a curve, in which the abscissa represented the temperatures, and the ordinates the pressures, and thereby found the law by which they were governed, sufficiently near for my then purpose. It was not till the years 1773-4, that I found leisure to make further experiments on this subject, of which, though I do not consider the results as accurate, I shall give an account here, as they were in point of date prior to any others that I was then acquainted with.

† A tin pan, of about five inches in diameter and four inches deep, had a hole made in its bottom, near one side, and in this hole was soldered a socket somewhat conical, which nearly fitted a barometer tube with which the experiments were to be made. This tube was about 36 inches long, and had a ball at one end about 1½ inches diameter, the contents of which were nearly equal to those of the stem of the tube; some paper was lapped round the tube near the ball, and it was forced tight into the conical socket of the pan, so that the ball was within the latter, at such a height that it might be immersed in water. The tube and pan were then inverted, and the ball was filled with clean mercury, and the stem with distilled water fresh boiled. The tube was re-inverted, so that the ball and pan were uppermost; the lower end of the tube being shut by the finger, the water ascended into the ball, and the mercury occupied the tube. The lower end of the latter being then placed in a cistern of mercury, and released from the finger, the mercury and water descended, and the ball was left partly empty: being agitated in this position, and let stand some time, much air was extricated from the water; the tube was inclined as much as it could be, and again inverted, the air let out, and its place supplied with boiling water. It was again placed with the ball uppermost, the end of the tube stopped, the pan filled with hot water which was made to boil by means of a lamp, the lower end of the tube being

placed in the cistern, and released from the finger, the mercury descended into the cistern, but upon the water in the pan being suffered to cool, partly rose again into the tube. Much air was thus liberated, and more was got rid of by agitation, in the manner of the water-hammer, and by leaving it standing for some time erect, until at last I got it so free from air, that when I raised it upright, it supported a column of mercury 34 inches high; and no vacuum was formed until it was violently shaken, when it fell down suddenly and settled at 28.75 inches, but upon being inclined, a speck of air always remained, though, when it was expanded by a pillar of mercury 27 inches high, this speck was not larger than a pin's head.

† In this state, when the tube was perpendicular, I found the mercury to stand at 28.75 inches, the column of water above it was about 6½ inches, = half an inch of mercury. The whole then being 29.25 inches, when the stationary barometer stood at 29.4, the difference, or pillar supported by the elasticity of the steam = 0.15 inch. The water in the pan was then heated exceedingly slowly by a lamp, and stirred continually by a feather to make the heat as equal as possible. The results are shown in the following table:—

At this time (1774) I tried a set of experiments in the same manner on a saturated solution of common salt. When this solution was perfectly saturated by boiling, and was put into the tube, it precipitated a quantity of salt which disturbed the experiment. I was therefore obliged to take it out, and filter it, during which process it attracted moisture from the air, and appeared, by its boiling point, not to be perfectly saturated. Though it was more free from air than water is, yet it parted from what it contained with great difficulty, and would part with none when shaken as a water-hammer, though it opened in all parts of the liquor. The result of this experiment is contained in the annexed table:—

In the same manner I tried a set of experiments upon spirit of wine, the results of which are contained in the annexed table:—

Upon considering the probable cause of the difference, especially in the lower heats, between my experiments and those of Mr Southern, related in his letter annexed to this essay, I can only reconcile them by supposing that the stationary barometer, with which the comparison was made, had its scale placed 0.2 of an inch too low, and by adding that quantity to the elasticities in table 1st, they approach nearly to Mr Southern's experiments.

If that conjecture is adopted, the same addition will be necessary to tables 2d and 3d, as they were compared with the same stationary barometer.

To determine the heats at which water boils when pressed by columns of mercury above 30 inches, a tube of 55 inches long was employed; one end was put through a hole in the cover of a digester, and made tight by being lapped round with paper, and within the digester the end of the tube was immersed in a cistern of mercury. A thermometer was fixed in another opening, so that the bulb was in the inside of the digester, and the stem and scale without; and the bulb was kept half an inch from the cover of the digester by a wooden collar. The cover being fixed on tight, and the digester half filled with water, it was heated by means of a large lamp.

† The air in the upper part of the digester expanding by heat, the column of mercury in the tube was considerably raised by that expansion before the water boiled. The air was let out, and the water heated to boiling; still, however, some air remained, for the mercury stood at 213½°. That deduction being made, the following table shows the heats and corresponding elasticities.

In making these experiments, the digester was heated very slowly, and the heat was kept stationary as much as was possible at each observation, so that the whole series occupied some hours. The degrees of elasticity were observed by my friend Dr Irvine, whilst I observed those of the thermometer in all these experiments.

With the whole of the observations, I was, after all, by no means satisfied, as I perceived there were irregularities in the results which my more urgent avocations did not permit me to explore the causes of and to correct.

The matter remained in that state till 1796, when I requested Mr Southern to try them over again, in the

Steam. performance of which he was assisted by Mr William
Mr Watt's Creighton. The results of these observations are con-
Experi- tained in Mr Southern's letter to me, which follows this
ments. memoir; and, from the very great care with which the
experiments were made, the known accuracy of both Mr
Southern and Mr Creighton, and the agreement of the
experiments with one another, I have reason to believe
them as nearly perfect as the subject admits of. The
method he adopted of trying the elasticities above the
temperature of boiling water by a piston, accurately fitted
to a cylinder, is much to be preferred to that adopted by
Dr Robison, and is more manageable under great elas-
ticities than that of a long pillar of mercury.

Mr South- 27. The reference which is here made applies to the
ern's Ex- following letter from Mr Southern* to Mr Watt:—

“DEAR SIR,—The experiments of which the particular
circumstances are hereafter related, were made in 1803,
with the view of ascertaining chiefly the density of steam
raised from water under different pressures above that of
the atmosphere, an apparatus having then been made for a
different purpose, which seemed pretty well adapted to
this object.

“Besides the experiments now to be related, in which
the temperature of steam raised under high pressures was
observed in 1803, others had been made some years before,
in 1797 and 98, for that purpose only; and, as they were
made with the greatest circumspection, both the manner
of making them and their results may be here described,
as may also the results of other experiments, made with
equal care, to ascertain the temperature of steam raised
under low pressures.

“The instrument used in the former was a Papin's
digester, similar to what you had used; the leading differ-
ences being in adapting a metallic tube to it to con-
tain the thermometer, or rather as much of it as con-
tained mercury, in the manner mentioned in the beginning
of this letter, and instead of a valve, by the load on which
to measure the elasticity of the contained steam, a nicely
bored cylinder was applied, with a piston fitting it, so as
to have very little friction, and to the rod of this was
applied a lever, constructed to work on edges like those
of a scale-beam, by which the resistance against the
elastic force of the steam could be accurately determined;
and at your suggestion, to be assured that no inaccuracy
had crept into the calculation, by which this resistance,
through the medium of the lever, was ascertained, an
actual column of mercury of 30 inches high was substi-
tuted, and the correspondence was found to be within
$\backslash frac\{1\}\{100\}$ of an inch.

“The observations at each of the points of pressure noted
were continued some minutes, the temperature at each
being alternately raised and lowered, so as to make the
pressure of the steam on the under side of the piston
alternately too much and too little for the weight with
which it was loaded; and thence a mean temperature was
adopted, the extremes of which were, as well as I now
recollect, not more than half a degree on each side of it.
The load on the piston, including its own weight, &c.,
&c., was calculated to be successively just equal to 1, 2, 4,
and 8 atmospheres of 29.8 inches of mercury each, and
the temperature of the steam was varied as above till
that of each point was determined; the results were
thus:—

“The experiments for ascertaining the temperature of
steam below the atmospheric pressure were made with an
apparatus essentially similar to that which you originally
used, and with scrupulous care and attention: and I met
with the same incidents as you had done; such as, the
production of a bubble of air whenever, after any experi-
ment, the tube was inclined to refill the ball; and also the
extraordinary suspension of a column of mercury of 35
inches vertical height, and of 7 inches of water above
that, although the counterpoise was only that of the at-
mosphere, then under 30 inches. I found also that the
tube required a considerable degree of tabouring or
shaking to make the column subside and leave a space in
the ball. This phenomenon was not produced till after
much pains taken in inverting and re-inverting the tube
again and again, nor till it had been suffered, after these
operations, to stand for three or four days undisturbed in
the exhausting position, and then discharging the air that
had been accumulating in the interval.

“The results to be found in the table below, were de-
duced from the observations as you had done—viz., by
adding to the height of the column of mercury in the tube
(ascertained by a gauge floating on the surface of the
mercury in the basin), that of the water above it, or rather
of an equivalent column of mercury, and subtracting their
sum from the height of the common barometer at the time.
All these results were taken from observations made after
the apparatus had been so perfectly exhausted of air as to
produce the phenomenon just mentioned.

“The following formula will be found to give the elas-
ticity belonging to a given temperature, and vice versa,
with a sufficient degree of accuracy for most purposes,
within the range of the experiments, at least, from which
they have been formed.

Let $t$ = temperature, $e$ = elasticity, in inches of mercury;
$T\; =\; t\; +\; 52$, and $E\; =\; e\; -\; \backslash frac\{1\}\{10\}$, $m\; =\; 94250,000000$;

Then

5.14

“But as the calculation is most easily performed by
logarithms, let $L$ signify the logarithm of the quantity to
which it is prefixed:

“The following table shows the observed elasticities,
those derived from calculation by the formula, and the
differences of the two, which appear to me to be as small
as can be expected, taking a general view.

* In all these experiments Mr Southern was assisted by Mr William Creighton.

"I believe it is now generally considered that the temperature 212° is that of water boiling when the barometer is at 30 inches instead of 29.8; and if, in the above algebraic expressions, the following alterations be made, the results from the formulae will correspond with the adjustment of that point, and fully as well with the experiments generally.

"Let $T\; =\; t\; +\; 51.3$; the index of the power and of the root be 5.13, instead of 5.14; and $m\; =\; 87344.000000$. So the two last equations will be: $5.13\; LT\; -\; 10.94123\; =\; LE$; and $\backslash frac\{LE\; +\; 10.94123\}\{5.13\}\; =\; LT$.

"The table will stand as follows, supposing the thermometer had been graduated for 212° to correspond with 30 inches of the barometer:—

"I remain, with the greatest esteem and respect, Dear Sir, Your very obedient Servant,

"Glasgow, 20th March, 1814." "JOHN SOUTHERN."

"To JAMES WATT, Esq., Heathfield."

28. In the Memoirs of the Royal Academy of Berlin for 1782, there is an account of some experiments made by M. Achard on the elastic force of steam, from the temperature 32° to 212°. They agree extremely well with those mentioned here, rarely differing more than two or three-tenths of an inch. He also examined the elasticity of the vapour produced from alcohol, and when the elasticity was equal to that of the vapour of water, he found that the temperature was about 35° lower. Thus, when the elasticity of both was measured by 28.1 inches

of mercury, the temperature of the watery vapour was 209°, and that of the spirituous vapour was 173°. When the elasticity was 18.5, the temperature of the water was 189.5°, and that of the alcohol 154.6°. When the elasticity was 11.05, the water was 168°, and the alcohol 134.4°. Observing the difference between the temperature of equally elastic vapours of water and alcohol not to be constant, but gradually to diminish, in M. Achard's experiments, along with the elasticity, it became interesting to discover whether, and at what temperature, this difference would vanish altogether. Experiments were accordingly made by the writer of this article, similar to those made with water. They were not made with the same scrupulous care, nor repeated as they deserved, but they furnished rather an unexpected result. The following table will give the reader a distinct notion of them.

29. Dr Dalton appears to have been the first to escape Dr Dalton's Experiments. His apparatus is the most simple and refined of any that have been employed for temperatures below 212°, and his experiments are those which, to the present day, have the greatest authority. Dr Dalton's first experiments were published in 1793, in his Meteorological Essays; afterwards, when more extended, in the Manchester Memoirs, 1802; then in the first volume of his System of Chemistry, 1808; and finally in the second volume of the same work, 1827. The following is the account given by himself, of his early experiments, in the Manchester Memoirs.

"My method is this: I take a barometer tube AB, (Fig. 13.) perfectly dry, and fill it with mercury just boiled, marking the place (30) where it is stationary; then, having graduated the tube, I pour a little water, or any other liquid, the subject of experiment, into it, so as to moisten the whole inside: after this I again pour in mercury, and carefully inverting the tube, exclude

all air; the barometer, by standing some time, exhibits a portion of water of $\backslash frac\{1\}\{10\}$ or $\backslash frac\{1\}\{15\}$th of an inch, W, on the top of the mercurial column; because, being lighter, it ascends by the side of the tube, which may now be inclined, and the mercury will rise to the top, manifesting a perfect vacuum from air. I next take a cylindrical glass tube CD, open at both ends, of two inches diameter and fourteen inches in length, to each end of which a cork is adapted, perforated in the middle, so as to admit a barometer tube to be put through, and to be held fast by them; the upper cork, C, is fixed two or three inches below the top of the tube, and is one-half cut away, so as to admit water, &c., to pass by, its service being merely to keep the tube steady. Things being thus circumstanced; water of any temperature may be poured into the wide tube, and thus made to surround the upper part or vacuum of the barometer, and the effect of temperature in the production of vapour within can be observed from the depression of the mercurial column at the top. In this way I have had water as high as 155° surrounding the vacuum; but as the higher temperatures might endanger a glass apparatus, instead of it I used the following:—

Having procured a tin tube CD, four inches in diameter, and two feet long with a circular plate soldered to one end, having a round hole in the centre, like the tube of a reflecting telescope, I got another smaller tube of the same length soldered into the larger, so as to be in the axis or centre of it; the small tube was open at both ends, and on this construction water could be poured into the large vessel to fill it, while the central tube was exposed to its temperature. Into this central tube I could insert the upper half of a syphon barometer, and fix it by a cork, the top of the narrow tube, also, being corked—thus the effect of any temperature under 212° could be ascertained, the depression of the mercurial column being known by the ascent in the exterior leg of the syphon. The force of vapour from water between 80° and 212°, may also be determined by means of an air-pump, and the result exactly agrees with those determined as above."

"After repeated experiments by all those methods, and a careful comparison of the results, I was enabled to digest the following:—

Table of the Force of Steam from Water in the temperatures from 32° to 212° (1802.)

Dr Dalton afterwards resumed the experimental examination of this subject, and was induced to modify these numbers slightly, as will be seen from our final table.

30. Passing over the experiments of Schmidt, Goldner, Dr. and others, as presenting no important differences from Experiments of some of these we have already noticed, we come to those of Dr Ure, published in the Philosophical Transactions of 1818, and made at Glasgow during 1817. The adjoining figures represent his apparatus.

Fig. 15. represents the construction used for temperatures under and a little above the boiling point. Figs. 16. and 17. are those used for higher temperatures, the last being the more convenient of the two; each was suspended from a lofty window ceiling, and placed in a truly vertical position, by means of a plumb line. Dr Ure gives the following account of his mode of experimenting. "One simple principle pervades the whole train of experiments—which is, that the progressive increase of elastic force developed by heat from the liquid, incumbent on the mercury at L, is measured by the length of column which must be added over L, the primitive level below, in order to restore the quicksilver to its primitive level above, at l. These two stations or points of departure are nicely defined by a ring of fine platina wire, twisted firmly round the tube.

"At the commencement of the experiment, after the liquid, well freed from air, has been let up, the quicksilver is made a tangent to the edge of the upper ring, by cautiously pouring mercury, in a slender stream, into the open leg of the syphon B, the level ring below is then carefully adjusted.

"From the mode of conducting my experiments, there remained always a quantity of liquid in contact with the vapour, a circumstance essential to accuracy in this research.

"Suppose the temperature of the water or the oil in A to be 32° Fahrenheit, as denoted by a delicate thermometer, or by the liquefaction of ice; communicate heat to the cylinder A, by means of two argand flames, playing gently on its shoulder at each side. When the thermometer indicates 42°, modify the flames, or remove them so as to maintain a uniform temperature for a few minutes. A film or line of light will now be perceived between the mercury and the ring at l, as is seen under the vernier of a mountain barometer, when it is raised a few feet off the ground; were the tube at l and L, of equal area, or were the relation of the areas experimentally determined, then the rise of the quicksilver above L would be one-half, or a known submultiple of the total depression, equivalent to the additional elasticity of the vapour at 42° above that at 32°. Since the depressions, however, for 30 or 40° in this part of the scale are exceedingly small, one-half of the quantity can scarcely be ascertained with suitable precision, even after taking the above precautions; and besides, the other sources of error, or at least embarrassment, from the inequalities of the tube, and from the lengthening space occupied by the vapour, as the temperature ascends, render this method of reduction very ineligible.

"By the other plan we avoid all these evils; for whatever additional elasticity be communicated to the vapour above l, it will be faithfully represented and measured by the mercurial column, which we must add over L, in order to overcome it and restore the quicksilver under l, to its zero or initial level, when the platina ring becomes once more a tangent to the mercury. At E, a piece of

cork is fixed between the parallel legs of the syphon to sustain it, and to serve as a point by which the whole is steadily suspended. For temperatures above the boiling point, the part of the syphon under E is evidently superfluous, merely containing in its two legs a useless weight of equipoised mercury. Accordingly, for high heats, the apparatus, Figs. 16 or 17, is employed, and the same method of procedure is adopted; the aperture at O, Fig. 17, admits the bulb of the thermometer, which rests as usual on E. The recurved part of the tube is filled with mercury, and then a little liquid is passed through it to the sealed end. Heat is now applied by an argand flame to the bottom of C, which is filled with oil or water, and the temperature is kept steadily at 212° for some minutes. Then a few drops of quicksilver may be required to be added to D', till L and E' be in the same horizontal plane. The further conduct of the experiment differs in no respect from what has been already described—

the liquid in C is progressively heated, and at each stage mercury is progressively added over L to restore the initial level or volume at E', by equipoising the progressive elasticity. The column above L being measured, represents the succession of elastic forces: when this column is wished to extend very high, the vertical tube requires to be placed for support in the groove of a long wooden prism.

The height of the column in some of my experiments being nearly twelve feet, it became necessary to employ a ladder to reach its top. I found it to be convenient, in Dr Ure's this case, after observing that the column of vapour had attained its primitive magnitude, to note down the temperature with the altitude of the column, then immediately to pour in a measured quantity of mercury nearly equal to three vertical inches, and to wait till the slow progress of the heating again brought the vapour in equilibrio with this new pressure, which at first had pushed the mercury within the platina ring at L. When the lower surface of the mercury was again a tangent to this ring, the temperature and altitude were both instantly observed.

This mode of conducting the process will account for the experimental temperatures being very often odd and fractional numbers. I present them to the public, as they were recorded on the instant in that particular repetition of the experiment which I consider most entitled to confidence. To trim and fashion the results into an orderly looking series, would have been an easy task; but, in my opinion, this is a species of deception very injurious to the cause of science, and a deviation from the rigid truth of observation, which ought never to be made for any hypothesis: we shall afterwards have ample opportunities of exposing the fallacy of such premature geometrical refinements.

The thermometers were constructed by Creighton with his well-known nicety, and the divisions were read off with a lens, so that $\backslash frac\{1\}\{10\}$th of a degree could be distinguished. After bestowing the utmost pains in repeating the experiments, during a period of nearly two months, I found that the only way of removing the little discrepancies which crept in between contiguous measures, was to adopt the astronomical plan of multiplying observations, and deducing truth from the mean. It is essential to heat with extreme slowness and circumspection the vessels A, B, C. One repetition of the experiment occupies on an average seven hours.

The apparatus employed in obtaining these results, has

The Elastic Force of the Vapour of Water in inches of Mercury, obtained from Experiments by Dr Ure.

the peculiar advantage, over all others, that the mercurial column is never heated. It is the concurrent opinion of all chemical philosophers, that caloric travels downwards in liquids with extreme slowness and difficulty. Indeed, Count Rumford's experiments led him to infer, that heat could not descend in fluids at all.

It is evident that, in my constructions, figures 15, 16, and 17, only that small portion of quicksilver within the

vessels A, B, and C, will be affected by the heat, but the measuring column is beyond the reach of its influence.

31. A series of experiments on high-pressure steam was Taylor and subsequently made by Mr Philip Taylor, but he has not described his apparatus. A similar series was also made by Professor Arsbenger of Vienna. As their results may be useful for comparison, we have united them in the following table:—

Steam. Taylor's and Arsberger's Experiments on High-Pressure Steam.

32. We now come to the most imposing series of experiments hitherto conducted. In 1823, the government of France having resolved to legislate on the means for obtaining security in the use of steam-engines, consulted the Academy of Sciences, upon the mode of most effectually promoting the public safety, without placing useless restraints on commercial enterprise and manufacturing industry. The examination into the state of knowledge concerning the phenomena of vapour at elevated temperatures, which resulted from this application, having brought the imperfections of this part of science prominently into notice, the Academy were induced to undertake a long and laborious enquiry, not entirely free from personal danger, into the law connecting temperature with the pressure of steam. The commission consisted of the illustrious members of the Academy, Baron de Prony, Arago, Girard, and Dulong; and the results of their investigation, finished in 1829, are given in the tenth volume of the Memoirs of the Academy of Sciences, printed in 1831. These experiments, conducted principally by the MM. Arago and

sources of the Academy. The precautions adopted to ensure minute accuracy, entitle them to confidence, no less than the names of two philosophers, so well versed in experiments of a similar nature. They were carried as high as to the twenty-fourth atmosphere of pressure.

The experiments were made in one of the courts of the Observatory. Fig. 18 represents a section of the principal portions of the apparatus. The boiler a consists of a cylindrical body, having its axis vertical; the two ends forming top and bottom are spherical segments, strongly riveted to the body, the whole being made of the finest plate iron. The material of the cylindrical part is half an inch thick, the top and bottom being considerably thicker. The aperture at the top, six inches in diameter, was closed by a plate of wrought iron, an inch and three-quarters thick, overlapping the hole, about two inches all round, and having on its lower surface a projecting ring, adapting it to a groove on the upper side of the top of the boiler: between these two surfaces was interposed a thick ring of lead, and the cover was then strongly screwed down by six steel bolts, the nuts of which had head-washers, so that, on screwing the whole together, the cover became hermetically closed. This experimental boiler was built in a furnace of considerable size and mass, intended to produce a temperature of the requisite constancy; x x are bars upon which the fire rests; y is the flue leading to the chimney.

The other parts of the apparatus connected with the boiler are b b, a lever, safety-valve, and weights; y y (fig. 19) the thermometer scales; and w w reservoirs of cold water, for maintaining uniform temperatures on the vertical parts of the instruments.

Fig. 19.

Fig. 18.

Dulong, were on a scale of magnitude and expense suited to the munificence of the French government and the re-

During the process of proving the boiler by a hydraulic pump, the common safety-valve, when used as an instrument for measuring with precision the pressure of the fluid in the boiler, was observed to give very erroneous indications, and the necessity of a more delicate apparatus was demonstrated. The improved index of pressure, made use of in the experiments, is shown in fig. 18. For measuring the great pressures to be used, a tube of mercury, 80 feet high, would have been requisite; but there was used, as a substitute for it, a glass tube x x, closed at the upper end, filled with dry atmospheric air, and having a length of only five feet seven inches, and an internal diameter of $\backslash frac\{1\}\{2\}$ of an inch, and of a thickness nearly equal to its diameter. It was so arranged as to furnish a convenient manometer, capable of giving the same indications, by the contraction of the contained air, as would have been given in similar circumstances, by a column of mercury of the height due to the diminished volume of the air. The graduation of this manometer, however, presented new difficulties.

These difficulties were successfully encountered by the skill and ardour of the academicians. Every one knows that it is impossible to obtain a glass tube of considerable length and magnitude which shall have a tolerably cylindrical interior; and that there are a number of practical

difficulties, which render it impossible to obtain even such a tube as that of a common thermometer, which shall possess the uniformity necessary to a good instrument. To make the proper allowance for this inevitable imperfection, the academicians easily might have adopted the same method as that used in the case of thermometer tubes, by determining the volume of successive small portions of its interior; but even this would have furnished a very partial remedy for the evil, because it had not been ascertained that the space occupied by the air in the manometer would diminish in bulk exactly in the proportion of the increase of compressing force, or of the corresponding increase in the height of the equivalent column of mercury. Two problems were therefore to be resolved at once, the elimination of the error of the tube, and the determination of the elasticity of air under high pressures. Both of them were satisfactorily accomplished, by the following laborious research.

As a preliminary measure, it was resolved to graduate the manometer, and determine the law of the elastic force of air under high pressures, by direct comparison with a column of mercury, from 75 to 80 feet in height. Such an experiment required a suitable locale and a stupendous apparatus. Among the buildings of the Royal College of Henri Quatre, there may be observed an old square tower, sole relic of the ancient church of Sainte Genevieve: there exist still in the interior three vaulted floors, pierced in the centre, and affording the very supports that were required for the erection of this stupendous mercurial gauge. In the centre of this opening there was raised a squared tree of the required height, and to this it was determined to attach the glass tube of 80 feet in height. To form a single glass tube of so great length was impossible; its own weight, when constructed, under the pressure of the mercury, would have endangered its existence. The glass column was built of separate portions, united in mastie, with great care, in viroles of steel. Each portion of tube was suspended in the air by an exact counterpoise, acting over pulleys fixed to the tree; and the whole of the parts were so united in equilibrio, that each sustained only its own weight, and the pressure of the mercury due to the height of the superior portion of the column. A homogeneous metallic scale was attached, and its divisions read by a vernier, as in the common barometer.

The manometer to be graduated, and this column of mercury, were both connected by tubes with a strong cylindrical vase f, holding about 100 lbs. of mercury. When thus placed in communication, a column of water was forced into the vase above the mercury by a hydraulic pump, and the pressure thus produced raised the metal with equal force up into the glass tube column on the one hand, and into the manometric tube on the other. The point to which the air was compressed was read off by a vernier, and the corresponding height of the mercury having been determined, it was manifest that the same degree of compression of the less instrument would ever after serve as the index of an equivalent column of mercury. In this manner the whole tube was graduated by careful experiment. The result of this graduation was satisfactory and very instructive. In forming the scale of the manometer, no room was left for errors of practical execution; and the comparison of the volume of the air with the height of the mercurial column demonstrated the diminution of the volume of the air to be precisely in the ratio of the pressure, so that the law of Marriotte is rigidly correct, even when extended to the extreme case, where the air is reduced to less than $\backslash frac\{1\}\{3\}$ part of its usual volume.

This preliminary process having been successfully terminated, the enormous column of glass was now laid aside, and the manometer, with its reservoir of mercury, transported to the court of the Observatory, for the purpose of being attached to the experimental boiler. Figure 18 shows the manometer in situ. An iron tube d d', g g', com-

posed of gun barrels welded together, connects the cover of the boiler a, with the reservoir of the manometer f, so as to conduct the pressure of the steam to the experimental surface, which formerly had sustained the mercurial column. The vacant space above the mercury was filled with water, which, by condensation from a stream of water on the outside, was kept full to the constant height v. A column of water contained in the glass tube x x, and constantly replenished, preserved the column of air, and other parts of the apparatus, at a constant temperature, indicated by a thermometer. A tube o p, of glass, communicating with the reservoir of mercury above and below, indicates, on the scale l m, the variation of level arising from the recession of the mercury into the manometer tube.

To ascertain the temperature of the water and steam of the boiler, it had been considered sufficient in the ruder experiments of earlier observers to insert thermometers directly into the boiler itself. Every one who has an acquaintance with these instruments knows, that any difference of pressure on the glass produces a false indication of the instruments, so that even the few inches of mercury in the instrument itself, when inverted, alter its indications, and a slight pressure of the finger would raise it a degree; the inaccuracy of the old method, when used under a pressure of 70 or 80 feet of mercury, or 450 pounds on every inch of the immersed surface of the instrument would have been great. The French academicians avoided this error, by immersing strong iron tubes t t, (figs. 18 and 19,) in the water and steam, in which the thermometers, surrounded by liquid metal, were kept in close communication with the heat of the fluids, without exposure to their force. By adopting only very slow variations of temperature, the error arising from the motion of heat was rendered insensible.

The following Table contains the results of Thirty of the most unexceptionable Experiments:—

A table of temperatures, from 1 to 50 atmospheres, calculated in coincidence with the experiments of the French academicians, and adapted to English measures, is given by us in Article 57, for the purpose of convenient practical reference.

33. The latest series of experiments on the elastic force of high-pressure steam, we owe to America. At the request of the Hon. S. D. Ingham, Secretary of the Treasury of the United States, a committee of the Franklin Institute, of the State of Pennsylvania, was appointed "to examine into the causes of the explosions of the boilers used on board of steam-boats, and to devise the most effectual means of preventing the accidents, or of diminishing the extent of their injurious effects." Among other subjects, such as the strength of boilers, the construction of safety-valves, to which we shall refer in another place, this committee took into consideration the elastic force of high-pressure steam at different temperatures. Funds were placed at their disposal by the House of Representatives, and the committee consisted of such a combination of scientific and practical men, as to give high authority to their results. On the 1st day of November, 1830, the subject was placed in the hands of the following gentlemen:—Professor Alex. Dallas Bache, Mr Benjamin Reeves, Mr W. H. Keating, Mr M. W. Balwin, Mr S. V. Berrick, and Isaiah Sukens.

We shall enter more fully on the description of their apparatus of experiment than we should otherwise have done, because we shall have frequent reference to make to the whole of their experiments, not only in this article, but in our article on the Steam-Engine, where we treat of explosions of boilers and their causes.

The boiler used by the committee is represented in figs. 20, 21, 22. It is a cylinder, twelve inches in internal diameter, two feet ten inches and a quarter in length within, and a quarter of an inch thick, of rolled iron, with the ends rivetted in the usual manner. Fig. 21 is a side view. Figs. 20 and 22 are end views of the boiler,

and of the apparatus connected with it. The boiler was placed horizontally in a furnace, the fire surface extending about halfway round the cylinder. The furnace was arranged for a charcoal fire, the grate bars extending the whole length of the boiler, and the fire being applied nearly the whole length. The draught entered by an

opening, closed in the usual manner, and left the furnace through a flue placed at one end and side of the boiler. In fig. 20, A is the ash-pit door, B the furnace door, and in 21 and 22, C is the furnace chimney.

In order to examine, readily, the interior of the boiler during the progress of the experiments, each end was provided with a glass window (D, figs. 20 and 22). The glass used was three-eighths of an inch thick. The openings in the ends, which were rectangular, were two and a half by one and three quarters inches wide.

Three gauge cocks were placed in the front end of the boiler; their positions will be particularly stated hereafter; they are shown in figs. 20 and 21, at a, b, and c.

To the same end and by the side of the gauge cocks, a glass water gauge (w, x, figs. 20 and 21) was attached, a particular description of which will be given in the detail of experiments made to compare its performance with that of the gauge cocks.

To supply the boiler with water, a forcing pump EE

FG, figs. 21 and 22, was placed near the back end. This pump was of the ordinary construction, with a solid plunger and conical valves; the diameter of the pump was one inch, and the play of the piston one inch and three-quarters. The diameter of the pipe FG, by which the water was conveyed from the pump to the boiler, was three-hundredths of an inch. By a coupling screw, this pipe could be connected with either of the stop cocks d e, fig. 22, in the back end of the boiler: the opening of these cocks was two-hundredths of an inch in diameter.

To ascertain the elasticity of the steam within the boiler, a closed steam gauge (H, figs. 21 and 22), was used, a particular description of the construction, &c. of which will be given. This instrument was placed upon the same stand (I, figs. 21 and 22) which supported the pump, so that the same experimenter could observe its indications and attend to the working of the pump. The cistern of the gauge was connected by a flexible pipe f g, with the upper part of the boiler.

The safety-valve is shown on the top of the boiler (K, fig. 21), midway between the ends. The graduation of it required much pains, and will receive a separate discussion.

Near the safety-valve is represented (at L, fig. 21,) the fasible plate apparatus, consisting of a sliding plate of iron, moved by a lever. On the other side of the safety-valve are the thermometers (M and N, fig. 21) plunged into iron tubes to give the temperature of the steam and water within the boiler. Above this appears the reservoir O, containing the water intended to maintain the scales of the thermometers at a constant temperature. All these parts require a more detailed description.

The steam gauge consisted of a glass tube closed at the upper, and open at the lower end, which passed steam-tight into a reservoir for mercury: when this reservoir was connected with the boiler the pressure of the steam raised the mercury into the gauge tube, compressing the air which the tube contained. The first mercurial gauge which was made, was broken by a sudden access of surcharged steam, in the experiments upon that subject, and was replaced by a second one. The method of graduation, and in general the description of the second gauge, will serve also for the first; the details, only varied slightly.

The glass gauge tube was 26.43 inches in length. To the lower end was connected an iron ferule, terminated above by a projecting ring. This ring was pressed upon the upper end of the pipe h, by a coupling screw, which served to form a tight juncture between the gauge and the cistern. The cistern i was a cylindrical vessel of cast iron, having the two projecting tubes h and k, upon which screws were cut: the first of them has been alluded to as giving a passage to the glass tube of the gauge; the second was coupled, by the pipe f g, to the boiler.

The gauge tube was not of precisely equal diameter throughout, and it was judged more accurate to graduate small portions of it into equal volumes. This was done by introducing equal measures of air from the point of a sliding-rod gas measure (Hare's); this operation was performed repeatedly, and by multiple measures, to verify the results, until the marks made for the equal volumes, on a paper scale attached to the tube, coincided, in the various trials. The lengths of the spaces occupied by the equal volumes were then carefully measured upon the brass scale to be used with the gauge. The slight differences between the lengths given by adjacent parts of the tube, showed that it might be considered as divided into so many small portions of uniform diameter. The mercury rising into the gauge tube from the cistern when pressure is applied, the level of the cistern is necessarily depressed; the amount of the correction for this depends upon the relation between the areas of the cistern and tube, supposed uniform. The areas of the cistern were found to be, within the limits of its use, sensibly the same;

those of the tube might be so assumed for such a purpose: the ratio was therefore found by filling the gauge tube with mercury, and pouring this into the cistern, noting the Experiment produced; comparing this with the mean length of the tube, the ratio of depression in the gauge for elevation in the tube was found to be as .01 to 1. The air within the tube was next carefully dried by the introduction of a receptacle of chloride of calcium, of the same length with the tube; the air having been in contact with this substance for a sufficient time, the receptacle was withdrawn through the mercury over which the drying had been effected; the tube was next placed over a dish of mercury, in the receiver of an air-pump, and the air withdrawn, until, on re-admitting air to the receiver, the mercury rose in the tube above the iron ferule.

The gauge tube was next introduced into the cistern, the level of which, corresponding to the zero of the brass scale was then arranged, and the point of the scale at which the mercury stood was ascertained, the barometer and thermometer being noted.

It was intended in the experiments to keep the pipe from the gauge to the boiler cool, so that it might contain water, and thus give a nearly constant pressure upon the mercury of the cistern, besides preventing the exposure of the apparatus to heat; the height of this column, above the level of the cistern, was therefore ascertained, after the gauge was put in its place by screwing the cistern i to the stand.

All the elements for calculating the elasticity of the steam within the boiler, from the height of the mercury of the gauge, were thus known; the temperature of the apparatus being supposed constant.

The elastic force of the steam within the boiler, together with the column of water in the steam-pipe, balances the elasticity of the compressed air within the gauge, together with the column of mercury above the level of that in the cistern. This level is not the original zero, but lower than that, by the depression produced by the rise of mercury in the gauge tube. The depression of the mercury changes the level above which the pressure of the column of water in the steam-pipe is measured, but the change in the pressure, by the column of water, is altogether inconsiderable. The law of the elastic force of dry air, which has been recently shown, by Dulong and Arago, to be accurate, at pressures from one to fifty atmospheres, was made use of in determining the elasticity of the air in the gauge: this elasticity is inversely as the space occupied by the air. From the data already obtained, and upon the principles just stated, a table was calculated, by which the observed heights of the gauge were converted into the corresponding pressures in inches of mercury or in atmospheres. The calculations were rendered rather tedious by the unequal diameter of the bore of the tube, on account of which equal lengths did not correspond to equal volumes. The usual method of calculation was resorted to, namely, to determine, by rigid calculation, the pressures, for points sufficiently near each other, and then to interpolate for intermediate heights.

The foregoing remarks take for granted that the temperature of the air in the gauge, as well as that of the mercury, remains constant; to secure this, an arrangement was adopted similar to that employed by Dulong and Arago for the same purpose. The gauge and scale were surrounded by a glass tube l, cemented below into a brass cap m, which had an opening in the side, communicating with a discharge pipe n, fig. 21. The tube was attached above, by an air-tight juncture, to a tin vessel P, of considerable capacity, compared with the tube. Water being introduced into the glass tube surrounding the gauge, the flow through this tube was regulated by a stop-cock o, placed at the end of the discharge pipe, the cistern above being filled with water.

To ascertain the temperature of the column of water

surrounding the gauge, a thermometer (p, fig. 22) with a very small bulb, was attached to the scale at the middle of its height: by this instrument, the flow of water through the casing of the gauge was regulated so as to keep the temperature nearly constant, and any deviations from a constant temperature were ascertained and noted, that the proper correction might be applied. The correction for the expansion of the air in the gauge, by a rise in its temperature during the progress of the experiments, was made according to the rules furnished by the rate of expansion of the gases, as determined by Gay Lussac, extended to compressed air by the experiments of Davy. The correction for the changes of height of the mercurial column, within the range to which the temperature was suffered to increase, could not have been appreciable if acting entirely, and the counteracting effect of the expansion of the glass further justified its being neglected. For similar reasons no reference was made to the effects of heat on the mercury in the cistern i, on the cistern itself, and on the water within the pipe communicating with the boiler.

In most of the researches of the committee, refinements in the mode of using the common thermometer would have been out of place. Results which might be obtained with little additional labour, and which would be interesting in both a practical and scientific point of view, were not to be neglected, and to some of them great accuracy was essential. In the questions of the first class, the thermometers were provided with wooden scales, and were graduated by immersion up to the point at which the scale commenced, the scale and upper part of the tube being exposed to the air: this was proper, as they were intended to be immersed in mercury nearly up to the scale. These instruments were examined after coming from the maker's hands, and the instrumental error ascertained. The tubes in which the thermometers were placed, and which contained mercury, were at first placed horizontally in one of the ends of the boiler; this had the advantage of rendering the tube for indicating the temperature of the water entirely independent of the steam, and thus any difference between the temperature of one and the other might be more effectually ascertained, than when the tube giving the temperature of the water passed through the steam. The position of these instruments interfered so much with other parts of the apparatus, and so much inconvenience and danger of error was experienced from the separation of the column of mercury in the thermometer, that these tubes were not used after the first weeks of experiment, and two vertical tubes, placed as already shown, were substituted for them.

The thermometers used, when the relation between the temperature of the steam and water, and the elasticity of the steam were to be observed in conjunction with some experiments of the subjects more directly under the cognizance of the committee, had much pains bestowed upon them.

The scales (M and N) were metallic, and surrounded by glass tubes, fitting into a cup a', through the bottom of which the stem of the thermometer passed water tight; a pipe b c, fig. 20, from the side of each cup, and provided with a stop-cock d, regulated the flow of water through the enveloping tubes: a tight connexion above with a reservoir (O) served, as in the case of the gauge, to supply the tubes with water. Small thermometers on the back of the scale of the large ones, showed the temperature of the water which surrounded them. The enveloping tubes being filled with water at 60°, the position of the boiling point of water and of the fusing point of tin, were used to verify the accuracy of graduation. The latter point, which is high upon the scale of the thermometer, having been very accurately determined, serves as an excellent check upon the graduation. The greatest error within the limits just stated, was in one instrument, three-fourths of a degree, and in the other one degree of Fahrenheit. The scales were graduated from two to two degrees, one quarter of a degree being readily estimated upon them. The corrections required by this examination were made through the medium of a table prepared for the purpose. In order to call the attention to the temperature of the water surrounding the scales, this temperature was recorded from time to time, when the height of the thermometers was observed. At no time did the rise of temperature, permitted in the water, make it necessary to apply a correction for the expansion of the scale. None was required for the cooling effect of the water around the stem upon the mercury, owing to the method of verifying the scale.

The other parts of the apparatus, less general in their use, as the water-gauge, safety-valve, fusible plate apparatus, &c., will be more conveniently described in connexion with the experiments for which they were devised.

34. With this apparatus, and these precautions, a series of experiments were made, the results of which are contained in the following tables:—

The Table, No. 1, contains the temperature observed by the thermometer in the water, corrected for the error of the graduation; the temperature of the scale of the thermometer, with a view to show that it was not allowed to vary too considerably; the observed height of the mercury in the gauge, reduced to its mean height; the temperature

TABLE, No. 1.—Of the Elastic Force of Steam at different Temperatures.

* This observation shows the height of the gauge before the experiment, corrected for the height of the barometer.
† Mean of four observations. ‡ Mean of two observations. § Mean of two observations.

of the air in the gauge; its volume at the observed temperature; the volume reduced to 48°, the temperature of graduation of the gauge at which the column of mercury, equivalent to an atmosphere, is very nearly 30 inches; the elasticity of the compressed air, in inches of mercury; the correction in the height of the column of mercury, for the depression produced in the cistern below; the height thus corrected: the height, after subtracting the sensibly constant number for the column of water between the level of the steam-pipe from the boiler and the cistern of the gauge; the total elasticity in inches of mercury; the elasticity in atmospheres. The first line of numbers in the table is merely introduced for the convenience of presenting certain data required for subsequent calculation; it gives the height of the mercury in the gauge before beginning the observations, after correcting for the height of the barometer.

A curve traced to represent these observations, the ordinates representing the pressures, and the abscissæ the

temperatures, is quite regular, until the temperature corresponding to eight atmospheres is attained, when it rises abruptly. This fact was explained, by examining the experiments of the American Committee. It was found that the cement used in attaching the glass tube to its ferule had become softened, and had permitted the tube to rise. This defect was remedied and its recurrence prevented. It was then determined to repeat the entire series of observations, and to carry them as high as could be done, with reasonable convenience, aiming particularly, to embrace the range of working pressures of the American engines.

The results are contained in the following table in which the observed data, and calculated numbers, are arranged as in the last table. This table extends to 9.91 atmospheres, and to the temperature of 352° Fahrenheit.

Care was taken that the elasticities were increased not too rapidly, and the last numbers obtained, were verified by keeping the temperature sensibly constant for a considerable time.

TABLE No. II.—Of the Elastic Force of Steam at Different Temperatures.

* This observation shows the corrected height of the gauge before the experiments.

There is one observation, namely, that at 329½°, which is certainly recorded erroneously; but omitting this one, the rest which are given, present a very tolerable regularity in the curve traced to represent them. For the sake of adding to the force of these results, the scattered

observations of temperatures and pressures incidentally made during the other experiments of the committee, are brought together in the annexed table.

A column is added to the table, to show the number of observations employed in obtaining the results.

TABLE No. III.—Of the Elastic Force of Steam at Different Temperatures.

* This observation shows the corrected height of the gauge before the experiments.

This table enables us to go as low as 1.43 atmospheres and is strikingly accordant with the two others as far as they extend in common.

A curve which would be traced by the following table, which may be considered to represent the mean of the foregoing, would differ little more than one-tenth of an atmosphere in any part of the range, from the observations, omitting one noticed in the first, and another noticed in the second table; the pressures in general differing less than one-tenth of an atmosphere from the observed pressures.

Table of the Elastic Force of Steam from One to Ten Atmospheres.

To compare our results with those given by the committee of the French Academy, we have traced a curve, from the above table, and another from those of the thirty observations, selected by the committee of the Academy, from their experiments which are below ten atmospheres. The curve of our observations, passes at low pressures nearer to the line AB than that of the French experimenters, and after coinciding at the medium pressures of the table, crosses the latter, differing at 10 atmospheres 5 degrees, or at 352½ degrees .65 of an atmosphere.

The difference here noticed is too considerable to be admitted as within the limits of errors in the apparatus or in observation. Having an authority of so much weight against them, the committee have been driven to examine their results very closely. The care employed in the graduation of the gauge seems to exclude the idea of error from it; the upper portion of the scale was divided to .05 of an inch, and could easily be read to half of that distance, making about .1 of an atmosphere at the highest pressure attained. A specific correction for capillarity was ascertained and employed. In one point of manipulation, namely, the method employed to dry the air, the committee differed from what was usual, and though they think there is reason to confide in that method, they have examined what effect would be produced if air were saturated with moisture. Recent experiments on the passage of gases, out and into vessels placed over mercury, and observations connected with them, warrant, moreover, a suspicion, that dry air standing in a glass vessel over mercury, the surface of which is covered by water, may become impregnated with vapour. The effect of such a source of error they have calculated in the highest and lowest results of table No. II. and find it to be as follows:—

For 248½° the tension of the vapour is 1.96 instead of 1.97, and 352 " " 9.78 " 9.91. Differing from the numbers given in table No. II. by .01 and .13 of an atmosphere.

This supposition is thus shown to be inadequate to explain the discordance, and must, in fact, be deemed, to a certain extent, gratuitous.

The committee have next compared the results furnished by the safety-valves graduated independently of the gauge, and these, as has already been shown, gave calculated pressures four per cent and ten per cent higher

than the pressures indicated by the gauge. From these independent experimental data we have then an evidence that our results are, probably, not too high."

SECT. III.—ON THE MATHEMATICAL LAW WHICH CONNECTS THE ELASTIC FORCE OF VAPOUR WITH ITS TEMPERATURE.

* 35. An inference which may be drawn from all these Dr. experiments is, that Nature seems to affect a certain law in the dilatation of aeriform fluids by heat. They seem to be dilatable nearly in the proportion of their present dilatation. For, if we suppose the vapours to resemble air in having their elasticity in any given temperature proportional to their density, we must suppose that if steam of the elasticity 60, that is, under a pressure of 60 inches of mercury, were subjected to a pressure of 30 inches, it would expand into twice its present bulk. The augmentation of elasticity, therefore, is the measure of the bulk into which it would expand, in order to acquire its former elasticity. Taking the increase of elasticity, as a measure of the bulk into which it would expand under one constant pressure, we see that equal increments of temperature produce nearly equal multiplications of bulk. Thus, if a certain diminution of temperature, diminishes the bulk of steam ¼, another equal diminution will very nearly diminish this new bulk ¼. Thus, in our experiments (Art. 25), the temperatures being in arithmetical progression, having equal differences, we see that the corresponding elasticities are very nearly in the continued proportion of 1 to 2, thus:

Now, although extreme temperatures differ considerably from this law, still we see that there is a considerable approximation to it; and it will frequently assist us, to recollect that within these limits an increase of 30° of temperature nearly doubles the elasticity and bulk of watery vapour.

This law obtains exactly in air and other gases, all of which are subject to the Boylean law, or law of Marriotte, as it is called, and have their elasticity proportional to their bulk inversely. If the bulk were always augmented in the same proportion by equal augmentations of temperature, the elasticities would be accurately represented by the ordinates of a logarithmic curve, of which the temperatures are the corresponding abscissæ; and we might contrive such a scale for our thermometer, that the temperatures would be the common logarithms of the elasticities, or of the bulks having equal elasticity; or, with our present scale, we may find such a multiplier $m$, for the number $t$ degrees of our thermometer (above the temperature where the elasticity is equal to unity), that this multiple shall be the common logarithm of the elasticity $F$; so that

36. As Dr Dalton was one of the earliest to investigate the properties of steam by well-contrived experiment, he has likewise been the most successful in obtaining precise and accurate views of those general relations which connect this with co-ordinate branches of physical knowledge. His experimental researches have been the model of imitation to all subsequent investigators. His apparatus was simple, his artifices were highly refined, and his processes elegant and precise; and, consequently, the results of his labour were immediately transferred to the works of highest philosophical character on the Continent and at home, and became part of the staple of accurate science. But his philosophical views were not so readily and widely received, and the fault lay, in part, with their author himself. He had overreached the ex-

isting condition of the other branches of contemporaneous science; and in taking for granted the accuracy of the existing state of knowledge, he proceeded to raise a theoretical structure on ground not yet sufficiently ascertained and determined. The result has been, as might have been anticipated, that now, when the progress of accurate knowledge has altered the conditions on which his system was based, his theory, becoming inapplicable to the facts, has been thrown aside, and, instead of having been modified, as it ought to have been, in conformity with the advancement of science, it has been hastily abandoned or undeservedly neglected.

From an extensive and laborious review of all that has since been added to the stores of our experimental facts on the properties of vapour, we have been conducted to this conclusion, that of all the views that have been taken of the constitution and laws of vapour, Dr Dalton's are those from which we may gain the clearest and most adequate conceptions; and therefore we have undertaken the task of reviewing the subject, and of making those changes and modifications which are now required to represent with fidelity and precision the advanced state of our knowledge.

If we examine any series of even the earlier experiments on the vapour of water (such as those in Art. 27), we cannot fail to recognise a certain degree of regularity in the progress of the increasing force of the vapour as the temperature is successively augmented. At the temperature of freezing water, the force of its vapour being taken at two-tenths of an inch, we see that it becomes more than doubled by raising the temperature $22\backslash frac\{1\}\{2\}^\{\backslash circ\}$; this again is rather more than doubled at $22\backslash frac\{1\}\{2\}^\{\backslash circ\}$ of additional heat; and this is again exactly doubled by a third addition of $22\backslash frac\{1\}\{2\}^\{\backslash circ\}$. But another addition of $22\backslash frac\{1\}\{2\}^\{\backslash circ\}$ of heat scarcely doubles the pressure; and $22\backslash frac\{1\}\{2\}^\{\backslash circ\}$ more fall still further short of producing that effect; so that, while the increase of the force of the steam takes place rapidly, with equal additions of heat, the rapidity of the increase does not maintain a constant proportion, but slowly diminishes as the temperature ascends. This will be plainer in the following table:—

accuracy, what would take place if we were to add another increment of $22\backslash frac\{1\}\{2\}$ degrees. We should then diminish the number in the third column by $\backslash frac\{2\}\{100\}$, and by doubling the Dr Dalton's, and having regard to this diminution from the preceding numbers, $2-\backslash frac\{2\}\{100\}$ we should have at

234$\backslash frac\{1\}\{2\}$ 45.00 2—$\backslash frac\{2\}\{100\}$ —9
And again at 257 63.90 2—$\backslash frac\{38\}\{100\}$ —8

It was in this way that Dr Dalton examined his experiments, and proceeded to form his tables, so as to include not only those points which he had already examined by experiment, but to fill up the vacancies, and extend them beyond the range which his actual observation had reached. He thus completed the table which we have already given. This was much more accurate than any previous table, and, being more extensive, formed a valuable addition to our knowledge.

This simple method of interpolation by which Dr Dalton constructed his table, although it suited perfectly, the limited object which he at that time had in view, and coincided with the limited range of his observations, was not of a sufficiently general description to stretch far beyond that sphere. It is obvious, that if his progression were continued much further, it would come to an end of itself; because the constant diminution of the proportion in the third column would bring it down to nothing, and so the march of the method would close and retrograde, and would thus bring the method of the formation into opposition with the march of the fact, for the force of the vapour continues to increase. Dr Dalton was himself the first to recognise the limited applicability of his method of interpolation to wide ranges of temperature; and, accordingly, in his lectures on heat, delivered at Edinburgh and Glasgow in 1807, and in his New System of Chemistry, published 1808, he developed those larger and more matured views which had grown up in his mind during a longer and more thorough investigation of the subject.

It does not belong to this article to consider the nature, and decide on the merits of Dr Dalton's theory of temperature; nor is a perfect acquaintance with that theory of any further use in understanding his views of the constitution of vapour, than to enable us to perceive how he was led from the former to the latter. For the validity of his views regarding steam, it is indeed of no consequence whether the theory of temperature from which it was originally deduced, be true or erroneous. The general laws which he has determined for elastic vapours, form the well-settled foundation on which any theory of temperature, true or false, must in some measure ultimately rest.

The only circumstances in regard to temperature which it is proper to keep in view, are these: that the present thermometer used to indicate temperature is not to be regarded as an exact measure of the quantity of heat producing that temperature. This is shown from the circumstance, that the same quantity of fuel which heats water $10^\{\backslash circ\}$ from $180^\{\backslash circ\}$ to $190^\{\backslash circ\}$, will not heat it from $80^\{\backslash circ\}$ to $90^\{\backslash circ\}$, an equal interval. From considerations of this nature it was evident that the divisions of the common scale were too large near the bottom, and too small in the higher portions; and Dr Dalton evinced this difference to be so great, that $72^\{\backslash circ\}$ of the common scale below the freezing point of water down to the freezing point of mercury, were to be reckoned as equivalent to as many as $207^\{\backslash circ\}$ of Dalton's scale. Proceeding on this view, it was necessary to find the ratio of these two series of indications, the indications of Dalton's and of Fahrenheit's scale; and he accordingly found that the progression of Fahrenheit's scale was in a high geometrical proportion to the increments of true temperature of the new one. On this principle he proceeded to construct his new scale of temperature—of which the following is a specimen.

From this simple collocation of results, a principle of progression is manifested. The number of degrees in the first column increases at each step by $22\backslash frac\{1\}\{2\}$ degrees, and the number in the second column on the same line is nearly doubled every step. At first, as the third column shows, it is more than doubled by $\backslash frac\{17\}\{100\}$; next time it is more than doubled by $\backslash frac\{9\}\{100\}$, and next it is doubled exactly; after this, however, it falls $\backslash frac\{6\}\{100\}$ short of being doubled, next time by twice that quantity, and so on, till we find at last that it falls short of doubling every time by about 8 or 9 hundredths for every $11\backslash frac\{1\}\{2\}$ degrees. Although, therefore, we may at first be disappointed in finding that the reduplication does not proceed with the regularity of a law of nature, still it is satisfactory to know that the deviation from this progression is itself the subject of a tolerably simple law, so as to enable us to predict, with some measure of

By this new scale of temperature it was found that many of the apparent anomalies in the effects of heat were resolved, and the complex relations of its phenomena rendered very simple. Amongst others, the most important were the phenomena of vapours, as it was found that, on the new scale of temperature, the elastic force of different vapours increased almost exactly in a uniform ratio to equal increments of heat.

But the further progress of experimental science soon raised up serious grounds of objection to this view. It was found that Dr Dalton had rated the inaccuracies of the present scale somewhat too high. His results were thus rendered inapplicable to the advanced state of some branches of thermal science; and his theory, instead of being modified and improved, was first hastily discredited and then summarily dismissed. Unable to follow the theory to its whole extent, it was abandoned even when it had furnished a safe guide thoroughly to explicate the intricacies of obscure truth.

It is now, therefore, necessary to examine the views of those who have endeavoured to form adequate representations of the mathematical law which connects the elastic force of vapour with its temperature. We shall first of all examine the methods and views which they have adopted, and then consider whether there may not be deduced from the clear theoretical views of Dr Dalton, tested and modified by the results of modern experiment, mathematical expressions of a character, at once less empirical, and more closely in accordance with observed phenomena.

37. M. de Prony was the first to represent, by a purely empirical formula, the law which governs the relation between the temperature and the elasticity of aqueous vapour. It was derived by him, in 1796, from the experiments of M. de Betancourt, and constructed according to a method of interpolation, which he afterwards presented to the Academy of Sciences, and which they have placed among the Mémoires des Savans Etrangers.

The Formula which he has thus obtained is

y being the height of the mercurial column of pressure. x the temperature.

The same formula holds in the case of alcoholic vapour, the numerical co-efficients being changed, and a constant quantity $A\; =\; 1.126447$ subtracted from the result.

$\backslash mu\; =\; -0.04853,\; \backslash lambda\; =\; 0.02393,\; \backslash sigma\; =\; 0.0467,\; f\; =\; 2.60249$
$\backslash mu\text{'}\; =\; -0.63414,\; \backslash lambda\text{'}\; =\; 0.09653,\; \backslash sigma\text{'}\; =\; 0.0294,\; f\text{'}\; =\; 1.64909$
This formula was afterwards improved by its author, and presented in the following more elegant and convenient shape.

Where $z$ is the mercurial column of pressure, $x$ the tem-

perature centigrade, and $\backslash mu,\; \backslash mu\text{'},\; \backslash mu\text{'}\text{'},\; \backslash mu\text{'}\text{'}\text{'},\; \backslash mu\text{'}\text{'}\text{'}\text{'}$ equidistant constants derived from experiment. For water, these values are:—

and hence he has formed the numbers which we have united in a subsequent table.

M. de Prony's formula for the vapour of alcohol is—

the constants being

These numbers refer to the centigrade thermometer, and to an atmosphere of 0.7577 metres in height.

These formulae indicate some singular phenomena at high temperatures, which have not been observed in recent experiments, and may therefore be deemed anomalies of the formulae themselves rather than the legitimate results of the experiments they were intended to represent. The formulae are, besides, much too opulent to be useful.

38. The experiments of Dr Dalton are adopted by La-Laplace in the fourth volume of the Mécanique Céleste, where we find him applying them to the calculation of the influence of the aqueous vapour of the atmosphere upon astronomical refractions. As an empirical formula agreeing sufficiently with Dr Dalton's experiments, he adopted the following approximation:—

$f$ being the force at any temperature $n$ of the centesimal scale, reckoned from the point of ebullition, and $p$ the pressure of the atmosphere = 0.76 metres; or that we have only to add to the log. of 0.76 the quantity $n\; \backslash cdot\; 0.015417\; -\; n^2\; \backslash cdot\; 0.0000625826$ and we have the log. of the common tabular logarithm of the corresponding elasticity at $n$ centesimal degrees of temperature.—Méch. Cel. iv. 273.

These numbers agree very well with the observations they were intended to represent, from 0° to 100° centigrade, but are found inaccurate above and below these points.

39. M. Biot, adopting still the methods and experiments of Dr Dalton, found it necessary to modify the formula in order to obtain a closer approximation to truth. Using the notation in which we have expressed Dr Dalton's method of calculation, Biot considers

as a first approximation; of which the logarithmic form is $\backslash text\{Log.\; \}\; f\; =\; \backslash text\{log.\; \}\; 30.\; +\; n\; \backslash text\{\; log.\; \}\; a$; which would always give the logarithm of the elastic force, provided the ratio were accurately constant; but, as it is variable in Dr Dalton's observed numbers, it would be convenient to represent the variation of the logarithm of the elastic force thus:—

$\backslash alpha,\; \backslash beta,\; \backslash gamma$, being constants derived from experiments thus—and setting out from 100° cent. as the zero—

By substituting successively these values in the formula we get

From these three equations we can readily obtain the three values wanted of $\backslash alpha$, $\backslash beta$, and $\backslash gamma$, and which we find to be

and hence the whole equation

is now determined in English inches for the centigrade thermometer; and in order to compare it with the French observations, it is only necessary to remember that 30. in. = 0.7679 French metres, and to transform it thus:

and in the common table of logarithms

which are almost identical with Laplace's formula (C), the degrees being reckoned positively from 100° cent. downwards, and negatively upwards.

In degrees Fahrenheit and English inches, the formula in this shape becomes—

These formulae are far from representing the results of late experiments at high temperatures, although, within the limits of one atmosphere, they accord pretty closely with Dr Dalton's early observations.

40. In the first volume of the new series of the Philosophical Magazine, Mr Ivory has given a formula constructed to represent empirically the experiments of Dr Ure. It is—

The application of this formula is laborious. It is of exactly the same nature with that of Laplace and Biot, and only represents the observations of Dr Ure within their narrow limits; extended to higher temperatures, it seems to deviate considerably from the truth, as may be seen from our table (Art. 57.)

41. Schmidt and Soldner, reviewing Dr Dalton's experiments, have each constructed a formula to represent them:

Soldner's formula is—

42. In the Edinburgh Journal of Science for 1829, Mr Tregaskis has given a theorem, which furnishes a rough approximation to experiment. It is this: that $\backslash frac\{1\}\{2\}$ of the temperature above 32°, added to vapour, will double its elasticity.

43. M. Roche, Professor of Mathematics at Toulon, sent to the Academy of Sciences, in 1828, a memoir on this subject, in which he proposes a formula, deduced from general principles. This formula is—

This formula agrees closely with the French experiments.

44. Dr Thomas Young invented a species of formula entirely new. Abandoning altogether the formula in which one of the variables is involved as an exponent, and abandoning altogether the views from which formulae of this kind had been derived, he assumed an expression which is apparently perfectly arbitrary, and which has been adapted empirically to the experiments of Dr Dalton. It is this:

t being reckoned above 212 Fahr. and F being the force in inches of mercury. Hence we get inversely:

For very small changes of temperature, Dr Young's formula becomes

e being the corresponding slight variation of pressure from 30 inches, which corresponds, within three-thousandth parts, with the mean between Deluc's correction 1.598, and Shuckburg's 1.70, or 1.645e.

Notwithstanding the simplicity of the form of this expression, and the facilities which it presents for ready calculation, it is impossible to adopt it, as it deviates widely and rapidly from the results of observation when extended to high temperatures. Induced, however, by the simplicity of the expression, and not a little influenced, it may be, by the high authority of a name that will ever be distinguished among the most distinguished of those who have contributed immortal truths to the treasures of physical science, the example of Dr Young has drawn after it many followers. Southern, Creighton, Coriolis, Tredgold, Arago, and Dulong, have successively attempted to modify the formula of Young, so as to twist it into some measure of conformity with observed phenomena—we shall see with how little success.

45. Mr Creighton adopted a similar formula to represent Ure's experiments, only changing the constant exponent from 7 to 6; so that, making F the force of steam in inches of mercury — 0.09, and the temperature of Fahrenheit + 85° = t, we have

46. Mr Southern represented his experiments by the formula

Or,

And,

47. Mr Tredgold simply reinstated Creighton's exponent, altering the co-efficient to bring it nearer to those experiments with which he was acquainted when his work was written; but it is inaccurate at high temperatures, and like that of Creighton.

48. To adapt the formula to more recent experiments, M. Coriolis (in his work Du Calcul de l'Effet des Machines, 4to, 1829) changed the exponent to 5.355, making it in French measures,

reckoning from 0° cent. in atmospheres of 0.76 metres of mercury.

49. The French Academy of Sciences have finally reduced the index to 5; finding that number, represent their experiments at high temperatures, they adopted the following expression:

to give the elasticity in atmospheres of 0.76 metres, the temperature being in centesimal degrees, of course

50. In conclusion, the committee of the Franklin Insti-

Steam. tute have found it necessary to reinstate the index 6, of
Creighton, only modifying Dr Young's constant multiplier,
Formula so as to obtain

51. It may be useful to collate these formulae, and for
this purpose they are assimilated in notation as follows—
F being the elastic force due to a certain temperature t.

Robison's Formula.

Prony's Formula.

Laplace's Formula.

Biot's Formula.

Ivory's Formula.

Schmidt's Formula.

Soldner's Formula.

Roche's Formula.

Dr Thomas Young's Formula.

Creighton's Formula.

Southern's Formula.

Tredgold's Formula.

Coriolis' Formula.

Commission of the French Academy.

Committee of the Franklin Institute.

52. From his earlier experiments Dr Dalton constructed
a scale of true temperature, in which the point of freezing
mercury is placed at 175°, and in the method he there
adopts, the increments of the scale of true temperature are
as the square roots of the corresponding expansions of the
mercury from its point of maximum density. This scale
was soon made the subject of a close experimental scruti-
ny by Messrs Dulong and Petit, and afterwards of less
accurate, though more acrimonious, strictures by Dr
Ure.

This scale was, in fact, slightly inaccurate, because
it was founded on the comparatively incorrect data of
the experimental physics of that date. It is, however,
scarcely fair to institute a comparison between the re-
sults of a theory based on certain phenomena and the
results of experiments which the improvement of our
knowledge has entirely altered. It were less unjust to
the theory, and more wise as regards the interest of
philosophy, first to examine how far it would have been
modified by recent discoveries and then to compare its

results with the legitimate consequences of the data on
which it rests. It ought also to be recorded, that Dr
Dalton published, in the third part of his Chemical
Philosophy in 1827, the corrected experimental results to
which he had been conducted by the improved methods of
observation, and the increased experience of thirty years
which had elapsed from his first experiments, while
modern writers continue to use the old numbers which
should have been altogether discarded.

Adopting, then, Dr Dalton's recent experiments below
the point of ebullition of water, and the experiments of
the French and American Institutes above that point
to 24. atmospheres, let us see what theory the views of
Dr Dalton would conduct us to, setting out from these
improved data.

Now, Dr Dalton found that, in his experiments, a certain
progression of temperatures was accompanied by a certain
progression of elastic force; but his range of experiment
being too small, he adopted an erroneous progression, by
which, reckoning this progression as rising from the
freezing point of mercury, and proceeding as the square
roots of the equal expansions of mercury above that point,
gave 175° as the point corresponding to the zero of the
scale and the origin of his progression.

53. In examining this subject again, I have found that
this gradation of temperatures, though not exact in
truth, is analogous to one which may be deduced from
the best experiments—and equally from Dr Dalton's and
those of the French Academy. The law at which I have
arrived is this—that if we reckon the temperatures from
the point of congelation of mercury in a logarithmic
series, the elastic force of steam forms a similar geo-
metric series to these intervals of temperature. This would
indicate that equal intervals of temperature are those
which expand the substance of the thermometer through
equal fractional parts of its bulk, instead of equi-differen-
tial parts as at present, so that, instead of the common
arithmetical series as at present, viz.,

$C\; +\; d\; +\; 2d\; +\; 3d\; +\; 4d\; +\; \backslash dots\; \backslash dots\; \backslash dots\; n\; d,\; \backslash dots\; \backslash dots\; \backslash dots\; a$
we should have the temperatures represented by the geo-
metrical series

$(C\; +\; d)\; \backslash cdot\; (1\; +\; d^2\; +\; d^4\; +\; d^6\; +\; \backslash dots\; \backslash dots\; \backslash dots\; d^n)\; \backslash dots\; \backslash dots\; \backslash dots\; b$
and then the corresponding elasticities would be the
geometrical series

$(F\; +\; h)\; \backslash cdot\; (1\; +\; h^2\; +\; h^4\; +\; h^6\; +\; \backslash dots\; \backslash dots\; \backslash dots\; h^n)\; \backslash dots\; \backslash dots\; \backslash dots\; \backslash gamma$

54. Let us, therefore, endeavour to obtain the values of
two such series, so as to coincide with the best experi-
ments. For this purpose we put the series $\backslash gamma$ into the
form

$a\; m^n\; =\; f\; \backslash dots\; \backslash dots\; \backslash dots\; \backslash delta$

and the series representing the progression of tempera-
tures into the form

$\backslash frac\{t\}\{r\}\; =\; p^n\; \backslash dots\; \backslash dots\; \backslash dots\; \backslash epsilon$

then since (2)

$\backslash text\{Log.\; \}\; a\; +\; \backslash text\{log.\; \}\; m\; \backslash cdot\; n\; =\; \backslash text\{log.\; \}\; f$

or making $a$ the unit of pressure, for simplicity, we get

$n\; =\; \backslash frac\{\backslash text\{log.\; \}\; f\}\{\backslash text\{log.\; \}\; m\}$

whence by substitution in $\backslash epsilon$ when $q\; =\; n$, that is, when the
elastic force is that which corresponds to the temperature
$q$, we get

$\backslash frac\{t\}\{r\}\; =\; p\; \backslash frac\{\backslash text\{log.\; \}\; f\}\{\backslash text\{log.\; \}\; m\}\; \backslash dots\; \backslash dots\; \backslash dots\; \backslash zeta$

therefore

$\backslash text\{Log.\; \}\; t\; =\; \backslash frac\{\backslash text\{log.\; \}\; p\}\{\backslash text\{log.\; \}\; m\}\; \backslash cdot\; \backslash text\{log.\; \}\; f\; +\; \backslash text\{log.\; \}\; \backslash epsilon\; \backslash dots\; \backslash dots\; \backslash dots\; \backslash eta$

and

When, therefore, $p$ and $r$ are determined for a given value of $m$, the relative is obtained. If we take the value $m\; =\; 2$, and if we take from the experiments of the French Academy and Franklin Institute, values of $t$ and $f$ above and below 30 inches, or unity, which is the value of $m$, and let these values be $t\text{'}$, $t\text{'}\text{'}$, $f\text{'}$, $f\text{'}\text{'}$; then from (5) we have

We have only, therefore, to assume $r$ so as to satisfy these conditions. Now,

that is to say, if we reckon temperature from some given point $c$ above or below the usual zero, viz. at the freezing point of mercury, like Dr Dalton's scale of temperature, and use the elastic force at $212^\backslash circ$ as our unit of pressure, we have then only to take $f\text{'}$ and $f\text{'}\text{'}$ from the tables of experiment, and give such a value to $c$ as will satisfy the conditions. But as Dr Dalton places that zero at $175^\backslash circ$ we get

We have still to find $p$ the index of progression, corresponding to the values of $f\text{'}$ and $f\text{'}\text{'}$ in the experiments.

If we take the value of $m\; =\; 2$, then since by (5)

From the French experiments we get $p\; =\; 1.103$, whence by substitution, $r$ being $=\; 387^\backslash circ$, $p\; =\; 1.102$, $m\; =\; 2$, we have

From Dr Dalton's experiments we get

From the combination of Dr Dalton's experiments below 30 inches, with the mean between those of the French Academy and the Franklin Institute, we get

It is from equation S and equation T that we have constructed the large table (Art. 56) in which the results of these formulae are compared with experiment; the formula for high-pressure steam being compared with the mean of the French and American experiments; but, as they do not extend below $212^\backslash circ$, that part is compared with Dr Dalton's table. The coincidence of these formulae with experiment turns out to be much closer than could possibly have been expected where the discrepancies of experiments from each other are so great. The experiments of Dr Ure deviate from those of Dr Dalton, below

the pressure of the atmosphere, as much as .33, and the greatest deviation of the formulae is .08. At pressures above the atmosphere the maximum deviation in the first ten atmospheres between the French and American experiments amounts to $6.4^\backslash circ$, while the maximum deviation of the formulae is only $1.1^\backslash circ$.

It is, however, remarkable, that in all the experiments hitherto made, the law of elasticity below the atmospheric pressure appears to deviate considerably from that above the atmosphere—perhaps it may arise from the circumstance that the experiments below atmospheric pressure have been made with different apparatus, having errors of a different kind from those made at high pressures above the atmosphere.

From these equations we easily deduce the following formula in a shape convenient for calculation.

These formulae converted into rules are as follows:—

To find the pressure corresponding to any given temperature of steam above $212^\backslash circ$—

Rule. To the temperature add $121^\backslash circ$, find the logarithm of that sum, subtract from this logarithm the number 2.522442, and multiply the remaining number by 6.42, the product is the logarithm of the pressure in atmospheres of 30 inches of mercury.

To find the temperature of steam, having any given pressure greater than that of the atmosphere—

Rule. Find the logarithm of the pressure in atmospheres, multiply it by 0.1557634, add to the product 2.522442; the sum is the logarithm of the temperature, from which, if $121^\backslash circ$ be subtracted, the remainder will be the temperature on the common scale.

Example. To find the temperature at which high pressure steam will exert a force greater than the atmosphere by 195 lbs. on the inch—

502.306 is the number of which 2.7009689 is the log. 121. being subtracted

381.306° is the temperature on Fahrenheit's scale at which the elastic force of steam has a pressure of 14 atmospheres; an elastic force of $13\; \backslash times\; 15$. lb. excess of pressure above the atmosphere on each square inch, = 195. lbs.

To find the pressure corresponding to any given temperature of steam below $212^\backslash circ$—

Rule. To the temperature add $175^\backslash circ$, find the logarithm of that sum, subtract from this logarithm the number

Steam. 2.587711, and multiply the remainder by 7.71307; the product is the logarithm of the pressure in decimal parts of an atmosphere; which, if multiplied by 15. will give pounds on the square inch, and by 30. inches of mercury.

To find the temperature at which steam will have a given elastic force less than that of the atmosphere—

Rule. Find the logarithm of the pressure in decimal parts of an atmosphere, multiply it by 0.12965, add to the product 2.5877110; the sum is the logarithm of the temperature which will be expressed in degrees of Fahrenheit's scale, if 175. be subtracted from it.

Example. To find the pressure of steam at 175°.

The next No. is 0.412837, its log. 1.6157786

12.384 inches of mercury is the pressure;
being 17.616 inches of mer. below the atmos.
30.000

By these rules the following table is calculated.

Table of the Elastic Force of Vapour in inches of Mercury, at different temperatures, according to our Formula S² below 212° and T² above it.

55. The formulae thus given are in such perfect accordance with our best experimental knowledge, that we cannot withhold our assent from the correctness of the principles from which they have been deduced. At the same time, we desiderate very much a better series of experiments than we yet possess, as the range of doubtful temperature above 212° is far wider than the present perfect state of experimental science, and our improved means of observing, can at all warrant. The discrepancies between the experiments above and below 212° show, that the two series should if possible be performed with identical apparatus.

The formulae we have obtained have been founded on the hypothesis, that bodies expand nearly equal proportions of bulk in equal intervals of true temperature; and we have found that the elastic force of steam increases in equal proportions, from equal increments of temperature, reckoned in true intervals from the bottom of the scale.

Our formula should, however, be capable of being reduced into a form closely resembling those which have preceded it, in so far as these have represented approximately the experiments they were made to represent; thus the formula of Laplace and his followers is of the form

So, in like manner, we should obtain from Equation T the following:

which is easily presented in a form absolutely the same.

In like manner, it may be presented at once in the form adopted by Dr Thomas Young and all his followers, viz.

for, if we take our formula S₂

we get, resuming the natural number,

Or, if we take formula T₂, we get

We thus find, that the old formulae have all approximated in a greater or less degree to the representation of

the very hypothesis on which ours has now been formed. They thus add greatly to the probability of its truth.

We do not, however, mean to assert, that the zero of the mercurial thermometer is absolutely at 175° or 121° below the present 0. or that the progression of the temperatures has been fixed accurately for mercury or for vapour. On the contrary, we have seen that the discrepancies of the results obtained by different physical experimenters are great, and do not admit of obtaining un-

changeable numerical indices of progression, either of the temperatures or the corresponding elastic force. The existence of these two progressions, and their character, has, we think, been established, and our research has the effect of confirming, the profound views of Dr Dalton, which have, we think, been ill understood and insufficiently appreciated.

56. The following table exhibits some formulae and experiments collated:

Table of the force of Steam at different temperatures from 0° to 500°.

Table of the Force of Steam at different Temperatures (continued).

The first and last columns contain the successive temperatures, as far as 240°, and after that the number of atmospheres of pressure; and their reference extends wholly across the table.

Col. II. contains the later experiments of Dr Dalton, interpolated, where necessary, down to 240°; the remainder of that column is from the experiments of the French Academy.

Col. III. contains the experiments of Dr Ure.

Cols. IV. V. and VI. contain the formulae of Dr Young, Mr Ivory, and Mr Tredgold.

Cols. VII. VIII. IX. and X. contain the experiments of Southern, Robison, Watt, and the Franklin Institute.

Col. XI. contains the numbers given by our formulae.

Col. XII. exhibits the differences between the experiments of Drs Dalton and Ure.

Col. XIII. exhibits the differences between the experiments of the French Academy and the Franklin Institute.

Col. XIV. exhibits the deviations of Tredgold's formula from Dalton's experiments, down to 240°; and below that point, from the mean of the experiments of the French Academy and the Franklin Institute, interpolated where required.

Col. XV. exhibits the difference between our formulae and the best experiments, Dr Dalton's being taken down to 240°; and the mean between those of the French Academy and the Franklin Institute from that point to the end of the table—interpolations being used when necessary.

SECTION IV.—ON THE CONSTITUTIONAL CALORIC OF STEAM, ITS DENSITY AND VOLUME AT DIFFERENT TEMPERATURES, AND ITS GENERATION AND CONDENSATION.

57. Having now ascertained the force that may be given to steam by heating water in a confined space, so that we can always obtain any force we desire by raising it to the proper temperature, we have next to enquire what quantity of heat is necessary to produce steam of that temperature and force. The answer to this question is, to determine the quantity of fuel necessary to generate steam of a given power, and direct the economic application of that power.

The quantity of caloric necessary to transform a given quantity of water into steam of the same temperature, is called the caloric of elasticity of that substance. The quantity of heat which it will contain at any given temperature is called its capacity for heat; and the relation which subsists between the quantity of heat which some well-known body, such as water or air, gives out or acquires, in a given change of temperature, and that which any other substance will give out or acquire in the same circumstances, is called the specific caloric of that substance.

58. Of the quantity of the caloric of elasticity of any substance, of its capacity for containing heat, and of its specific caloric, the thermometer gives us no information. An instrument for doing so may be called, as a distinction, the calorimeter. Thus the thermometer measures the intensity of heat—the calorimeter its quantity.

If in three several vessels there be contained, at the temperature of 32°, a pound weight of three different fluids, water in one, mercury in the second, and oil in the third; and if the heat of an alcohol lamp be applied, first of all to the mercury, then to the water, and next to the oil, a thermometer being inserted in each, it will require much longer time to heat the water to 212° of the thermometer than the mercury, twenty-five times as much alcohol being burnt in the process; whereas the oil will be warmed to the same temperature on the thermometer, by half the quantity of caloric which is necessary to heat the water to 212°. Therefore, the capacity of water for heat is said to be twenty-five times as great as that of mercury, and twice as great as the capacity of oil; and the specific heats of these substances, in relation to water, are thus represented:

Water, 1.000; oil, 0.520; mercury, 0.040 nearly.

If, instead of taking equal weights of these substances, we had filled equal vessels with them, and then applied the quantity of water necessary to heat them to equal temperatures, we should have had the capacities and specific heat of equal volumes, instead of equal masses as formerly; and it would have been found, that the quantity of alcohol required to heat them to the same temperature, was only half as much for the mercury as the water, and greater for the water than the oil, in the proportion of 20 to 9; showing that the capacity of water was still the

greatest, and that the specific caloric, in equal volumes of these substances, are nearly

Water, 1.000; oil, 0.450; mercury, 0.550.

The capacities for heat, and the specific caloric of different substances, may be determined by cooling as well as heating them. An ounce of ice thrown upon a pound of mercury will cool it or an equal weight of oil much more than water; and, in general, the quantity of ice, or of cold water, or of cold air, or any other fluid required to cool a body, will exactly correspond to the quantity of caloric required to heat it to the same degree of temperature. A calorimeter measures the quantity of ice which must be melted in cooling different substances, and is described in our article "HEAT." The following are the results of such of the most valuable experiments upon this subject as are appropriate to our present enquiry:

Table of the specific Caloric in different substances.

69. Besides the capacity of different bodies for heat, and the specific heat of each at given temperatures, there is another condition of heat still more striking, and of which the thermometer gives no indication. It is this: that the same substance, at different times, may contain different quantities of caloric, and yet the thermometer in both cases give the same indication of temperature. Ice at 32°, which is in the process of melting, and while its bulk is diminishing by one-tenth part, receives as much caloric as would raise its temperature, when melted, to 172°; and after having received it all, remains still at the same temperature as before, indicating 32° on the thermometer. In like manner, when the particles of the water have acquired so much sensible heat as to raise its temperature to 212°, it may receive as much more heat as would have raised its temperature 950° or 960°, if it had continued to be shown by the thermometer; but the water now assuming the state of steam, the thermometer indicates no accession, but remains in the water or in the steam still at the temperature of 212°. In these two conditions, there-

* In these two columns air is assumed as unity, the first being the specific heat under equal weights, and the second under equal volumes.

fore, when the particles of ice are leaving the solid and taking the liquid form, and again passing out of the liquid into the vaporous state, a large accession of caloric passes into the substance without being detected by the thermometer; this heat, insensible to the thermometer, and manifested only by the calorimeter, is called LATENT HEAT. The doctrine of latent heat was discovered by Dr Black.

The quantity of heat thus latent in the mass of a solid, when it assumes the liquid state, is called the caloric of fluidity. The latent caloric of a liquid passing into vapour is called the caloric of elasticity or vaporization.

60. The determination of the latent heat of ordinary steam is a problem of considerable practical difficulty. It may be obtained rudely by very simple contrivances. If a lamp, which burns with tolerable uniformity, be applied to a vessel containing cold water, at the temperature of 32°, so long as to heat it to 212°, the boiling point, and if the lamp be then weighed and the consumption of oil ascertained by the loss of weight; and if the lamp be still applied to the boiling water so as to keep it constantly in ebullition until the whole has been converted into steam; the steam passing off at the same temperature as the water, it will be found, when the whole water has been boiled away, or converted into steam, that 6 times as much oil has been consumed, or that 6 times as much heat has been employed in the conversion of the water into steam as was required formerly to heat the water from 32° to 212°, or to give it 180° of temperature; so that 6 times 180° or 1080°, will appear to have been absorbed or carried off in the steam of 212°—that is, the latent heat of steam is 1080°.

Otherwise, the same determination may be obtained, if the steam, when passing off from the boiling water, be led carefully in a pipe to a vessel of cold water, so as to take from it the heat which it has thus carried off; if the water to which the heat of the steam is given out be at a temperature of 32° and of 6 times the quantity of the water from which the steam was formed, the whole of it will be heated by the caloric of the steam to 212°, showing that the quantity of caloric of the steam amounts to what gives 180° to 6 times the quantity of water; giving, as formerly, 6 times 180° or 1080° as the amount of the latent heat of the steam.

It is to Mr Watt that we owe the earliest determination of the latent heat of steam. Dr Black endeavoured to ascertain this point by the first of the methods we have pointed out, by comparing the time of raising the temperature of water a certain number of degrees, with the time of boiling it off a certain number of degrees; but his result was not correct, being only 800°. Mr Watt's result for the latent heat of steam was 1006° 79.

Mr Southern's experiments were made in 1803; and he was assisted in them by Mr William Creighton, and communicated them to Mr Watt for an appendix to this article. He obtained the number 950°. The thermometers employed in his experiments were made and graduated with the greatest care, the tubes having been accurately measured as to the proportional capacity of their different parts.

A similar series of experiments was afterwards made by M. Schmidt, who determined the heat latent in steam to be about 5.33 times that necessary to heat water from 32° to 212° = 5.33 times 180° or 960 nearly.

Count Rumford determined the latent heat of steam by condensing it in a calorimeter formed by pushing a long spiral steam pipe through a vessel of cold water, by which he obtained 1040.8 as the latent heat of steam of water.

M. Despretz in the Annales de Chimie et Physique, gives 955.8° as the result of his experiments on the latent heat of steam.

Lavoisier and Laplace make the latent heat of steam 1000°.

From the experiments of Gay Lussac and of M. Clement and Desormes, the number 990° is generally used by the French to represent the latent heat of steam. The diversity of the results obtained from experiments made by so many excellent experimenters, with so much precaution, is remarkable—to eliminate from them the precise truth with certainty is not within our present resources of analysis. There is high probability in favour of the numbers 990. or 1000., as representing nearly enough the latent heat of steam, being 5.555 times the caloric of boiling water, its whole caloric reckoned from 32° being 6.666 times that of boiling water.

61. A doctrine of great simplicity is now pretty generally held as expressing with an accuracy quite within the limits of experimental precision, the result of our knowledge of the heat latent in steam. It is found that in steam of great elasticity and of corresponding high temperature, the heat latent is in quantity less; and that, on the contrary, when steam is of lower elastic force and of lower temperature than at 212°, its latent heat is greater than at 212°. And it appears that we are warranted in the conclusion first suggested by Mr Watt and afterwards by Dr Dalton, that the whole amount of caloric in a given quantity of elastic vapour remains the same at all temperatures and under all pressures. When the volume of the vapour is great the greater is its capacity and the less its temperature; while, by compressing it into smaller space, its elasticity is increased and its temperature raised. The doctrine is thus expressed, that the sum of the sensible and latent heat of vapour is a constant quantity. M. Despretz has extended this to the vapours of several other fluids.

There is another expression for the law of the constitutional heat of the vapour, which is, in the language of the Atomic Theory, that every atom of a fluid in the state of vapour possesses, under every degree of elasticity and pressure, the same quantity of caloric. This doctrine leads to very important consequences both of a theoretical and practical nature.

It follows immediately from this doctrine, that if a quantity of vapour have once been formed by adding to the liquid the quantity of caloric necessary to the constitution of the vapour, the same particles of matter surrounded by the same spheres of caloric may pass through all gradations of density, and through all gradations of temperature, without either parting with caloric or obtaining fresh supplies. Vapour of the temperature of 212°, as it rises from water boiling in the open air, may be collected in a vessel and compressed by the force of 30 inches of mercury into half its bulk, it will become steam of a higher temperature, viz., 250° from the increased quantity of caloric in the diminished volume, and in this case the latent heat will only be 970° instead of 1000°. If compressed still further into again one half of that bulk, the temperature will rise to 292°, and leave only 920° latent. Compressed still further into half of the last-mentioned space, that is into $\backslash frac\{1\}\{4\}$ of its original bulk, the temperature is raised to 339°, leaving only 873° latent; and another step would raise the temperature to 392°, leaving only 820° latent; less than seven steps more would bring the steam into less than its original bulk of water, with a temperature of between 900° and 1000° of sensible heat, and

Steam. an amount of latent heat not much greater than its original proportion of sensible heat, or 212°. In this case, we should have steam as heavy as water and as hot as flame.

Latent Heat. If, on the contrary, this process were reversed, and the steam produced at 212° under the pressure of an atmosphere permitted to expand in vacuo to double its bulk, a portion of the sensible heat would become absorbed into the spheres of caloric around the atoms of water, increasing the latent heat by 32°, and diminishing the sensible heat to 180°. The bulk being again doubled, and the steam expanded to four times its original bulk, the temperature would sink to 150°, and three more repetitions of the expansion would give a vapour of 71° temperature, and 1141° of latent heat.

This expansion and contraction of the steam, accompanied by diminished temperature, is exactly what would exist if our atmosphere, instead of oxygen and nitrogen, were wholly composed of vapour of water. Suppose the temperature of the ocean to be 1000, an atmosphere of vapour would be raised of 2000 times the weight of the present atmosphere: the under part of this atmosphere, compressed by the superincumbent weight, would be of great density, but in ascending, the diminished pressure would be attended with diminished temperature, until at last a cloud of white ice would be seen floating on the surface. Must not the sun, from his intense heat, be a body of this nature, having an atmosphere of enormous depth, on the summit of which the beautifully crystalline and sparkling crust is continually preserved by its diminished temperature in a state of renewed whiteness?

Specific gravity, density, and volume of Steam. 62. The specific gravity, density, and volume occupied by steam at different temperatures, have been correctly determined by experiment; and it has been ascertained that the expansion of vapour follows the law of the expansion of other gases by heat; viz., the law of Dalton and Gay Lussac, that all gases expand from 1. to 1.375 in bulk, by 180° of temperature, or 480 for each degree of Fahrenheit; and, secondly, that steam obeys the law of Boyle and Mariotte, contracting in volume proportionally to pressure. It is first of all necessary to know what bulk a given quantity of water converted into steam will occupy at a given pressure, and the application of these laws will determine the specific gravity, density, and volume at all other pressures and temperatures.

Gay Lussac's experiments. 63. The experiments of Gay Lussac upon this subject are simple, elegant, and satisfactory. His apparatus is as follows:—A chauffer, F, contains burning fuel, by which heat is communicated to B C, a bath of mercury. A spherule A, of thin glass, hermetically closed, contains a given weight of water. G is a glass tube of considerable diameter, filled with pure dry mercury, and inserted in the bath, after which the spherule, A, containing the water, is allowed to ascend to the top of the mercury, and is then broken by concussion, so that a given quantity of water is thus placed in the Torricellian vacuum at the top of the mercury. By the fuel in F heat is then communicated upwards, by the fluids, to the whole apparatus, and to the water in the summit of the tube G; and the mercury descends until the whole of the water is converted into steam, after which it ceases to descend in the same rapid proportion to the increase of temperature. This change shows that the whole of the water is evaporated, and the heat must again be allowed gradually to diminish, until the depression of the mercury corresponds to the temperature indicated in our table of Elastic

Force. The capacity of the tube, G, is shown by divisions on its surface previously fixed, and the height of the mercurial column by a graduated rule and vernier r r, supported on the edge of the bath. The thermometers, h h, Denon indicate the temperature of the fluids.

By means of this apparatus, Gay Lussac has determined the specific gravity of steam, to be .625, air being 1000; that is to say, steam from boiling water is lighter than common air in the proportion of 5. to 8.

64. Dr Dalton's recent experiments make the weight of a Dr. cubic foot of air at 60° = 535.68 grains; therefore a cubic foot of common steam weighs 334.8 grains at 60°, under a pressure of 30 inches of mercury; but as this pressure would convert it into water, the true weight will be found, by the law of Mariotte, thus:

the true weight, in grains, of a cubic foot of steam at 60°, and under the former pressure due to its own elasticity in vacuo; but if we wish to know the weight of a cubic foot of steam at 212°, we must use the law of Gay Lussac and Dalton, thus:

254.3 grains is, therefore, the weight of a cubic foot of steam, as it passes off from water boiling in the air at 212°.

But the weight of one cubic inch of water at 60° is 253 grains; therefore, the weight of a cubic inch of water at 60° is almost exactly equal to one cubic foot, or 1728 cubic inches, of steam.

Hence we find, that the particles of water, when they form steam, are so much repelled by their spheres of caloric, as to be kept at twelve times their original distance from each other; that, in this gaseous state, water is 1728 times rarer than when liquid; and that one gallon of water, with the requisite supply of caloric, will make 1728 gallons of steam.

65. The source from which caloric is obtained for the conversion of water into steam, is either the heat of the sun, the central heat of the earth, or of artificial fires. It is upon the intensity and quantity of this heat that the elastic force, temperature, density, and volume of the steam obtained for any particular purpose must depend; and it is therefore an important point to determine how it is to be obtained.

The most important and common sources of heat for the production of steam, are the combustion of coal, charcoal, wood, resin, and oil. Many experiments have been made upon the quantities of caloric given out during their combustion; but the results vary much with the methods of applying the heat. The six following are some of the results of Dr Dalton's experiments; the rest are selected from the best authorities:

The numbers in the last column represent the number of pounds of water at 32°, which will be heated to 212°, when the fuel is applied in the most economical manner; and hence the quantity of fuel to heat any other quantity

of water any number of degrees, can be found by the common arithmetical rules of proportion.

The quantity of water at 212°, which will be converted into steam, may be found, by dividing the number of pounds of water in the table by 5.55. Thus, from the table—

1 lb. of Newcastle coal gives 180° to 55.5 lbs. of water. Therefore, 1 lb. of Newcastle } 55.5 = 10 lbs. of water.
coal converts into steam, } 5.55

This is to be taken as the effect that may be produced if there be no material loss of heat; and in the Cornish engines I find that even 10.5 lbs. are actually accomplished.

In general, however, for the purposes of ordinary manufactures, in Lancashire, Staffordshire, and the vicinity of London, it appears that not more than 6.6 lbs. of water are converted into steam by one pound of coal; so that not more than 33.3 lbs. of water are heated with ordinary boilers from 32° to 212°. The following table may be taken as the numbers usually given to represent the actual state of practice. But a late investigation by Mr Parkes shows, that in the best constructions of boilers now used in Cornwall, Warwickshire, and elsewhere, these effects are nearly doubled:

1 lb. of the best coal is generally required to heat
33.3 lbs. of water from 32° to 212°. 1 lb. .... 6.6 lbs. of water at 212° into steam.
and 1 lb. .... 5.5 lbs. of water at 32° into steam.
2 lbs. nearly ..... one cubic foot of water from 32° to 212

11 lbs. nearly ..... one cubic foot of water at 212° into steam.

13 lbs. nearly ..... one cubic foot of water at 32° into steam.

Now, as a gallon contains ten pounds of water, it follows that

1 lb. of coal will raise 3½ gallons of water from 32° to the boiling point.

5 lbs. of coal will convert 3½ gallons of water at 212° into steam.

6 lbs. of coal will convert 3½ gallons of water at 32° into steam.

We have given these approximate numbers for practical use, in the application of steam to some of the ordinary purposes and processes of art and domestic use, upon which we are about to enter; and they are such as may, with very ordinary care, be safely calculated on. But for a full exposition of the processes, and principles, and mechanical arrangements connected with the best methods of generating steam from fuel, we must refer to the article "STEAM-ENGINE," where the generation and condensation of steam find their most important uses.

It may perhaps be proper to remark, that a boiler, which is there called a boiler of one, two, or three horses' power, is one which is capable of raising one, two, or three cubic feet of water into steam in an hour. Whatever, therefore, be the application for which steam is wanted, if twenty cubic feet of water per hour are required to be converted into steam, a twenty horse-power boiler is that

Dr Dalton's Table of the Density of Air and Steam.

Gay Lussac's Table of the Density and Volume of Steam.

Steam. which must be procured for the purpose—and of course from 220 to 260 pounds of the best coal will be consumed in that time.

SECTION V.—THE APPLICATION OF OUR KNOWLEDGE OF THE PROPERTIES, PHENOMENA, AND LAWS OF STEAM, TO PRACTICAL AND ECONOMICAL PURPOSES.

Application of Steam. 1. Warming apartments and buildings by steam. 2. Heating greenhouses &c., by steam. 3. Evaporating solutions, drying fabrics, paper, gunpowder, grain, &c., by steam. 4. Warming baths, boiling liquids, and distilling by steam. 5. Preparation and economy of wholesome food by steam. 6. The steam-engine.

1. Warming Apartments by Steam.

Warming by Steam. 66. One of the most important applications of steam in the economy of fuel, is its employment as a vehicle for transferring to a distance, and distributing uniformly, the heat of a fire for the purpose of warming an apartment or building. Its great efficiency for this purpose arises from the largeness of its capacity for caloric, because, as it holds a quantity of caloric equal to 1000 degrees, it will communicate as much heat as a mass of red hot iron; and it will have this advantage over the iron, that it can carry this heat to a distance without a similar loss; because, the heat being latent, will not be given out until it arrive at its destination and become condensed, when the whole of its 1000° will be usefully applied.

The manner in which warming by steam is to be effected, is this. At a convenient part of the building, and as low as possible, there is to be placed a close steam boiler of the ordinary construction. From this boiler a small steam pipe is to be carried to the part of the building which is to be warmed. This small pipe should be pretty thick, and carefully rolled round with a fillet of flannel to a quarter of an inch thick, and the boiler should be wholly covered with bricks and plastered over to keep it warm. This smaller steam pipe should have an area of one square inch for every six gallons of water that the boiler can boil off in an hour. Pipes of a larger size are to be laid round the room above the floor, or under the floor if apertures be left to allow a free circulation of warmed air to enter the room; but the best method we have seen is, to make the surbase, which passes round the room of thin iron plate or copper having the external figure of the surbase, and sufficiently strong to withstand the pressure of the steam, which strong tin plate or copper of $1\backslash frac\{1\}\{2\}$ lb. to the foot will sufficiently effect, if the surbase be not more than about 4 inches square. Into these larger pipes the steam is to be conducted, and in them the steam will be condensed into water, and will give 1000° of heat to the colder air of the room which is in contact with the outside of these pipes. In doing so the steam being condensed into water, small pipes of lead or tin must be provided, for the purpose of bringing back this condensed water into the boiler; and, in order that they may act well, care is to be taken that a gentle slope, of about an inch in 20 feet, be given to all the pipes. The condensed water being thus conducted back to the bottom of the boiler, it will there be replenished with heat, and in the form of steam will again carry up its supply of 1000° to the apartment, again to be given off as formerly to the room, and then returning once more to the boiler, a continual circulation of the same particles of water, giving out in each circuit a quantity of heat equal to red hot iron, is uniformly and gently imparted to and diffused equally over the apartment. The pipe which brings the steam from the boiler may be called the feeding pipe, the pipes which give off the heat the radiating pipes, and the pipes which lead back the water to the boiler the return pipes. We have already given dimensions for the feed

pipe. The return pipes need not be more than $\backslash frac\{1\}\{4\}$ of the diameter of the feed pipe, but an increase of size could do little harm, and may have the effect of preventing accidental obstruction: the boiler will require to have a by-pipe of water added now and then to supply accidental waste; and a safety-valve on the boiler is indispensable. A self-regulating feeder, such as that mentioned in the article STEAM-ENGINE, among the apparatus of boilers, is also to be recommended where it can be readily attained. It is necessary, however, to give directions at greater length for the dimensions of the warming or radiating pipes, as it is upon their proper construction and arrangement that the efficiency of the apparatus entirely depends; and the apparatus has frequently failed from the want of proper precaution. The radiating pipes in the room are generally too small. It is their extent of surface, and the free circulation of air round them, which determines how much of the heat will be given out, and how rapidly. From very accurate experiments I am induced to conclude, that a room containing 500 cubic feet of air, and exposing 400 feet of surface, may be maintained at a temperature of 20° above that of the air without—that is to say, at 60° in the inside of the room when the atmosphere is at 40° without—for a space of twelve hours, by the evaporation of 2 gallons of water, and at the expense of about three pounds of coal of the value of one farthing. But this supposes that there is no ventilation, and that the air of the room is never changed; whereas, the presence of one individual would render it necessary to introduce nearly 400 cubic feet of external air every hour. Now, the heat of 20° given to 400 cubic feet of air would require the evaporation of 3 gallons of water; and, therefore, the evaporation of 3 gallons of water would be required for such a room, and 3 gallons for every person in it, if properly ventilated, and for every 2 gallons there should be at least one square foot of radiating surface; so that such a room, occupied by one person, would require a surface of warming pipe equal to $2\backslash frac\{1\}\{2\}$ square feet, and so on for every such room and occupant, for a space of 12 hours in the day.

Thus, the evaporation of 1 gallon per day for every 400 feet of surface, with a difference of temperature of 20° from the external air, and $1\backslash frac\{1\}\{2\}$ gallons per day for each person, and 1 square foot of radiating surface, is a standard from which we easily calculate.

A room 30 feet long, 20 feet wide, and 10 feet high, has a surface of 2200 feet, which would require $5\backslash frac\{1\}\{2\}$ gallons; six people would require 9 gallons; therefore, $14\backslash frac\{1\}\{2\}$ gallons of water and $7\backslash frac\{1\}\{2\}$ feet of radiating surface will heat the well-ventilated room 12 hours for 6 persons at an expense of 25 lbs. of coal, or about threepence per day; or a whole house, occupied by 6 persons, may be warmed, if 30 feet high, 30 feet wide, and 30 feet deep, at tenpence a-day, the price of coals being twenty shillings a-ton.

It is scarcely necessary to add, that the radiating pipes may be best constructed of thin copper, and ought to be roughened and blackened on the outside.

In the same way the calculation may be made for any other room, building, and number of occupants.

For more extensive and minute information on the subject of Warming, the reader is requested to consult the article "WARMING AND VENTILATION," in the Encyclopedia.

The form in which the radiating surface may be distributed admits of variety.

Provision must be made for the expansion and contraction of the pipes.

The arrangement of steam in the apartment to be heated is of some consequence. It is, we have already stated, sufficiently out of the way in the surbase, but, in that case, much heat passes out into the walls and wood. It may stand on the hearth like a stove, and consist

of concentric cylindric chests within each other (Fig. 24), filled with steam, and allowing air to ascend by the sides and through the intervening spaces, so as to be made warm, and the outer surface radiating directly on the surface exposed. It may be tubular, and coiled in a large quantity in a small box fitted up externally in the appearance of a cabinet or pedestal, thus: (Figs. 25 and 26); the feed pipe commencing at the top of the coils, and the return pipe passing off from the bottom. As the conducting power of tin is nearly equal to that of iron, a quantity of tin pipe will suffice, and be more economical than iron or copper. This coil may be placed in a cabinet or pedestal of the form of Fig. 26, and the warm air will have free egress through the wire work of the panel.

The next diagram (Fig. 27.) shows an arrangement of copper steam-vessels, by which an extensive surface is very efficiently exposed to the air, the condensed water being drawn off at the bottom.

There is one case in which warming by steam may be employed with especial advantage, and where it is frequently neglected—where the power of steam is already employed to drive machinery. Let the engine employed be what is called high-pressure, or non-condensing, in which the steam escapes from the engine and is passed off into the air; and, instead of the common plan, let the steam from the engine be conveyed in pipes through the apartments to be warmed, and let the diameter of the pipe

gradually increase towards the end of its circuit, and finally terminate in a hot-water pipe, which may also circulate in the building and there will be given out the whole original heat of the steam after having done its work in the steam-engine, and that as effectually as if there had been no steam-engine at all, and the whole power of the engine will thus be clear saving. This will be the case to a still greater extent if the steam-engine work expansively, and may further be increased if the pipes be so formed as to constitute an aerial condenser. For further information on this subject see article STEAM-ENGINE.

2. Warming Hothouses, Greenhouses, &c. by Steam.

67. The principles which regulate this application of Green-steam are similar to those mentioned already in Art. 66, houses, &c. and steam possesses the same advantages in the distribution of heat for this purpose, which it does in the cases already mentioned. The warmth thus distributed is freed from those risks of injury to the vitality of the plants, which accompany the old method of warming by hot air flues, in which a contaminated and unwholesomely dry air and unequal temperature were inevitably produced, and an occasional annoyance from smoke. The warmth given out by the steam is of uniform intensity throughout the whole length of the glass; it occupies very small space—one furnace and chimney is all that is required for any extent of range of glasshouses, as the steam may be conveyed to any usual distance in well swathed pipes without sensible loss. The saving thus effected by the concentration of the fire, and by its equable distribution, has been found to produce an economy of more than one-third of the fuel commonly used. At Sion House, the seat of the Duke of Northumberland, there are nearly a thousand feet in length of glasshouses heated by one such apparatus. The boiler and chimney may also be placed at a convenient distance from the houses—a circumstance which contributes much to the beauty of this arrangement.

Those who wish to study the details of this subject are referred to Mr Loudon's Horticultural Works, and to the article "HORTICULTURE," in this Encyclopedia. The following are the mechanical principles and arrangements that belong exclusively to this article.

Our first subject of enquiry, is into the amount of heat requisite to sustain the glass at a given temperature higher than that of the external air; if we take the temperature of the atmosphere at 35°, and that of the hot-house at 65°, giving 30° of difference, we shall have a case approaching near to that of a glasshouse in winter. In order to determine this question, which can only be ascertained by experiment, the author has examined a case upon a large scale, which may furnish a standard of comparison.

Steam. The large palm house of the Botanic Garden of Edin-
burgh is an octagonal structure, 60 feet in diameter, and
45 feet high. Excepting stone pillars at the angles, and
Green- between the windows, of about three feet wide, the whole
houses, &c. building is glass, presenting a surface of almost exactly
warmed by 5000 square feet of glass. The quantity of fuel required
Steam. in a cold atmosphere, having a mean of 35°, is 1344
pounds for 24 hours; being at the rate of 269 lbs. of coal
for every 1000 feet of glass in a day, or 11.2 pounds an
hour for 1000 feet, or .0112 of a pound per hour for each
foot.

To confirm this observation, it was thought proper to
examine another house of different dimensions. The
eastern wing of the great range of houses is warmed by a
fire, which consumes half that quantity of coals, of about
half the value, being dross of a bad quality; so that its
consumption is about $\backslash frac\{1\}\{2\}$ of the other in real value, being
about 672 lbs. of dross, equivalent to 336 lbs. of good coal
in 24 hours. Now, the exposed glass of this wing amounts
to about 1376 square feet, being at the rate of 243 lbs. to
1000 feet—a result which is sufficiently near to the other,
to allow us to assume 250 or 260 lbs. of fuel in 24 hours
for each thousand feet, as a standard of tolerable accuracy
in such cases. It may be useful to add, that these houses
are in tolerably sheltered situations, and that the glass
faces in every direction, so as to be acted on with toler-
ably uniformity.

Hence we have the following results:

Temperature of the air 35°—temperature of the hot-
house 65°.

Heat sustained 24 hours, by 250 lbs. of good coal, for
1000 square feet of glass.

Heat sustained 1 hour, by 10.4 lbs. of good coal for 1000
feet of glass.

Heat sustained 1 hour, by 0.0104 lbs. of good coal, for
one foot of glass.

To find what amount of steam will be required to warm
such a house, we have only to apply the calculations of
Art. 66; 10.4 lbs. of good coal will convert seven gallons
of water into steam; therefore, a boiler of one horse's
power is necessary to evaporate a sufficient quantity of
water for the supply of steam for each 1000 feet of glass;
that is to say, for the palm-house alone, a boiler of five
horses' power would be required to furnish steam; and
this supposes the hot water of the condensed steam to be
returned into the boiler immediately; and if this were not
the case, six horses' power would be the size of boiler
ordered for this purpose: hence—

To warm a hothouse by steam, there is required the
boiler of a steam-engine, reckoned at one horse's power for
every thousand feet of glass.

The method of distributing the heat through the rooms
of the hothouse, is not a matter of so nice calculation as
in a common apartment. There is much greater convenience
for this purpose in a greenhouse than a common
room, on account of the necessary vacuities under the
ranges and beds. In general, a single circuit of steam-pipe
four inches in diameter, round the apartment, with a re-
turn pipe of equal dimensions laid parallel to it, is sufficient.

It is, however, of great importance to provide a remedy
for one of the practical inconveniences of steam. When
the fire is not very carefully tended, as during the night,
the steam in the boiler falls below the proper point, and
the supply instantly ceases. This is remedied by the
following method. Cast iron boxes, a foot square and
five or six feet deep, are filled with stones, and ranged
round the forcing rooms—a pipe passing into each of them
communicates a supply of steam to them, and the stones
they contain. The caloric entering first of all with rapid-
ity into the stones, is afterwards given out gradually by
them to the house; and the advantage of this arrangement
is, that even if neglect or accident were to occasion a tem-

porary cessation of the supply, this heat would still continue
to be given out from the matter of the boxes for several hours
until the defect might be remedied. These boxes of stones
perform the same function for caloric that a fly-wheel
does for mechanical power—absorbing it when in excess,
and giving it out again when deficient.

It is a valuable hint to economy, which Mr Macnab
has put in execution in his houses at the Botanical Gar-
den, that the boiler flues should be extended to the green-
house after they have left the boiler; the remaining heat
is thus given out to the hothouse, and the last degree of
saving accomplished. With this arrangement a smaller
boiler will suffice, but it will not always be convenient;
neither can a greater length than about 30 feet of flue be
advantageously used in this way.

3. On Evaporating and Drying Solutions, Cloths,
Paper, Grain, Gunpowder, &c., by Steam.

68. We have already observed how well the peculiar-
ities of steam enables us to make use of it as a vehicle
for the collection, transference, and distribution of heat.
In addition to the facility with which it may be carried
to a distance, and the uniformity of temperature resulting
from it, we have this further adaptation to the purposes
now under consideration, that the temperature can at no
time become so great as to produce injury, or deteriorate
the substances to which it is applied. Hence it follows
that thickened liquids, strong solutions, and any porous
solid matter impregnated with fluid may be evaporated,
and wholly separated from the fluid, without incurring
the danger and suffering the deterioration resulting from
direct application of the fire. And, further, by the proper
application of steam, as a conductor of heat, liquids may
be warmed, evaporated, and even boiled in vessels of wood,
which is in some cases, as in brewing and delicate distri-
butions, a matter of much importance.

When any mixture or solution of a solid in water is to
be evaporated by steam, it may be done in some of the
following ways.

(1.) The vessel containing the solution may have
two bottoms, the interval between them being filled
with water and steam, and the solution resting on the
upper one, the fire is applied to the under one; thus the
steam and water intervening between the solution and
the fire, the latter is protected, as well as the vessel
itself, from being burnt when the process has nearly
attained the necessary degree of dryness; and the process
of communicating the heat from the fire to the water
takes place in the following manner; the fire generates
in the water bubbles of steam, which ascend from the
lower to the higher bottom of the vessel, which is in
contact with the solution and acquires its temperature,
and, giving off their heat to the upper bottom, are condensed,
and fall down again to the lower bottom to acquire the
accession necessary to rise, once more, in steam to the
top. This plan has been successfully used in making salt;
and it is necessary to have a safety and an atmospheric
valve attached to the space between the lower and upper
bottoms. The quantity of heat required for the purpose
of evaporating the water of the mixture is neither
increased sensibly, nor diminished by the intervention of
the steam between the bottoms; the number of gallons of
water to be evaporated from the solution will determine
the quantity of heat by Art. 66.

(2.) A second method of producing evaporation is to intro-
duce among the mixture a steam pipe, so as to wind amongst
it either in the form of a helix, like a cork-screw or worm
of a still, or to perform such a circuit as shall expose a
large quantity of surface, with tolerable uniformity, to the
fluid for the absorption of heat from the steam. Copper
is the best material for the tubes, and wooden tanks lined
with lead or tin will contain the mixture. The fuel

required for evaporation will be 5 lbs. for every 3½ gallons of water to be evaporated, and the steam boiler must have one horse's power for every 6 gallons to be evaporated in an hour.

(3.) A third method is the invention of Mr Goodlet of Leith. The substance to be evaporated is forced by means of a pump into a long copper pipe, which enters a close steam boiler, and after winding through it so frequently as to expose a sufficient surface for a sufficient length of time to acquire the necessary supply of heat from the boiler, again passes out from the boiler and discharges its heated contents into an appropriate reservoir, again, if necessary, to be passed once more through the same process by the force pump. In this case the substance is brought to the steam boiler for evaporation instead of having the steam brought to it, and thus any loss of heat during the transit of the steam is prevented.

A steam kiln for drying grain has been used with great success by the same person. The grain is spread out on the iron floor of a large room—this floor is perforated with a multitude of small openings, or formed of a very fine grating; immediately under the floor steam pipes of 6 inches diameter lie parallel to each other at small intervals apart, and radiate heat directly to the floor and the grain, and also to the surrounding air, which in this hot state ascends through the grain; numerous large ventilators being provided for the escape of the vapour thus impregnated with moisture, after it has ascended through the grain. This method has been found effectual, and is attended with less risk of injury than the ordinary one.

In the processes of drying and printing cloths and fabrics of various kinds, rapid and complete drying is of much importance. This is effected principally in what is called a drying frame: this consists of a dozen of tin cylinders, a foot in diameter and 6 or 8 feet long; these cylinders are closed completely by two hemispherical ends, and are placed upon an axis in a frame, so as to revolve in contact with each other. Steam is conducted into all these cylinders by a pipe passing through the axis, which is hollow, and the joint is made steam-tight by a stuffing-box similar to that of the cover of a steam-engine cylinder. A piece of cloth dripping from the dye-vat is passed through the frame once and is then perfectly dry. Of course, the quantity of steam required for this process is proportional to the number of pounds of water to be evaporated from each piece, that is, to the difference between the weight of a piece when wet and when dry. The number of lbs. being ascertained, the fuel and power of the boiler are found from article 66.

In the manufacture of paper the process of drying by steam is beautifully exhibited. The wet pulp, laid out on the web of wire cloth, is gradually strained as it approaches the large hollow cylinders, around which it winds for half a minute and then comes off perfectly dry finished paper, ready for use. This process is minutely described in the article "PAPER" of this Encyclopedia.

4. Warming Baths, Boiling Liquids, Distilling by Steam.

69. To heat water rapidly, and in considerable quantity, by means of a fire placed at some distance, is a problem frequently proposed; and steam for such a case is an excellent vehicle for the heat. Let there be placed in a steam boiler 10 gallons of water, and heated into steam; these 10 gallons conveyed to a reservoir or bath of water at a distance, by a small lead pipe, will heat nearly 55 gallons of water from 32° to the boiling point, or 165 gallons from 40° to 100°, the usual temperature of a warm bath. A bath of the ordinary construction will require about 160 gallons, and the said 10 gallons will require 18 lbs.

of coal, value sixpence. Besides this, the sides of the bath, if made hollow, may be warmed by the introduction of steam between the lining and the outside, at the same time that the water is warmed; and the apartment may further be heated with steam pipes from the same apparatus.

The most effectual method of communicating the heat of steam to water is to pass the open end of the steam pipe from the boiler directly into the water to be heated, so as at once to mix with it. The mouth of the pipe, properly regulated by a stopcock, should enter at the bottom, and be directed from one end of the bath along one side towards the other, and thus the impetus of the steam on entering will communicate to the water a circulation highly conducive to an equable distribution of heat.

A very simple apparatus of this kind may be placed in the same apartment with the bath itself. A boiler 4 feet long and 2 feet deep, with a fire covering a square of 18 inches, will heat such a bath in three quarters of an hour. A copper boiler will be most effective, and the steam pipe should first be matted with bandages of flannel, and then stitched with canvass painted, from the boiler to where it enters the bath.

In establishments where there are many baths, a reservoir at the top of the building may be kept constantly filled with water ready to descend into the baths, the reservoir being supplied with heat from the steam pipe of a boiler placed in the outer buildings, or some other suitable place. It is to be recollected that the boiler used must be what is called a one, two, or three horse power boiler, according as one hundred, two hundred, or three hundred gallons per hour are wanted; and so on for every additional hundred gallons of water at 100° of temperature—one horse power of boiler for one hundred gallons.

The same process may be used for heating and even for boiling a liquid or solution, in which no injury will result from adding the steam of this condensed water to the liquid. In a dye-work, or other work where much boiling and many solutions are used, steam boiling in this manner is a process of great convenience and value. The vessels containing dyes of various kinds, including leys and solutions of various substances, in which cloths, yarns, wools, and various materials are to be boiled, are ranged around a spacious apartment. Around this apartment, attached to the wall, circulates a steam pipe of two inches diameter, from which smaller branch pipes go off to each of the boilers from the nearest point, and pass down to the bottom of their respective vessels. The exit of the steam is governed by a stopcock under the hand of the operator; and by this means he can easily, by allowing a smaller or greater supply of steam to enter the liquid, produce a uniform and gentle simmer or excite an instantaneous and tumultuous ebullition. Two great advantages give this method much superiority over the common mode of boiling by the direct action of the fire. The condensed steam supplies to the solution exactly as much water as is lost by evaporation, so that it remains of the same strength through a protracted process, and there is no injury sustained by allowing the substances immersed to settle down and rest at the bottom.

Where it is not allowable for the caloriferous steam to be condensed in contact with the liquid to be warmed or boiled, we must resort to the method of heating by surface; that is to say, the steam must be conveyed through the mass of liquid by a pipe, or other conductor best fitted to give out the heat or retain the water. A very thin pipe of the purest soft copper is best for this purpose; 2 inches in diameter and $\backslash frac\{1\}\{32\}$ to $\backslash frac\{1\}\{16\}$ of an inch in thickness will be found good dimensions, and a square foot of surface for every 10 gallons of water to be boiled per hour will be required. For some purposes, it will be enough to wind the pipe in a spiral

Steam. round the inside of the vessel to be heated; but if the vessel be large, numerous pipes must pass through the liquid. For boiling 1000 gallons per hour, 200 feet of copper pipe 2 inches in diameter are required.

Boiling Liquids by Steam. In distillation by steam, the same method of communicating heat to the liquid to be distilled is employed as already described in boiling. But the vapours of other liquids, having less specific caloric than water, a smaller quantity of steam from the boiler will be required to evaporate than to evaporate an equal quantity of water; thus the heat of one gallon of water will evaporate and cause to be distilled 2 gallons of alcohol, 3 of sulphuric ether, and 4 of turpentine. It is a curious phenomenon, of which distillers should avail themselves in carrying off the vapour in distillation, that although alcohol floats on water, and ether on alcohol, nevertheless the vapour of water floats above vapour of alcohol, and vapour of alcohol above vapour of ether.

5. Preparation of Food by Steam.

Cooking by Steam. 70. The last of the applications of steam which we shall here examine is that which was historically the first, its application to cooking and other domestic uses. This invention makes its appearance in the following record of the Royal Society of London.

At a meeting of the Council of the Royal Society,
December 8, 1680.

Papin's Digester. Ordered, that a book intitled A New Digester, or Engine for softening Bones, &c., written by Denys Papin, Doctor of Physick, and Fellow of this Society, be printed and published.

CHR. WREN.
This work on the New Digester was accordingly published in 1681; "Containing the description of its make and use in these particulars, viz., Cookery, Voyages at Sea, Confectionary, Making of Drinks, Chymistry and Dyeing, with an Account of the Price a good big Engine will cost, and of the profit it will afford."

The following list will show the extent to which the learned doctor had proceeded in applying steam to the improvement of the dietetic art. It is copied from the Index to the work. (1.) How to know the quantity of pressure in the Digester. (2.) How to know the degree of heat. (3.) How meat may be kept upon the fire three times as long as is necessary to make it ready, and yet it will not be spoiled. (4.) The same experiment made upon bones. (5.) How to boil mutton. (6.) How to boil beef. (7.) How to boil lamb. (8.) How to boil rabbits. (9.) How to boil pigeons. (10.) How to boil fish. (11.) How to boil pulse. (12.) How to make jelly, very cheap. (13.) Glue for glasses. (14.) Harts-horn turned like Parmesan cheese. (15.) A macquerel kept without salt. (16.) Salt water as good for nourishment as fresh water. (17.) To make sweetmeats at a cheap rate, and of a new taste. (18.) To make two sorts of drink with the same fruit. (19.) To make a new sort of wine. (20.) Tinctures drawn in the hundredth part of the time usually required for them. (21.) New ways for distilling. (22.) How to hatch chickens. (23.) How to save the labour of grinding cochineal. (24.) To dye with thick juices. (25.) To make horn and tortoiseshell soft for a great while."

This catalogue of uses of steam we shall shortly run over, as the modern uses of steam for cookery are principally applications of Dr Papin's methods, and as valuable economization in the preparation of food on a large scale has resulted from them, especially in the extraction of

highly nutritious food from bones. A digester on the principle of Dr Papin is used in every modern kitchen.

"Description of the Digester and how to use it safely."

"A A is a brass (or copper) cylinder, hollow within, shut at the bottom and open at the top. B is another cylinder inverted upon it. C C are two appendices or ears cast to the cylinder A A, as the trunnions of a piece of ordnance. D D are two pieces of iron put upon the appendices at one end, and the iron bar E E at the other. F F are two screws, which serve to press both the cylinders A A, B B, against one another. G is another hollow cylinder, made of glass, pewter, or some other materials, fitted to receive those things that are to be included in the cylinders A A, and B B, with water all round it.

"To use this engine with convenience and ease, it ought to be fitted in a furnace built on purpose for it, and should go on as far as the appendices C C; so the fire being underneath, and the screws well fastened, and a piece of moistened paper laid between the cylinders at the joint i i to make it steam tight, you may boil your meat as long as you please without danger of wasting it by the exhalation of the volatile parts.

"To know the quantity of the inward pressure, you must have a little pipe open at both ends as H H: this being soldered to a hole in the cover B B, is to be stopped at the top with a little valve P, exactly ground to it. This must be kept down with an iron rod I M, one end of which must be put into an iron staple M, fastened to the bar E E, and the other end kept down by a weight N to be hung upon it nearer or further from the valve according as you would keep it less or more strong, after the manner of an ordinary Roman balance, or steelyard.

"To know the degree of heat, I hang a weight to a thread about 3 feet long, and I let fall a drop of water into a little cavity made for that purpose at the top of it, and I tell how many times the hanging weight will move to and fro before the drop of water is quite evaporated.

"Experiment. Having filled my pot with a piece of a breast of mutton, and weighed five ounces of coals, I lighted my fire, and by blowing gave such a heat that a drop of water would evaporate in 4 seconds, the inward pressure being about 10 times stronger than the atmosphere: I let the fire go out of itself, and then the mutton was very well done, the bones soft and the juice a strong jelly. So that, having had occasion to boil mutton several times since, I have always observed the same rule, and never have missed to have it in the same condition, which I take to be best of all."

Beef required 7 ounces of coal and the same heat, and the beef was very well boiled, although there were more parts of the bones not quite softened. Lamb, rabbits, and pigeons, mackerel, pike, and eel, were subjected to the same process; whence the doctor infers that the bones of young beasts require almost as much fire as those of old ones to be boiled, that rabbit bones are harder than those of mutton, that tough old rabbits may be made as good as tender young ones by this means, that pigeons may be best boiled with a heat that evaporates a drop of water in 5 seconds, that mackerel was

Fig. 28.

cooked with gooseberries in a digester, the fish being good and firm, and the bones so soft as not to be felt in eating; and he particularly recommends, as an excellent dish cooked in this manner, cod fish and green peas.

The most important of Papin's experiments are those on the extraction of gelatine from bones, as now done on a large scale in France and in this country, as also the manufacture of essence of meat, soups, &c., especially suitable for long sea voyages.

"I took," says he, "beef bones that had never been boiled, but kept dry a long time, and of the hardest part of the leg; these being put into a little glass pot with water, I included in the engine, together with another little glass pot full with bones and water too, but in this the bones were ribs and had been boiled already. Having preste the fire till the drop of water would dry away in three seconds, and ten pressures, I took off the fire; and the vessels being cooled, I found very good jelly in both my pots; but that which had been made out of ribs had a kind of a reddish colour, which I believe might proceed from the medullary part, the other jelly was without colour like hartshorn jelly; and I may say, that having seasoned it with sugar and juice of lemon, I did eat it with as much pleasure, and found it as stomachical, as if it had been jelly of hartshorn. Mutton bones are better than beef bones; and he infers (1.) that one pound of beef bones afford about two lbs. of jelly; (2.) that it is the cement (gelatine) that unites the parts of the bones, which is dissolved in the water to make it a jelly, since after that, the bones remain brittle; (3.) that few glutinous parts are sufficient to congeal much water, for I found that when the jelly was dried, I had very little glue-ten? remaining; (4.) I used it to glue a broken glass, which did since that time hold very well, and can be washed as well as if it had never been broken; (5.) it is heavier than water, and sinks to the bottom; (6.) hartshorn produces five times its weight of jelly.

"From all these experiments, I think it very likely, that if people would be persuaded to lay by bones, gristles, tendons, feet, and other parts of animals that are solid enough to be kept without salt, whereof people throw away more than would be necessary to supply all the ships that England hath at sea, the ships might always be furnished with better and cheaper victuals than they used to have. And I may say, that such victuals would take up less room too, because they have a great deal more nourishment in them in proportion to their weight. They would also be more wholesome than salt meat. Vegetables, such as dried peas, may also be cooked by the steam of salt water without becoming salt."

We have entered thus fully upon the work of Doctor Denys Papin, and the properties of his digester for cooking, and extracting jellies by high-pressure steam, because it contains nearly all that is at present practised in the preparation of food by steam.

If to what has been already stated, we add, that if the steam of salt water be collected in a vessel kept cold on the outside, the condensed water will not be impregnated with salt, and may be used as food, the importance of steam in the economical and menial capacity of cook, will be sufficiently apparent. The supply of water to the crew of a steam vessel may be obtained in this manner, and an apparatus for thus procuring fresh water from the condensation of steam from salt water, has been used with advantage in ordinary ships.

Fig. 29 contains the steam-cooking apparatus used in modern kitchens; a a is a portion of the kitchen fireplace. In one of the divisions of it, b, is placed a steam boiler, furnished with the usual apparatus of feeding pipes, gauge cocks, &c. From this boiler a steam pipe, c c, is led along the back of the cooking table d d, and at certain

intervals, branch pipes, furnished each with a stopcock, project across the table at right angles to the main pipe.

The extremities of these branch pipes are conical, and made accurately to fit into conical sockets inserted into the cooking pans, one of which, e, is seen in its place on the table. These pans have each a double bottom, the lower one close, the upper one perforated; between the bottoms the socket before mentioned, through which the steam enters, is inserted. The manner of using this apparatus is simple. The article to be cooked is laid in its place on the perforated bottom of the pan, the lid is applied, and the pan is joined to one or other of the branch pipes, by its socket receiving the conical end of the pipe; the stopcock is now turned, and the matter in the pan is subjected to the action of the steam. Each pan has a crane in front, to allow of the condensed steam being drawn off.

The remaining part of the apparatus is the hot closet, f f. This consists of a steam-tight iron box, containing shelves, inserted in another iron box of dimensions so much greater as to allow of a considerable vacuity being between them; into this vacuity the steam from the boiler is permitted to flow, and give out its heat to the articles placed in the closet to receive it.

Fig. 30 contains a steam-apparatus for cooking the