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, &c. It is distinguished from smoke by its not having been produced by combustion, by not containing any foot, and by its being condensible by cold into water, oil, inflammable spirits, or liquids composed of these.

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 other juices which seem to have produced it, and the steam seems to be completely soluble in air, as salt is in water, compoing while thus united a transparent elastic fluid.

But in order to its appearance in the form of an opaque white cloud, the mixture with or dissemination in air seems absolutely 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 transparent at its first emission. This is rendered still more evident by fitting to the spout of the tea-kettle 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 through the whole length 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 be quite transparent. The visibility therefore of the matter which constitutes the steam is an accidental or extraneous circumstance, and requires the admixture with air; yet this quality again leaves it when united with air by solution. It appears therefore to require a differentiation in the air. The appearances are quite agreeable to this notion: for 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.

In the mean time, as 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 spout of the tea-kettle be surrounded with cold water, 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.

This is evidently the common process for distilling; and the whole appearances may be explained by saying, that the water is converted by heat into an elastic vapour, and that this, meeting with colder air, imparts to it the heat which it carried off as it arose from the heated water, and being deprived of its heat it is again water. The particles of this water being vastly more remote from each other than when they were in the tea-kettle, and thus being disseminated in the air, become visible, by reflecting light from their anterior and posterior surfaces, in the same manner as a transparent salt becomes visible when reduced to a fine powder. This disseminated water being presented to the air in a very extended surface, is quickly dissolved by it, as pounded salt is in water, and again becomes a transparent fluid, but of a different nature from what it was before, being no longer convertible into water by depriving it of its heat.

Accordingly this opinion, or something very like it, has been long entertained. Mutschchenbroeck expressly says, that the water in the form of vapour carries off with it all the heat which is continually thrown in by the fuel. But Dr Black was the first who attended minutely to the whole phenomena, and enabled us to form distinct notions of the subject. He had discovered that it was not sufficient for converting ice into water that it be raised to that temperature in which it can no longer remain in the form of ice. A piece of ice of the temperature \(32^\circ\) of Fahrenheit's thermometer will remain a very long while in air of the temperature \(50^\circ\) before it be all melted, remaining all the while of the temperature \(32^\circ\), and therefore continually absorbing heat from the surrounding air. By comparing the time in which the ice had its temperature changed from \(280^\circ\) to \(32^\circ\) with the subsequent time of its complete liquefaction, he found that it absorbed about 130 or 140 times as much heat as would raise its temperature one degree; and he found that one pound of ice, when mixed with one pound of water \(140\) degrees warmer, was just melted, but without rising in its temperature above \(32^\circ\). Hence he justly concluded, that water differed from ice of the same temperature by containing, as a constituent ingredient, a great quantity of fire, or of the cause of heat, united with it in such a way as not to quit it for another colder body, and therefore so as not to go into the liquor of the thermometer and expand it. Considered therefore as the possible cause of heat, it was latent, which Dr Black expressed by the abbreviated term LATENT HEAT. If any more heat was added to the water it was not latent, but would readily quit it for the thermometer, and, by expanding the thermometer, would show what is the degree of this redundant heat, while fluidity alone is the indication of the combined and latent heat.

Dr Black, in like manner, concluded, that in order to convert water into an elastic vapour, it was necessary, not only to increase its uncombined heat till its temperature is \(212^\circ\), in which state it is just ready to become elastic; but also to pour into it a great quantity of fire, or the cause of heat, which combines with every particle of it, so as to make it repel, or to recede from, its adjoining particles, and thus to make it a particle of an elastic fluid. He supposed that this additional heat might be combined with it so as not to quit it for the thermometer; and therefore so as to be in a latent state, having elastic fluidity for its sole indication.

This opinion was very consistent with the phenomenon of boiling off a quantity of water. The application of heat to it causes it gradually to rise in its temperature till it reaches the temperature \(212^\circ\). It then begins to send off elastic vapour, and is slowly expended in this way, continuing all the while of the same heat which it absorbs. The steam also is of no higher temperature, as appears by holding a thermometer in it. We must conclude that this steam contains all the heat which is expended in its formation. Accordingly the scalding power of steam is well known; but it is extremely difficult to obtain precise measures of the quantity of heat absorbed by water during its conversion into steam. Dr Black endeavoured to ascertain this point, by comparing the time of raising its temperature a certain number of degrees with the time of boiling it off by the same external heat; and he found that the heat latent in steam, which balanced the pressure of the atmosphere, was not less than 800 degrees. He also directed Dr Irvine of Glasgow to the form of an experiment for measuring the heat actually extracted from such steam during its condensation in the refrigeratory of a still, which was found to be not less than 774 degrees. Dr Black was afterwards informed by Mr Watt, that a course of experiments, which he had made in each of these ways with great precision, determined the latent heat of steam under the ordinary pressure of the atmosphere to be about 948 or 950 degrees. Mr Watt also found that water would distil with great ease in vacuo when of the temperature 70°; and that in this case the latent heat of the steam is not less than 1200 or 1300 degrees; and a train of experiments, which he had made by distilling in different temperatures, made him conclude that the sum of the sensible and latent heats is a constant quantity. This is a curious and not an improbable circumstance; but we have no information of the particulars of these experiments. The conclusion evidently presupposes a knowledge of that particular temperature in which the water has no heat; but this is a point which is still sub judice.

This conversion of liquids (for it is not confined to water, but obtains also in ardent spirits, oils, mercury, &c.) is the cause of their boiling. The heat is applied to the bottom and sides of the vessel, and gradually accumulates in the fluid, in a sensible state, uncombined, and ready to quit it and to enter into any body that is colder, and to diffuse itself between them. Thus it enters into the fluid of a thermometer, expands it, and thus gives us the indication of the degree in which it has been accumulated in the water; for the thermometer swells as long as it continues to absorb sensible heat from the water: and when the sensible heat in both is in equilibrium, in a proportion depending on the nature of the two fluids, the thermometer rises no more, because it absorbs no more heat or fire from the water; for the particles of water which are in immediate contact with the bottom, are now (by this gradual expansion of liquidity) at such distance from each other, that their laws of attraction for each other and for heat are totally changed. Each particle either no longer attracts, or perhaps it repels its adjoining particle, and now accumulates round itself a great number of the particles of heat, and forms a particle of elastic fluid, so related to the adjoining new formed particles, as to repel them to a distance at least a hundred times greater than their distances in the state of water. Thus a mass of elastic vapour of sensible magnitude is formed. Being at least ten thousand times lighter than an equal bulk of water, it must rise up through it, as a cork would do, in form of a transparent ball or bubble, and getting to the top, it dissipates, filling the upper part of the vessel with vapour or steam. Thus, by tossing the liquid into bubbles, which are produced all over the bottom and sides of the vessel, it produces the phenomenon of ebullition or boiling. Observe, that during its passage up through the water, it is not changed or condensed; for the surrounding water is already so hot that the sensible or uncombined heat in it, is in equilibrium with that in the vapour, and therefore it is not disposed to absorb any of that heat which is combined as an ingredient of this vapour, and gives it its elasticity. For this reason, it happens that water will not boil till its whole mass be heated up to 212°; for if the upper part be colder, it robs the rising bubble of that heat which is necessary for its elasticity, so that it immediately collapses again, and the surface of the water remains still. This may be perceived by holding water in a Florence flask over a lamp or chuffer. It will be observed, some time before the real ebullition, that some bubbles are formed at the bottom, and get up a very little way, and then disappear. The distances which they reach before collapsing increase as the water continues to warm farther up the mass, till at last it breaks out into boiling. If the handle of a tea-kettle be grasped with the hand, a tremor will be felt for some little time before boiling, arising from the little succussions which are produced by the collapsing of the bubbles of vapour. This is much more violent, and is really a remarkable phenomenon, 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 this succussion. The great resemblance of this tremor to the feeling which we have during the shock of an earthquake has led many to suppose that these last are produced in the same way, and their hypothesis, notwithstanding the objections which we have elsewhere stated to it, is by no means unfavourable.

It is owing to a similar cause that violent thumps are sometimes felt on the bottom of a tea-kettle, especially one which has been long in use. Such are frequently the brown crusts on the bottom with a stony concretion. This sometimes is detached in little scales. When one of these is adhering by one end to the bottom, the water gets between them in a thin film. Hence it may be heated considerably above the boiling temperature, and it suddenly rises up in a large bubble, which collapses immediately. A smooth thilling lying on the bottom will produce this appearance very violently, or a thimble with the mouth down.

In order to make water boil, the fire must be applied to the bottom or sides of the vessel. If the not boil until heat be applied at the top of the water, it will waste the fire away without boiling; for the very superficial particles are first supplied with the heat necessary for rendering them elastic, and they fly off without agitating the rest (A).

Since this disengagement of vapour is the effect of its

(A) We explained the opaque and cloudy appearance of steam, by saying that the vapour is condensed by coming into contact with the cooler air. There is something in the form of this cloud which is very inexplicable. The particles of it are sometimes very distinguishable by the eye; but they have not the smart star-like brilliancy of very small drops of water, but give the fainter reflection of a very thin film or vehicle like a soap bubble. If we attend also to their motion, we see them descending very slowly in comparison with the descent of a solid drop; and this vesicular constitution is established beyond a doubt by looking at a candle through a cloud of steam. It is seen surrounded by a faint halo with prismatical colours, precisely such as we can demonstrate by optical laws to belong to a collection of vehicles, but totally different from the halo which would be produced by a collection of solid drops. It is very difficult to conceive how these vehicles can be formed of watery particles, each of which was far- its elasticity, and since this elasticity is a determined force when the temperature is given, it follows, that fluids cannot boil till the elasticity of the vapour overcomes the preasure of the incumbent fluid and of the atmosphere. Therefore, when this preasure is removed or diminished, the fluids must sooner overcome what remains, and boil at a lower temperature. Accordingly it is observed that water will boil in an exhausted receiver when of the heat of the human body. If two glass balls A and B (fig. 1.) be connected by a flender tube, and one of them A be filled with water (a small opening or pipe b being left at top of the other), and this be made to boil, the vapour produced from 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 that the pipe by sticking it into a piece of tallow or bees-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, will cause it immediately to close and seal hermetically. We now have a pretty instrument or toy called 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 in it. When the water in A has thus been diffused, 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 and b cut in the pane, in such a situation that when A is full of water and preponderates, B is opposite to the hole b. Whenever the room became sufficiently warm, the vapour was formed in A, and immediately drove 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; and this amusement continued as long as the room was warm enough.

We know that liquors differ exceedingly in the temperatures necessary for their ebullition. This forms the great chemical distinction between volatile and fixed bodies. But the difference of temperature in which they boil, or are converted into permanently elastic vapour, under the preasure of the atmosphere, is not a certain measure of their differences of volatility. The natural boiling point of a body is that in which it will be converted into elastic vapour under no pressure, or in vacuo. The boiling point in the open air depends on the law of the elasticity of the vapour in relation to its heat. A fluid A may be less volatile, that is, may require more heat to make it boil in vacuo, than a fluid B: But if the elasticity of the vapour of A be more increased by an increase of temperature than that of the vapour of B, A may boil at as low, or even at a lower temperature, in the open air, than B does; for the increased elasticity of the vapour of A may sooner overcome the preasure of the atmosphere. Few experiments have been made on the relation between the temperature and the elasticity of different vapours. So long ago as the year 1765, we had occasion to examine the boiling points of all such liquors as we could manage in an air pump; that is, such as did not produce vapours which destroyed the valves and the leathers of the pistons: and we thought that the experiments gave us reason to conclude, that the elasticity of all the vapours was affected by heat nearly in the same degree. For we found that the difference between their boiling points in the air and in vacuo was nearly the same in all, namely, about 120 degrees of Fahrenheit's thermometer. It is exceedingly difficult to make experiments of this kind: The vapours are so condensible, and change their elasticity so prodigiously by a trifling change of temperature, that it is almost impossible to examine this point with precision. It is, however, as we shall see by and bye, a subject of considerable practical importance in the mechanic arts; and an accurate knowledge of the relation would be of great use also to the distiller: and it would be no less important to discover the relation of their elasticity and density, by examining their compressibility, in the same manner as we have ascertained the relation in the case of what we call aerial fluids, that is, such as we have never observed in the form of liquids or solids, except in consequence of their union with each other or with other bodies. In the article Pneumatics we took notice of it as something like a natural law, that all these airs, or gases as they are now called, had their elasticity very nearly, if not exactly proportional to their density. This appears from the experiments of Achard, of Fontana, and others, on vital air, inflammable air, fixed air, and some others. It gives us some presumption to suppose that it holds in all elastic vapours whatever, and that it is connected with their elasticity; and it renders it somewhat probable that they are all elastic, only because the cause of heat (the matter of fire if you will) is elastic, and that their law of elasticity, in respect of density, is the same with that of fire. But it must be

rounded with many particles of fire, now communicated to the air, and how each of these vehicles shall include within it a ball of air; but we cannot refuse the fact. We know, that if, while linseed oil is boiling or nearly boiling, the surface be obliquely struck with the ladle, it will be dashed into a prodigious number of exceedingly small vehicles, which will float about in the air for a long while. M. Sauffure was (we think) the first who distinctly observed this vesicular form of mists and clouds; and he makes considerable use of it in explaining several phenomena of the atmosphere. Steam. be observed, that although we thus assign the elasticity of fire as the immediate cause of the elasticity of vapour in the same way, and on the same grounds, that we ascribe the fluidity of brine to the fluidity of the water which holds the solid salt in solution, it does not follow that this is owing, as is commonly supposed, to a repulsion or tendency to recede from each other exerted by the particles of fire. We are as much entitled to infer a repulsion of unlimited extent between the particles of water; for we see that by its means a single particle of sea salt becomes disseminated through the whole of a very large vessel. If water had not been a visible and palpable substance, and the salt only had been visible and palpable, we might have formed a similar notion of chemical solution. But we, on the contrary, have considered the quaquaversum motion or expansion of the salt as a discrimination among the particles of water; and we have ascribed it to the strong attraction of the atoms of salt for the atoms of water, and the attraction of these last for each other, thinking that each atom of salt accumulates round itself a multitude of watery atoms, and by so doing must recede from the other saline atoms. Nay, we farther see, that by forces which we naturally consider as attractions, an expansion may be produced of the whole mass, which will act against external mechanical forces. It is thus that wood swells with almost insuperable force by imbibing moisture; it is thus that a sponge immersed in water becomes really an elastic compressible body; resembling a blown bladder; and there are appearances which warrant us to apply this mode of conception to elastic fluids.—When air is suddenly compressed, a thermometer included in it shows a rise of temperature; that is, an appearance of heat now redundant which was formerly combined. The heat seems to be squeezed out as the water from the sponge.

Accordingly this opinion, that the elasticity of steam and other vapours is owing merely to the attraction for fire, and the consequent discrimination of their particles through the whole mass of fire, has been entertained by many naturalists, and it has been ascribed entirely to attraction. We by no means pretend to decide; but we think the analogy by far too slight to found any confident opinion on it. The aim is to solve phenomena by attraction only, as if it were of more easy conception than repulsion. Considered merely as facts, they are quite on a par. The appearances of nature in which we observe actual recesses of the parts of body from each other, are as distinct, and as frequent and familiar, as the appearances of actual reproach. And if we attempt to go farther in our contemplation, and to conceive the way and the forces by which either the approximation or recesses of the atoms are produced, we must acknowledge that we have no conception of the matter; and we can only say, that there is a cause of these motions, and we call it a force, as in every case of the production of motion. We call it attraction or repulsion just as we happen to contemplate an access or a recess. But the analogy here is not only flight, but imperfect, and fails most in those cases which are most simple, and where we should expect it to be most complete. We can squeeze water out of a sponge, it is true, or out of a piece of green wood; but when the white of an egg, the tremella, or some gums, swell to a hundred times their dry dimensions by imbibing water, we cannot squeeze out a particle. If fluidity (for the reasoning must equally apply to this as to vapourousness) being owing to an accumulation of the extended matter of fire, which gradually expanded the solid by its very minute additions; and if the accumulation round a particle of ice, which is necessary for making it a particle of water, be so great in comparison of what gives it the expansion of one degree, as experiment obliges us to conclude—it seems an inevitable consequence that all fluids should be many times rarer than the solids from which they are produced. But we know that the difference is trifling in all cases, and in some (water, for instance, and iron) the solid is rarer than the fluid. Many other arguments, (each of them perhaps of little weight when taken alone, but which are all systematically connected) concur in rendering it much more probable that the matter of fire, in causing elasticity, acts immediately by its own elasticity, which we cannot conceive in any other way than as a mutual tendency in its particles to receive from each other; and we doubt not but that, if it could be obtained alone, we should find it an elastic fluid like air. We even think that there are cases in which it is observed in this state. The elastic force of gunpowder is very much beyond the elasticity of all the vapours which are produced in its deflagration, each of them being expanded as much as we can reasonably suppose by the great heat to which they are exposed. The writer of this article exploded some gunpowder mixed with a considerable portion of finely powdered quartz, and another parcel mixed with fine filings of copper. The elasticity was measured by the penetration of the ball which was discharged, and was great in the degree now mentioned. The experiment was so conducted, that much of the quartz and copper was collected; none of the quartz had been melted, and some of the copper was not melted. The heat, therefore, could not be such as to explain the elasticity by expansion of the vapours; and it became not improbable that fire was acting here as a detached chemical fluid by its own elasticity. But to return to our subject.

There is one circumstance in which we think our own experiments show a remarkable difference (at least in degree) between the condensible and incondensible vapours. It is well known, that when air is very suddenly expanded, cold is produced, and heat when it is suddenly condensed. When making experiments with sensible vapours; the hopes of discovering the connection between the elasticity and density of the vapours of boiling water, and also of boiling spirits of turpentine, we found the change of density accompanied by a change of temperature vastly greater than in the case of incondensible gases. When the vapour of boiling water was suddenly allowed to expand into five times its bulk, we observed the depression of a large and sensible air thermometer to be at least four or five times greater than in a similar expansion of common air of the same temperature. The chemical reader will readily see reasons for expecting, on the contrary, a smaller alteration of temperature, both on account of the much greater rarity of the fluid, and on account of a partial condensation of its water and the consequent disengagement of combined heat.

This difference in the quantity of fire which is combined in vapours and gases is so considerable, as to authorize us to suppose that there is some difference in the chemical constitution of vapours and gases, and that the connection connection between the specific bates of the vapour and the fire which it contains is not the same in air, for instance, as in the vapour of boiling water; and this difference may be the reason why the one is easily condensible by cold, while the other has never been exhibited in a liquid or solid form, except by means of its chemical union with other substances. In this particular instance we know that there is an essential difference—that in vital or atmospheric air there is not only a prodigious quantity of fire which is not in the vapour of water, but that it also contains light, or the cause of light, in a combined state. This is fully evinced by the great discovery of Mr Cavendish of the composition of water. Here we are taught that water (and consequently its vapour) consists of air from which the light and greatest part of the fire have been separated. And the subsequent discoveries of the celebrated Lavoisier show, that almost all the condensible gases with which we are acquainted consist either of airs which have already lost much of their fire (and perhaps light too), or of matters in which we have no evidence of fire or light being combined in this manner.

This consideration may go far in explaining this difference in the condensibility of these different species of aerial fluids, the gases and the vapours; and it is with this qualification only that we are disposed to allow that all bodies are condensible into liquids or solids by abstracting the heat. In order that vital air may become liquid or solid, we hold that it is not sufficient that a body be presented to it which shall simply abstract its heat. This would only abstract its uncombined fire. But another and much larger portion remains chemically combined by means of light. A chemical affinity must be brought into action which may abstract, not the fire from the oxygen (to speak the language of Mr Lavoisier), but the oxygen from the fire and light. And our production is not the detached basis of air, but detached heat and light, and the formation of an oxide of some kind.

To prosecute the chemical consideration of Steams farther than these general observations, which are applicable to all, would be almost to write a treatise of chemistry, and would be a repetition of many things which have been treated of in sufficient detail in other articles of this work. We shall therefore conclude this article with some other observations, which are also general, with respect to the different kinds of coercible vapours, but which have a particular relation to the following article.

Steam or vapour is an elastic fluid, whose elasticity balances the prelude of the atmosphere; and it has been produced from a solid or liquid body raised to a sufficient temperature for giving it this elasticity; that is, for causing the fluid to boil. This temperature must vary with the prelude of the air. Accordingly it is found, that when the air is light (indicated by the barometer being low), the fluid will boil sooner. When the barometer stands at 30 inches, water boils at the temperature 212°. If it stands so low as 28 inches, water will boil at 208°. In the plains of Quito, or at Gondar in Abyssinia, where the barometer stands at about 21 inches, water will boil at 195°. Highly rectified alcohol will boil at 160°, and vitriolic ether will boil at 88° or 89°. This is a temperature by no means uncommon in these places; nay, the air is frequently warmer. Vitriolic ether, therefore, is a liquor which can hardly be known in those countries. It is hardly possible to preserve it in that form. If a phial have not its stopper firmly tied down, it will be blown out, and the liquor will boil and be dissipated in steam. On the top of Chimborazo, the human blood must be disposed to give out air-bubbles.

We said some time ago, that we had concluded, from some experiments made in the receiver of an air-pump, that fluids boil in vacuo at a temperature nearly 120 degrees lower than that necessary for their boiling in the open air. But we now see that this must have been a gross approximation; for in these experiments the fluids were boiling under the prelude of the vapour which they produced, and which could not be abstracted by working the pump. It appears from the experiments of Lord Charles Cavendish, mentioned in the article PNEUMATICS, that water of the temperature 72° was converted into elastic vapour, which balanced a prelude of \( \frac{1}{4} \)ths of an inch of mercury, and in this state it occupied the receiver, and did not allow the mercury in the gauge to sink to the level. As fast as this was abstracted by working the air-pump, more of it was produced from the surface of the water, so that the prelude continued the same, and the water did not boil. Had it been possible to produce a vacuum above this water, it would have boiled for a moment, and would even have continued to boil, if the receiver could have been kept very cold.

Upon reading these experiments, and some very curious ones of Mr Nairne, in the Phil. Transf. vol. lvii. the writer of this article was induced to examine more particularly the relation between the temperature of the vapour and its elasticity, in the following manner:

ABCD (fig. 2.) is the section of a small digester made of copper. Its lid, which is fastened to the body with screws, is pierced with three holes, each of which had a small pipe folded 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 \( \frac{1}{4} \)th of an inch. There rested on the stalk at top of this valve the arm of a steelyard 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 steelyard on its top; so that when the weight was at the division 10, the prelude of the steelyard on the valve was just equal to that of a column of mercury 10 inches high, and \( \frac{1}{4} \)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 farther 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 132°. 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 following table, where column 1st expresses the elasticity of the steam, being the sum of 29.9, and the division of the steelyard; column 2d expresses the temperature of the steam corresponding to this elasticity.

<table> <tr> <th>I.</th> <th>II.</th> </tr> <tr><td>35 inches.</td><td>219°</td></tr> <tr><td>40</td><td>226</td></tr> <tr><td>45</td><td>232</td></tr> <tr><td>50</td><td>237</td></tr> <tr><td>55</td><td>242</td></tr> <tr><td>60</td><td>247</td></tr> <tr><td>65</td><td>251</td></tr> <tr><td>70</td><td>255</td></tr> <tr><td>75</td><td>259</td></tr> <tr><td>80</td><td>263</td></tr> <tr><td>85</td><td>267</td></tr> <tr><td>90</td><td>270\( \frac{1}{2} \)</td></tr> <tr><td>95</td><td>274\( \frac{1}{2} \)</td></tr> <tr><td>100</td><td>278</td></tr> <tr><td>105</td><td>281</td></tr> </table>

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 atmosphere. 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 too 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.

<table> <tr> <th>32°</th> <th>8</th> </tr> <tr><td>40</td><td>0.1</td></tr> <tr><td>50</td><td>0.2</td></tr> <tr><td>60</td><td>0.35</td></tr> <tr><td>70</td><td>0.55</td></tr> <tr><td>80</td><td>0.82</td></tr> <tr><td>90</td><td>1.18</td></tr> <tr><td>100</td><td>1.61</td></tr> <tr><td>110</td><td>2.25</td></tr> <tr><td>120</td><td>3.00</td></tr> <tr><td>130</td><td>3.95</td></tr> <tr><td>140</td><td>5.15</td></tr> <tr><td>150</td><td>6.72</td></tr> <tr><td>160</td><td>8.65</td></tr> <tr><td>170</td><td>11.05</td></tr> <tr><td>180</td><td>14.05</td></tr> <tr><td>190</td><td>17.85</td></tr> <tr><td>200</td><td>22.62</td></tr> <tr><td>210</td><td>28.65</td></tr> </table>

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. 3, hav.-Fig. 3- ing 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 flender bore, and was about fix 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.

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 mercury in the tube MN. Their correspondent stations are marked in the following table:

<table> <tr> <th>Temperature.</th> <th>Elasticity.</th> </tr> <tr><td>212°</td><td>0.0</td></tr> <tr><td>220</td><td>5.9</td></tr> <tr><td>230</td><td>14.6</td></tr> <tr><td>240</td><td>25.0</td></tr> <tr><td>250</td><td>36.9</td></tr> <tr><td>260</td><td>50.4</td></tr> <tr><td>270</td><td>64.2</td></tr> <tr><td>280</td><td>106.0</td></tr> </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.

<table> <tr> <th>Temperature.</th> <th>Elasticity.</th> </tr> <tr><td>32°</td><td>0.0</td></tr> <tr><td>40</td><td>0.1</td></tr> <tr><td>50</td><td>0.1</td></tr> <tr><td>60</td><td>0.35</td></tr> <tr><td>70</td><td>0.55</td></tr> <tr><td>80</td><td>0.82</td></tr> <tr><td>90</td><td>1.25</td></tr> <tr><td>100</td><td>1.6</td></tr> <tr><td>110</td><td>2.25</td></tr> <tr><td>120</td><td>3.0</td></tr> <tr><td>130</td><td>3.95</td></tr> <tr><td>140</td><td>5.15</td></tr> <tr><td>150</td><td>6.72</td></tr> <tr><td>160</td><td>8.05</td></tr> <tr><td>170</td><td>11.05</td></tr> <tr><td>180</td><td>14.25</td></tr> <tr><td>190</td><td>17.85</td></tr> <tr><td>200</td><td>22.02</td></tr> <tr><td>210</td><td>28.05</td></tr> <tr><td>220</td><td>35.8</td></tr> <tr><td>230</td><td>44.7</td></tr> <tr><td>240</td><td>54.9</td></tr> <tr><td>250</td><td>66.8</td></tr> <tr><td>260</td><td>80.3</td></tr> <tr><td>270</td><td>94.1</td></tr> <tr><td>280</td><td>105.9</td></tr> </table>

In the memoirs of the Royal Academy of Berlin for 1782, there is an account of some experiments made by Mr 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 found, that when the elasticity was equal to that of the vapour of water, the temperature was about 32° 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 temperatures of equally elastic vapours of water and alcohol not to be constant, but gradually to diminish, in Mr 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:

<table> <tr> <th>Temperature.</th> <th>Elasticity.</th> </tr> <tr><td>32°</td><td>0.0</td></tr> <tr><td>40</td><td>0.1</td></tr> <tr><td>60</td><td>0.8</td></tr> <tr><td>80</td><td>0.8</td></tr> <tr><td>100</td><td>3.9</td></tr> <tr><td>120</td><td>6.9</td></tr> <tr><td>140</td><td>12.2</td></tr> <tr><td>160</td><td>21.3</td></tr> <tr><td>180</td><td>34.</td></tr> <tr><td>200</td><td>52.4</td></tr> <tr><td>220</td><td>78.5</td></tr> <tr><td>240</td><td>115.</td></tr> </table>

We say that the result was unexpected; for as the natural boiling point seemed by former experiments to be fixed in all fluids about 120° or more below their boiling point in the ordinary pressure of the atmosphere, it was reasonable to expect that the temperature at which they ceased to emit sensibly elastic steam would have some quality of relation to their temperatures when emitting steam of any determinate elasticity. Now as the vapour of alcohol of elasticity 30 has its temperature about 36° lower than the temperature of water equally elastic, it was to be expected that the temperature at which it ceased to be sensibly affected would be several degrees lower than 32°. It is evident, however, that this is not the case. But this is a point that deserves more attention, because it is closely connected with the chemical relation between the element (if such there be) of fire, and the bodies into whose composition it seems to enter as a constituent part. What is the temperature 32°, to make it peculiarly connected with elasticity? It is a temperature assumed by us for our own convenience, on account of the familiarity of water in our experiments. Ether, we know, boils in a temperature far below this, as appears from Dr Cullen's experiments narrated in the Essays Physical and Literary of Edinburgh. On the faith of former experiments, we may be pretty certain that it will boil in vacuo at the temperature —14°, because in the air it boils at +106°. Therefore we may be certain, that the steam or vapour of ether, when of the temperature 32°, will be very sensibly elastic. Indeed Mr Lavoisier says, that if it be exposed in an exhausted receiver in winter, its vapour will support mercury at the height of 10 inches. A series of experiments on this vapour similar to the above would be very instructive. We even with that those on alcohol were more carefully repeated. If we draw a curve line, of which the abscissa is the line of temperatures, and the ordinates are the corresponding heights of the mercury in these experiments on water and alcohol, we shall observe, that although they both sensibly coincide at 32°, and have the abscissa for their common tangent, a very small error of observation may be the cause of this, and the curve which expresses the elasticity of spirituous vapour may really intersect the other, and go back wards considerably beyond 32°.

This range of experiments gives rise to some curious and important reflections. We now see that no particular temperature is necessary for water affuming the form of permanently elastic vapour; and that it is highly probable that it affumes this form even at the temperature 32°; only its elasticity is too small to afford us any sensible measure. It is well known that even ice evaporates (see experiments to this purpose by Mr Wilfon in the Philosophical Transactions, when a piece of polished metal covered with hoar-frost became perfectly clear by exposing it to a dry frosty wind).

Even mercury evaporates, or is converted into elastic vapour, when all external pressure is removed. The dim film which may frequently be observed in the upper part of a barometer which stands near a stream of air, is found to be small globules of mercury sticking to the inside of the tube. They may be seen by the help of a magnifying glass, and are the best test of a well made barometer. They will be entirely removed by causing the mercury to rise along the tube. It will lick them all up. They consist of mercury which had evaporated in the void space, and was afterwards condensed by the cold glass. But the elasticity is too small to occasion a sensible depression of the column, even when considerably warmed by a candle.

Many philosophers accordingly imagine, that spontaneous evaporation in low temperatures is produced in this way. But we cannot be of this opinion, and must still think that this kind of evaporation is produced by the dissolving power of the air. When moist air is suddenly rarefied, there is always a precipitation of water. This is most distinctly seen when we work an air-pump briskly. A mist is produced, which we see plainly fall to the bottom of the receiver. But by this new doctrine the very contrary should happen, because the tendency of water to appear in the elastic form is promoted by removing the external pressure; and we really imagine that more of it now actually becomes simple elastic watery vapour. But the mist or precipitation shows incontrovertibly, that there had been a previous solution. Solution is performed by forces which act in the way of attraction; or, to express it more safely, solutions are accompanied by the mutual approaches of the particles of the menstruum and solved: all such tendencies are observed to increase by a diminution of distance. Hence it must follow, that air of double density will dissolve more than twice as much water. Therefore when we suddenly rarely saturated air (even though its heat should not diminish) some water must be let go. What may be its quantity we know not; but it may be more than what would now become elastic by this diminution of surrounding pressure; and it is not unlikely but this may have some effect in producing the vehicles which we found it so difficult to explain. These may be filled with pure watery vapour, and be floating in a fluid composed of water dissolved in air. An experiment of Fontana's seems to put this matter out of doubt. A distilling apparatus AB (fig. 4.) was so contrived, that the heat was applied above the surface of the water in the alembic A. This was done by inclosing it in another vessel CC, filled with hot water. In the receiver B there was a sort of barometer D, with an open cistern, in order to see what pressure there was on the surface of the fluid. While the receiver and alembic contained air, the heat applied at A produced no sensible distillation during several hours: But on opening a cock E in the receiver at its bottom, and making the water in the alembic to boil, steam was produced which soon expelled all the air, and followed it through the cock. The cock was now shut, and the whole allowed to grow cold by removing the fire, and applying cold water to the alembic. The barometer fell to a level nearly. Then warm water was allowed to get into the outer vessel CC. The barometer rose a little, and the distillation went on briskly without the smallest ebullition in the alembic. The conclusion is obvious: while there was air in the receiver and communicating pipe, the distillation proceeded entirely by the dissolving power of this air. Above the water in the alembic it was quickly saturated; and this saturation proceeded slowly along the still air in the communicating pipe, and at last might take place through the whole of the receiver. The sides of the receiver being kept cold, should condense part of the water dissolved in the air in contact with them, and this should trickle down the sides and be collected. But any person who has observed how long a crystal of blue vitriol will lie at the bottom of a glass of still water before the tinge will reach the surface, will see that it must be next to impossible for distillation to go on in these circumstances; and accordingly none was observed. But when the upper part of the apparatus was filled with pure watery vapour, it was supplied from the alembic as fast as it was condensed in the receiver, just as in the pulse glass.

Another inference which may be drawn from these A certain experiments is, that Nature seems to affect a certain law in the law in the dilatation of aeriform fluids by heat. They dilatation seem to be dilatable nearly in proportion of their present dilatation. For if we suppose that the vapours aeriform fluids by 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, supporting 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 therefore 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 its bulk 1/4th, another equal diminution of temperature will diminish this new bulk 1/4th very nearly. Thus, in our experiments, the temperatures 110°, 140°, 170°, 200°, 230°, are in arithmetical progression, having equal differences; and we see that the corresponding elasticities 2.25, 5.15, 11.05, 22.62, 44.7, are very nearly in the continued proportion of 1 to 2. The elasticity corresponding to the temperature 260 deviates considerably from this law, which would give 88 or 89 instead of 80; and the deviation increases in the higher temperatures. But still we see that there is a considerable approximation to this law; and it will frequently assist us to recollect, that whatever be the present temperature, an increase of 30 degrees doubles the elasticity and the bulk of watery vapour.

<table> <tr> <th>That 4° will increase the elasticity from 1 to 1^1/2</th> <th>8</th> <th>10</th> <th>12.5</th> <th>18</th> <th>22</th> <th>24</th> <th>26</th> </tr> <tr> <td></td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> <td>1 to 1^1/2</td> </tr> </table>

This is sufficiently exact for most practical purposes. Thus an engineer finds that the injection cools the cylinder of a steam engine to 192°. It therefore leaves a steam whose elasticity is three fifths of its full elasticity, = 18 inches. But it is better at all times to have recourse to the table. Observe, too, that in the lower temperatures, i.e. below 110°, this increment of temperature does more than double the elasticity.

This law obtains more remarkably in the incoercible vapours; such as vital air, atmospheric air, fixed air, &c. all of which have also their elasticity proportional to their bulk inversely: and perhaps the deviation from the law in steams is connected with their chemical difference of constitution. 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 abscissae; 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 x of degrees of our thermometer (above that temperature where the elasticity is equal to unity), that this multiple shall be the common logarithm of the elasticity y; so that \(mx = \log y\).

But our experiments are not sufficiently accurate for determining the temperature where the elasticity is measured by 1 inch; because in these temperatures the elasticities vary by exceedingly small quantities. But if we take 11.04 for the unit of elasticity, and number our temperature from 170°, and make \(m = 0.010035\), we shall find the product \(mx\) to be very nearly the logarithm of the elasticity. The deviations, however, from this law, are too great to make this equation of any use. But it is very practicable to frame an equation which shall correspond with the experiments to any degree of accuracy; and it has been done for air in a translation of General Roy's Measurement of the Bafe at Hounslow Heath into French by Mr Prony. It is as follows: Let x be the degrees of Reaumur's thermometer; let y be the expansion of 10,000 parts of air; let e be 10, \(m = 2.7979\), \(n = 0.01768\); then \(y = e^{m + nx} - 627.5\). Now e being 10, it is plain that \(e^{m + nx}\) is the number, of which \(m + nx\) is the common logarithm. This formula is very exact as far as the temperature 60°: but beyond this it needs a correction; because air, like the vapour of water, does not expand in the exact proportion of its bulk.

We observe this law considerably approximated to in the augmentation of the bulk or elasticity of elastic vapours; that is, it is a fact that a given increment of temperature makes very nearly the same proportional augmentation of bulk and elasticity. This gives us some notion of the manner in which the supposed expanding of the bulk causes produces the effect. When vapour of the bulk 4 is expanded into a bulk 5 by an addition of 10 degrees of sensible heat, a certain quantity of fire goes into it, and is accumulated round each particle, in such a manner that the temperature of each, which formerly was m, is now \(m + 10\). Let it now receive another equal augmentation of temperature. This is now \(m + 20\), and the bulk is \(5 \times 5 / 4\) or 6\(1/4\), and the arithmetical increase of bulk is 1\(1/4\). The absolute quantity of fire which has entered it is greater than the former, both on account of the greater augmentation of space and the greater temperature. Consequently if this vapour be compressed into the bulk 5, there must be heat or fire in it which is not necessary for the temperature \(m + 20\), far less for the temperature \(m + 10\). It must therefore emerge, and be disposed to enter a thermometer which has already the temperature \(m + 20\): that is, the vapour must grow hotter by compression; not by squeezing out the heat, like water out of a sponge, but because the law of attraction for heat is deranged. It would be a very valuable acquisition to our knowledge to learn with precision the quantity of sensible heat produced in this way; but no satisfactory experiments have yet been made. M. Lavoisier, with his chemical friends, and colleagues, were busily employed in this inquiry: but the wickedness of their countrymen deprived the world of this and many other important additions which we might have expected from this celebrated and unfortunate philosopher. He had made, in conjunction with M. de la Place, a numerous train of accurate and expensive experiments for measuring the quantity of latent or combined heat in elastic vapours. This is evidently a very important point to the distiller and practical chemist. This heat must all come from the fuel; and it is greatly worth while to know whether any saving may be made of this article. Thus we know that distillation will go on either under the preface of the air, or in an alembic and receiver from which the air has been expelled by steam; and we know that this last may be conducted in a very low temperature, even not exceeding that of the human body. But it is uncertain whether this may not employ even a greater quantity of fuel, as well as occasion a great expense of time. We are disposed to think, that when there is no air in the apparatus, and when the condensation can be speedily performed, the proportion of fuel expended to the fluid which comes over will diminish continually as the heat, and consequently the density of the steam, is augmented; because in this case the quantity of combined heat must be less. In the mean time, we earnestly recommend the trial of this mode of distillation in vessels cleared of air. It is undoubtedly of great advantage to be able to work with smaller fires; and it would secure us against all accidents of blowing off the the head of the still, often attended with terrible consequences (b).

We must not conclude this article without taking notice of some natural phenomena which seem to owe their origin to the action of elastic steam.

We have already taken notice of the resemblance of the tremor and succussions observed in the shocks of many earthquakes to those which may be felt in a vessel where water is made to boil internally, while the breaking out of the ebullition is lifted by the cold of the upper parts; and we have likewise stated the objections which are usually made to this theory of earthquakes. We may perhaps resume the subject under the article VOLCANO; but in the mean time we do not hesitate to say, that the wonderful appearances of the Geyser spring in Iceland (see Huer; and ICELAND, No 3—5,) are undoubtedly produced by the expansion of steam in ignited caverns. Of these appearances we suppose the whole train to be produced as follows.

A cavern may be supposed of a shape analogous to CBDEF (fig. 5.), having a perpendicular funnel AB issuing from a depressed part of the roof. The part F may be lower than the rest, remote, and red hot. Such places we know to be frequent in Iceland. Water may be continually trickling into the part CD. It will fill it up to B, and even up to E e, and then trickle slowly along into F. As soon as any gets into contact with an ignited part, it expands into elastic steam, and is partly condensed by the cold sides of the cavern, which it gradually warms, till it condenses no more. This production of steam hinders not in the smallest degree the trickling of more water into F, and the continual production of more steam. This now presses on the surface of the water in CD, and causes it to rise gradually in the funnel BA; but slowly, because its cold surface is condensing an immense quantity of steam. We may easily suppose that the water trickles faster into F than it is expended in the production of steam; so that it reaches farther into the ignited part, and may even fall in a stream into some deeper pit highly ignited. It will now produce steam in vast abundance, and of prodigious elasticity; and at once push up the water through the funnel in a solid jet, and to a great height. This must continue till the surface of the water sinks to BD. If the lower end of the funnel have any inequalities or notches, as is most likely, the steam will get admission along with the water, which in this particular place is boiling hot, being superficial, and will get to the mouth of the funnel, while water is still pressed in below. At last the steam gets in at B on all sides; and as it is converging to B, along the surface of the water, with prodigious velocity it sweeps along with it much water, and blows it up through the funnel with great force. When this is over, the remaining steam blows out unmixed with water, growing weaker as it is expended, till the bottom of the funnel is again flopped by the water increasing in the cavern CBD. All the phenomena above ground are perfectly conformable to the necessary consequences of this very probable construction of the cavern. The feeling of being lifted up, immediately before the jet, in all probability is owing to a real heaving up of the whole roof of the cavern by the first expansion of the great body of steam. We had an accurate description of the phenomena from persons well qualified to judge of these matters who visited these celebrated springs in 1789.

STEAM-Engine, is the name of a machine which derives its moving power from the elasticity and condensibility of the steam of boiling water. It is the most valuable present which the arts of life have ever received from the philosopher. The mariner's compass, the telescope, gunpowder, and other most useful servants to human weakness and ingenuity, were the productions of chance, and we do not exactly know to whom we are indebted for them; but the steam-engine was, in the very beginning, the result of reflection, and the production of a very ingenious mind; and every improvement it has received, and every alteration in its construction and principles, were also the results of philosophical study.

The steam-engine was beyond all doubt invented by the marquis of Worcester during the reign of Charles II. This nobleman published in 1663 a small book entitled A CENTURY OF INVENTIONS; giving some obscure and enigmatical account of a hundred discoveries or contrivances of his own, which he extols as of great importance to the public. He appears to have been a person of much knowledge and great ingenuity: but his description or accounts of these inventions seem not so much intended to instruct the public, as to raise wonder; and his encomiums on their utility and importance

(b) We earnestly recommend this subject to the consideration of the philosopher. The laws which regulate the formation of elastic vapour, or the general phenomena which it exhibits, give us that link which connects chemistry with mechanical philosophy. Here we see chemical affinities and mechanical forces set in immediate opposition to each other, and the one made the indication, characteristic, and measure of the other. We have not the least doubt that they make but one science, the Science of Universal Mechanics; nor do we despair of seeing the phenomena of solution, precipitation, crystallization, fermentation, nay animal and vegetable secretion and assimilation, successfully investigated, as cases of local motion, and explained by the agency of central forces. Some thing of this kind, and that not inconsiderable, was done when Dr Cullen first showed how the double affinities might be illustrated by the assistance of numbers. Dr Black gave to this hint (for it was little more) that elegant precision which characterizes all his views. Mr Kirwan has greatly promoted this study by his numerous and ingenious examples of its application; and the most valuable passages of the writings of Mr Lavoisier, are those where he traces with logical precision the balancings of force which appear in the chemical phenomena. It is from the familiar balancings and consequent measurements, which may be observed and obtained in the present case, that we are to hope for admission into this almost unbounded science of contemplation. We have another link equally interesting and promising, viz. the production of heat by friction. This also highly deserves the consideration of the mathematical philosopher. ance are to a great degree extravagant, resembling more the puff of an advertising tradesman than the puristic communications of a gentleman. The marquis of Worcester was indeed a projector, and very importunate and mysterious withal in his applications for public encouragement. His account, however, of the steam-engine, although by no means fit to give us any distinct notions of its structure and operation, is exact as far as it goes, agreeing precisely with what we now know of the subject. It is No 68. of his inventions. His words are as follow: "This admirable method which I propose of raising water by the force of fire has no bounds if the vessels be strong enough: for I have taken a cannon, and having filled it three fourths full of water, and flint up its muzzle and touch-hole, and exposed it to the fire for 24 hours, it burst with a great explosion. Having afterwards discovered a method of fortifying vessels internally, and combined them in such a way that they filled and acted alternately, I have made the water spout in an uninterrupted stream 40 feet high; and one vessel of rarefied water raised 40 of cold water. The person who conducted the operation had nothing to do but turn two cocks; so that one vessel of water being consumed, another begins to force, and then to fill itself with cold water, and so on in succession."

It does not appear that the noble inventor could ever interest the public by these accounts. His character as a projector, and the many failures which persons of this turn of mind daily experience, probably prejudiced people against him, and prevented all attention to his projects. It was not till towards the end of the century, when experimental philosophy was prosecuted all over Europe with uncommon ardour, that these notions again engaged attention. Captain Savary, a person also of great ingenuity and ardent mind, saw the reality and practicability of the marquis of Worcester's project. He knew the great expansive power of steam, and had discovered the inconceivable rapidity with which it is reconverted into water by cold; and he soon contrived a machine for raising water, in which both of these properties were employed. He says, that it was entirely his own invention. Dr Defagulliers infers that he only copied the marquis's invention, and charges him with gross plagiarism, and with having bought up and burned the copies of the marquis's book, in order to secure the honour of the discovery to himself. This is a very grievous charge, and should have been substantiated by very distinct evidence. Defagulliers produces none such; and he was much too late to know what happened at that time. The argument which he gives is a very foolish one, and gave him no title to consider Savary's experiment as a falsehood; for it might have happened precisely as Savary relates, and not as it happened to Defagulliers. The fact is, that Savary obtained his patent of invention after a hearing of objections, among which the discovery of the marquis of Worcester was not mentioned: and it is certain that the account given in the Century of Inventions could instruct no person who was not sufficiently acquainted with the properties of steam to be able to invent the machine himself.

Captain Savary obtained his patent after having actually erected several machines, of which he gave a description in a book entitled The Miner's Friend, published in 1696, and in another work published in 1699. Much about this time Dr Papin, a Frenchman and fellow of the Royal Society, invented a method of dissolving bones and other animal solids in water, by confining them in clole vessels, which he called DIGESTERS, so as to acquire a great degree of heat. For it must be observed in this place, that it had been discovered long before (in 1684) by Dr Hooke, the most inquisitive experimental philosopher of that inquisitive age, that water could not be made to acquire above a certain temperature in the open air; and that as soon as it begins to boil, its temperature remains fixed, and an increase of heat only produces a more violent ebullition, and a more rapid waste. But Papin's experiments made the elastic power of steam very familiar to him: and when he left England and settled as professor of mathematics at Marpurg, he made many awkward attempts to employ this force in mechanics, and even for raising water. It appears that he had made experiments with this view in 1698, by order of Charles, landgrave of Hesse. For this reason the French affect to consider him as the inventor of the steam-engine. He indeed published some account of his invention in 1707; but he acknowledges that Captain Savary had also, and without any communication with him, invented the same thing. Whoever will take the trouble of looking at the description which he has given of these inventions, which are to be seen in the Acta Eruditorum Lipsiae, and in Leupold's Theatrum Machinarum, will see that they are most awkward, absurd, and impracticable. His conceptions of natural operations were always vague and imperfect, and he was neither philosopher nor mechanician.

We are thus anxious about the claim of those gentlemen, because a most respectable French author, Mr Bofut, says in his Hydrodynamique, that the first notion of the steam-engine was certainly owing to Dr Papin, who had not only invented the digester, but had in 1695 published a little performance describing a machine for raising water, in which the pistons are moved by the vapour of boiling water alternately dilated and condensed. Now the fact is, that Papin's first publication was in 1707, and his piston is nothing more than a float on the surface of the water, to prevent the waste of steam by condensation; and the return of the piston is not produced, as in the steam engine, by the condensation of the steam, but by admitting the air and a column of water to press it back into its place. The whole contrivance is so awkward, and so unlike any distinct notions of the subject, that it cannot do credit to any person. We may add, that much about the fame time Mr Amontons contrived a very ingenious but intricate machine, which he called ton's fire-a fire-wheel. It consisted of a number of buckets placed in the circumference of a wheel, and communicating with each other by very intricate circuitous passages. One part of this circumference was exposed to the heat of a furnace, and another to a stream or cistern of cold water. The communications were so disposed, that the steam produced in the buckets on one side of the wheel drove the water into buckets on the other side, so that one side of the wheel was always much heavier than the other; and it must therefore turn round, and may execute some work. The death of the inventor, and the intricacy of the machine, caused it to be neglected. Another member of the Parisian academy of sciences (Mr Delalandes) also presented to the academy a project of a steam-wheel, where the impulsive force of the va- pour was employed; but it met with no encouragement. The English engineers had by this time so much improved Savary's first invention that it supplanted all others. We have therefore no hesitation in giving the honour of the first and complete invention to the marquis of Worcester; and we are not disposed to refuse Captain Savary's claim to originality as to the construction of the machine, and even think it probable that his own experiments made him see the whole independent of the marquis's account.

Captain Savary's engine, as improved and simplified by himself is as follows.

A (fig. 6.) represents a strong copper boiler properly built up in a furnace. There proceeds from its top a large steam-pipe B, which enters into the top of another strong vessel R called the receiver. This pipe has a cock at C called the steam cock. In the bottom of the receiver is a pipe F, which communicates sidewise with the rising pipe KGH. The lower end H of this pipe is immersed in the water of the pit or well, and its upper part K opens into the cistern into which the water is to be delivered. Immediately below the pipe of communication F there is a valve G, opening when pressed from below, and shutting when pressed downwards. A similar valve is placed at I, immediately above the pipe of communication. Lastly, there is a pipe ED which branches off from the rising pipe, and enters into the top of the receiver. This pipe has a cock D called the injection-cock. The mouth of the pipe ED has a nozzle f pierced with small holes, pointing from a centre in every direction. The keys of the two cocks C and D are united, and the handle g h is called the regulator.

Let the regulator be so placed that the steam-cock C is open and the injection-cock D is shut; put water into the boiler A, and make it boil strongly. The steam coming from it will enter the receiver, and gradually warm it, much steam being condensed in producing this effect. When it has been warmed so as to condense no more, the steam proceeds into the rising pipe; the valve G remains shut by its weight; the steam lifts the valve I, and gets into the rising pipe, and gradually warms it. When the workman feels this to be the case, or hears the rattling of the valve I, he immediately turns the steam-cock so as to shut it, the injection-cock still remaining shut (at least we may suppose this for the present). The apparatus must now cool, and the steam in the receiver collapses into water. There is nothing now to balance the pressure of the atmosphere; the valve I remains shut by its weight; but the air incumbent on the water in the pit presses up this water through the suction-pipe HG, and causes it to lift the valve G, and flow into the receiver R, and fill it to the top, if not more than 20 or 25 feet above the surface of the pit water.

The steam cock is now opened. The steam which, during the cooling of the receiver, has been accumulating in the boiler, and acquiring a great elasticity by the action of the fire, now rushes in with great violence, and, pressing on the surface of the water in the receiver, causes it to shut the valve G and open the valve I by its weight alone, and it now flows into the rising pipe, and would stand on a level if the elasticity of the steam were no more than what would balance the atmospheric pressure. But it is much more than this, and therefore it presses the water out of the receiver into the rising pipe, and will even cause it to come out at K, if the elasticity of the steam is sufficiently great. In order to ensure this, the boiler has another pipe in its top, covered with a safety-valve V, which is kept down by a weight W suspended on a fleelyard LM. This weight is so adjusted that its pressure on the safety-valve is somewhat greater than the pressure of a column of water VK as high as the point of discharge K. The fire is so regulated that the steam is always issuing a little by the loaded valve V. The workman keeps the steam-valve open till he hears the valve I rattle. This tells him that the water is all forced out of the receiver, and that the steam is now following it. He immediately turns the regulator which turns the steam-cock, and now, for the first time, opens the injection-cock. The cold water trickles at first through the holes of the nozzle f, and falling down through the steam, begins to condense it; and then its elasticity being less than the prelude of the water in the pipe KEFDf, the cold water spouts in all directions through the nozzle, and, quick as thought, produces a complete condensation. The valve G now opens again by the prelude of the atmosphere on the water of the pit, and the receiver is soon filled with cold water. The injection-cock is now shut, and the steam-cock opened, and the whole operation is now repeated; and so on continually.

This is the simple account of the process, and will serve to give the reader an introductory notion of the operation; but a more minute attention must be paid to many particulars before we can see the properties and defects of this ingenious machine.

The water is driven along the rising pipe by the elasticity of the steam. This must in the boiler, and this must in every part of the machine, exert a prelude on every inch square inch of the vessels equal to that of the upright column of water. Suppose the water to be raised 100 feet, about 25 of this may be done in the suction-pipe; that is, the upper part of the receiver may be about 25 feet above the surface of the pit-water. The remaining 75 must be done by forcing, and every square inch of the boiler will be squeezed out by a prelude of more than 30 pounds. This very moderate height therefore requires very strong vessels; and the marquis of Worcester was well aware of the danger of their bursting. A copper boiler of six feet diameter must be nine-tenths of an inch thick to be just in equilibrium with this prelude: and the soldered joint will not be able to withstand it, especially in the high temperature to which the water must be heated in order to produce steam of sufficient elasticity. By consulting the table of the elasticity of steam deduced from our experiments mentioned in the preceding article, we see that this temperature must be at least 283° of Fahrenheit's thermometer. In this heat soft folder is just ready to melt, and has no tenacity; even spelter folder is considerably weakened by it. Accordingly, in a machine erected by Dr Desaguliers, the workman having loaded the safety-valve a little more than usual to make the engine work more briskly, the boiler burst with a dreadful explosion, and blew up the furnace and adjoining parts of the building as if it had been gunpowder. Mr Savary succeeded pretty well in raising moderate quantities of water to small heights, but could make nothing of deep mines. Many attempts were made, on the marquis's quis's principle, to strengthen the vessels from within by radiated bars and by hoops, but in vain. Very small boilers or evaporators were then tried, kept red hot, or nearly so, and supplied with a slender stream of water trickling into them; but this afforded no opportunity of making a collection of steam during the refrigeration of the receiver, so as to have a magazine of steam in readiness for the next forcing operation; and the working of such machines was always an employment of great danger and anxiety.

The only situation in which this machine could be employed with perfect safety, and with some effect, was where the whole lift did not exceed 30 or 35 feet. In this case the greatest part of it was performed by the suction-pipe, and a very manageable pressure was sufficient for the rest. Several machines of this kind were erected in England about the beginning of this century. A very large one was erected at a salt-work in the south of France. Here the water was to be raised no more than 18 feet. The receiver was capacious, and it was occasionally supplied with steam from a small salt-pan constructed on purpose with a cover. The entry of the steam into the receiver merely allowed the water to run out of it by a large valve, which was opened by the hand, and the condensation was produced by the help of a small forcing pump also worked by the hand. In so particular a situation as this (and many such may occur in the endless variety of human wants), this is a very powerful engine; and having few moving and rubbing parts, it must be of great durability. This circumstance has occasioned much attention to be given to this first form of the engine, even long after it was supplanted by those of a much better construction. A very ingenious attempt was made very lately to adopt this construction to the uses of the miners. The whole depth of the pit was divided into lifts of 15 feet, in the same manner as is frequently done in pump-machines. In each of these was a suction-pipe 14 feet long, having above it a small receiver like R, about a foot high, and its capacity somewhat greater than that of the pipe. This receiver had a valve at the head of the suction-pipe, and another opening outwards into the little cistern, into which the next suction-pipe above dipped to take in water. Each of these receivers sent up a pipe from its top, which all met in the cover of a large vessel above ground, which was of double the capacity of all the receivers and pipes. This vessel was close on all sides. Another vessel of equal capacity was placed immediately above it, with a pipe from its bottom passing through the cover of the lower vessel and reaching near to its bottom. This upper vessel communicates with the boiler, and constitutes the receiver of the steam-engine. The operation is as follows: The lower vessel is full of water. Steam is admitted into the upper vessel, which expels the air by a valve, and fills the vessel. It is then condensed by cold water. The pressure of the atmosphere would cause it to enter by all the suction-pipes of the different lifts, and press on the surface of the water in the lower receiver, and force it into the upper one. But because each suction-pipe dips in a cistern of water, the air presses this water before it, raises it into each of the little receivers which it fills, and allows the spring of the air (which was formerly in them, but which now passes up into the lower receiver) to force the water out of the lower receiver into the upper one. When this has been completed, the steam is again admitted into the upper receiver. This allows the water to run back into the lower receiver, and the air returns into the small receivers in the pit, and allows the water to run out of each into its proper cistern. By this means the water of each pipe has been raised 15 feet. The operation may thus be repeated continually.

The contrivance is ingenious, and similar to those which are to be met with in the hydraulics of Schottus, Sturmius, and other German writers. But the operation must be exceedingly slow; and we imagine that the expense of steam must be great, because it must fill a very large and very cold vessel, which must waste a great portion of it by condensation. We see by some late publications of the very ingenious Mr Blackey, that he is still attempting to maintain the reputation of this machine by some contrivances of this kind; but we imagine that they will be ineffectual, except in some very particular situations.

For the great defect of the machine, even when we occasions can secure it against all risk of bursting, is the prodigious waste of steam, and consequently of fuel. Daily experience shows, that a few scattered drops of cold water are sufficient for producing an almost instantaneous condensation of a great quantity of steam. Therefore when the steam is admitted into the receiver of Savary's engine, and comes into contact with the cold top and cold water, it is condensed with great rapidity; and the water does not begin to subside till its surface has become so hot that it condenses no more steam. It may now begin to yield to the pressure of the incumbent steam; but as soon as it descends a little, more of the cold surface of the receiver comes into contact with the steam, and condenses more of it, and the water can descend no farther till this addition of cold surface is heated up to the state of evaporation. This rapid condensation goes on all the while the water is descending. By some experiments frequently repeated by the writer of this article, it appears that no less than \( \frac{1}{2} \)ths of the whole steam is uselessly condensed in this manner, and not more than \( \frac{1}{2} \)th is employed in allowing the water to descend by its own weight; and he has reason to think that the portion thus wasted will be considerably greater, if the steam be employed to force the water out of the receiver to any considerable height.

Observe, too, that all this waste must be repeated in every succeeding stroke; for the whole receiver must be cooled again in order to fill itself with water.

Many attempts have been made to diminish this waste; but all to little purpose, because the very filling of the receiver with cold water occasions its sides to condense a prodigious quantity of steam in the succeeding stroke. Mr Blackey has attempted to lessen this by using two receivers. In the first was oil; and into this only the steam was admitted. This oil passed to and fro between the two receivers, and never touched the water except in a small surface. But this hardly produced a sensible diminution of the waste: for it must now be observed, that there is a necessity for the first cylinder's being cooled to a considerable degree below the boiling point; otherwise, though it will condense much steam, and allow the water to rise into the receiver, there will be a great diminution of the height of suction, unless the vessel be much cooled. This appears plainly by inspecting the table of elasticity. Thus, if the vessel be cooled no lower than 180°, we should lose one half of the pressure of the atmosphere; if cooled to 120, we should still lose \( \frac{1}{4} \)th. The inspection of this table is of great use for understanding and improving this noble machine; and without a constant recollection of the elasticity of steam corresponding to its actual heat, we shall never have a notion of the niceties of its operation.

The rapidity with which the steam is condensed is really astonishing. Experiments have been made on steam-vessels of fix feet in diameter and seven feet high; and it has been found, that about four ounces of water, as warm as the human blood, will produce a complete condensation in less than a second; that is, will produce all the condensation that it is capable of producing, leaving an elasticity about one-fifth of the elasticity of the air. In another experiment with the same steam-vessel, no cold water was allowed to get into it, but it was made to communicate by a long pipe four inches in diameter with another vessel immersed in cold water. The condensation was so rapid that the time could not be measured: it certainly did not exceed half a second. Now this condensation was performed by a very trifling surface of contact. Perhaps we may explain it a little in this way: When a mass of steam, in immediate contact with the cold water, is condensed, it leaves a void, into which the adjoining steam instantly expands; and by this very expansion its capacity for heat is increased, or it grows cold, that is, abstracts the heat from the steam situated immediately beyond it. And in this expansion and refrigeration it is itself partly condensed or converted into water, and leaves a void, into which the circumjacent steam immediately expands, and produces the same effect on the steam beyond it. And thus it may happen that the abstraction of a small quantity of heat from an inconsiderable mass of steam may produce a condensation which may be very extensive. Did we know the change made in the capacity of steam for heat by a given change of bulk, we should be able to tell exactly what would be the effect of this local actual condensation. But experiment has not as yet given us any precise notions on this subject. We think that this rapid condensation to a great distance by a very moderate actual abstraction of heat is a proof that the capacity of steam for heat is prodigiously increased by expansion. We say a very moderate actual abstraction of heat, because very little heat is necessary to raise four ounces of blood-warm water to a boiling temperature, which will unfit it for condensing steam. The remarkable phenomenon of snow and ice produced in the Hungarian machine, when the air condensed in the receiver is allowed to blow through the cock (see PNEUMATICS), shows this to be the case in moist air, that is, in air holding water in a state of chemical solution. We see something very like it in a thunder-storm. A small black cloud sometimes appears in a particular spot, and in a very few seconds spreads over many hundred acres of sky, that is, a precipitation of water goes on with that rapid diffusion. We imagine that this increase of capacity or demand for heat, and the condensation that must ensue if this demand is not supplied, is much more remarkable in pure watery vapours, and that this is a capital distinction of their constitution from vapours dissolved in air (A).

The reader must now be so well acquainted with what passes in the steam-vessel, and with the exterior results from it, as readily to comprehend the propriety of the changes which we shall now describe as having been made in the construction and principle of the steam-engine.

Of all places in England the tin-mines of Cornwall attempted flood most in need of hydraulic assistance; and Mr Sa, to improve vary was much engaged in projects for draining them, the steam- by his steam-engine. This made its construction and principles well known among the machinists and engine- engineers of that neighbourhood. Among these were a Mr Newcomen, an ironmonger or blacksmith, and Mr Cawley a glazier at Dartmouth in Devonshire, who had dabbled much with this machine. Newcomen was a person of some reading, and was in particular acquainted with the person, writings, and projects of his countryman Dr Hooke. There are to be found among Hooke's papers, in the possession of the Royal Society, some notes of observations, for the use of Newcomen his countryman, on Papin's boasted method of transmitting to a great distance the action of a mill by means of pipes. Papin's project was to employ the mill to work two air-pumps of great diameter. The cylinders of these pumps were to communicate by means of pipes with equal cylinders furnished with pistons, in the neighbourhood of a distant mine. These pistons were to be connected, by means of levers, with the piston-rods of the mine. Therefore, when the piston of the air-pump at the mill was drawn up by the mill, the corresponding piston at the side of the mine would be pressed down by the atmosphere, and thus would raise the piston-rod in the mine, and draw the water. It would appear from these notes, that Dr Hooke had dissuaded Mr Newcomen from erecting a machine on this principle, of which he had exposed the fallacy in several discourses before the Royal Society. One passage is remarkable. "Could he (meaning Papin) make a speedy vacuum under your second piston, your work is done."

It is highly probable that, in the course of this speculation, it occurred to Mr Newcomen that the vacuum he so much wanted might be produced by steam, and that this gave rise to his new principle and construction of the steam-engine. The specific desideratum was in Newcomen's mind; and therefore, when Savary's engine appeared, and became known in his neighbourhood many years after, he would readily catch at the help which it promised.

Savary, however, claims the invention as his own; but Switzer, who was personally acquainted with both, is positive that Newcomen was the inventor. By his principles (as a Quaker) being averse from contention, he was contented to share the honour and the profits with Savary, whose acquaintance at court enabled him to procure the patent in 1705, in which all the three were associated. Posterity has done justice to the modest inventor, and the machine is universally called NEWCOMEN'S

(A) But if it has been found that the condensation requires more cold water than what is allowed above, and it is suspected that the rapidity of condensing a large volume of steam by the cold surface of a vessel is overrated.