On the water passing off in this new form or condition, two very remarkable phenomena take place, namely, the fluid expands to a very great extent, the vapour occupying nearly 1700 times the space which the fluid occupied from which it was generated; and at the same moment, an immense quantity of caloric or heat enters into the water while becoming steam, and disappears; which heat, from the circumstance that it cannot be discovered by the thermometer, is usually called latent heat, in contradistinction to that which affects the thermometer, and which is accordingly named sensible heat, that is, heat whose effects are apparent in producing the movement of the fluid in the thermometer tube.

When the water has assumed the state of vapour, it is invisible, being as perfectly transparent as the atmospheric air; and in this form it becomes obedient to those laws which affect gaseous or aëriform bodies, supposing always that the usual increased temperature is maintained (212 degrees Fahrenheit) to preserve it in this new state; for, on withdrawing the caloric, it then returns to its liquid inelastic condition, which is termed condensation. This.elastic state of the vapour may be suddenly destroyed by bringing it in contact with a large quantity of cold water—a process essentially a part of the greater number of steam-engines.

In this state of vapour the temperature is 212 degrees, or the same as that of the water from which it is generated. This may be easily determined by placing a thermometer in the boiling water, and then in the steam which arises from it.

Under the usual conditions in which water is made to boil, as in an open vessel on the fire, the temperature indicated by the thermometer is commonly about 212 degrees, the water acquiring at that temperature sufficient elastic force to overcome the weight of the atmosphere. But it is to be observed, that the pressure of the air must tend to retard the water swelling out into vapour; it will follow, therefore, that if we reduce the pressure on the surface of the water, the escape into the state of vapour will take place at a lower temperature, as was first observed by Dr. Cullen, and subsequently more minutely detailed by the late Professor Robinson. The latter has, indeed, established the general proposition, that vapours are produced from fluids in vacuo (where all atmospheric pressure is removed) at 140 degrees of Fahrenheit below the temperature at which these fluids naturally pass into vapour, under the usual pressure of the air. Water, for instance, which usually boils at 212 degrees, in this case would boil at 72 degrees, a temperature of the atmosphere frequently observed in the summer months of this country; and ether, which boils at 96 degrees, a temperature nearly corresponding with that of the human body (being lower only by 2 degrees), in vacuo would boil at 44 degrees below zero, or at a temperature lower than that which would suffice to render mercury solid.

The thin aërial fluid called the atmosphere, or commonly the air, is a distinct material substance surrounding the globe, and possessing considerable weight. That the air is actually a material substance, may be easily shown by connecting a thin glass flask, provided with a good stop-cock, with the exhausting tube of an air-pump. The air can in this manner be withdrawn, and the flask will be found to weigh less than before. One hundred cubic inches of air, when perfectly dry, weigh, according to the very careful investigations of Dr. Prout, 31-0117 grains; the temperature of the air being 60 degrees Fahrenheit, and the pressure of the air, as indicated by the barometer, being equal to 30 inches of mercury.

If, instead of air (the oxygen and nitrogen which constitute the atmosphere), an atmosphere of mercury were to envelop the globe, which would have the same weight

of the sea; and if, in like manner, instead of the air, the fluid water were substituted, it would be nearly 34 feet above the level of the sea. Hence, we say that the pressure of the air is equal to a column of mercury 30 inches in height, or to a column of water 34 feet high; or, in other words, whatever extent of surface we have, the pressure of the atmosphere is equal to the pressure or weight of 30 inches of mercury, or 34 feet of water, over a similar surface.

The amount of this pressure, estimated by the extent of surface, is as 14-7 pounds on the square inch, or nearly 15 pounds. In other terms, the weight of air pressing on a square inch is 15 pounds, and the weight of the column of water is also 15 pounds. That is, the column of air whose basis is exactly a square inch, extending from the surface of the globe to the highest or extreme range of the atmosphere (nearly 45 miles), is equivalent to the column of mercury which is only 30 inches in height, or to a weight of 14-7 pounds.

It is this weight, then, which the water has to overcome before it pass into vapour. The greatest pressure of the atmosphere will be at the surface of the earth; and as we ascend in elevation above the sea level, this pressure will gradually decrease, less air being above us, and in a corresponding ratio the volume will be augmented.

By attending to these circumstances, we perceive that when the pressure is lessened, water boils at a lower temperature than 212 degress; and therefore, that we have not merely to consider the temperature to which the water is exposed, but also the amount of the weight of the atmosphere at the time, or the height of the mercury in the barometer tube. For example, at Quito, which is 10,000 feet above the level of the sea, water boils at 194 degrees Fahrenheit, while at Geneva, ebullition begins at 209 degrees, that city being 12 feet above the sea level.

The law, then, as regards the pressure of the atmosphere, simply is, that the boiling temperature is uniformly the same when the barometer is at the same height. If we employ the thermometer of Fahrenheit, it will be found that the boiling point is exactly 212 degrees if the barometer indicate 30 inches; but if the boiling point rise to 213 degrees, then the barometer also will ascend to about 303; and conversely, if it be nearly 211 degrees, the barometer conversely also will fall to about 294. It is obvious, then, from these facts, that the boiling point is an index of the height of the barometer, and, on the other hand, that the height of the barometer will give the point of ebullition according to the thermometer of Fahrenheit, or any other which may be used.

Experimentally, the effect of a diminution of pressure on the temperature at which water boils may be shown by the common air-pump. If a jar of water, at the temperature of 178 degrees, be placed under the large bell receiver, and the air be withdrawn so as to reduce the pressure very speedily, the water will be found to boil at the reduced temperature. The pressure at which this takes place, as measured by the barometer, is equal to half the ordinary weight of the air, or 74 pounds on the square inch. If the barometer be retained in the jar, it will be found to indicate 15 inches when the ebullition takes place. Should the barometer fall lower before the boiling commences, then it will also be noticed that the thermometer points to a lower temperature, corresponding always in an exact ratio.

Steam, or the vapour of water, when produced at the usual pressure of the atmosphere, is commonly deno minated low-pressure, in opposition to that which is formed at a higher pressure than that of the air, and accordingly named high-pressure steam. In common lan guage, however, the term low-pressure steam is applied to the steam which has even a force of several pounds on the square inch, and therefore formed at a temperature

higher than 212 degrees. The steam is in this case condensed in working the engine, and receives this general name because the pressure does not range higher than a few pounds.

ordinary pressure of that body will be exerted on the surface of the water. The water will therefore, as already. noticed, boil when the temperature 212 degrees of Fahrenheit is indicated by the thermometer. But if we now shut the stop-cock, so that there is no longer escape for the steam, the temperature of the water gradually rises, because the heat is continued, and the steam accumulating in the upper part of the boiler, exerts, first on the water, and immediately on the mercury beneath, a force or pressure equal to its increased elasticity. The mercury, is, however, in an open tube, or rather is placed between the extremity of an open tube and the water and its vapour. Accordingly, if the force of this vapour is greater than what is requisite to overcome the pressure of the atmosphere, the mercury will be forced into the tube, and in proportion to the increasing force which it possesses, will the mercurial liquid ascend. In proportion, then, as the heat continues to be applied, the mercury will be seen to ascend in the barometer tube, indicating the force which the steam exerts on the surface of the water in the boiler, while the actual amount of the heat at which the water is passing off into vapour will be shown by the thermometer.

In order to produce steam of greater pressure or force than that obtained by boiling water in the open air, means must be adopted to confine the vapour as it is generated from the water. If we have a stout copper vessel, containing a considerable quantity of water, and provided with stop-cocks which can be properly closed, and then expose it to heat, a quantity of vapour will be disengaged; but as it cannot fly off, all the stop-cocks being closed, it must necessarily, in proportion to its density, compress the fluid below, and proportionately prevent any further escape of vapour. But the heat being continued and increased, vapour will then rise, which in like manner will increase the degree of compression on the water, for the density of the first disengaged vapour will now be increased by this new accession of vapour, and the further formation of vapour will be checked until the heat is again so far increased as to be able to overcome resistance offered by the pressure of the vapours. In this manner vapour or steam, of any degree of elasticity, may be generated from water merely by having a firm and stout vessel capable of bearing great pressure, in which the vapour is to be formed.

The generation of steam in this manner, and the relation between the temperature at which the steam is produced and the pressure upon it, and consequent force or elasicity of the steam, may be illustrated by the apparatus represented in the adjoining cut, fig. 1. A firm copper vessel is procured, sufficient to bear a considerable heat and a great degree of pressure. It is provided with three apertures, as in the figure. The aperture at the sum

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But, as already stated, if the height at which the mercury stands corresponds in a distinct ratio with the temperature, it will be sufficient to ascertain either the one or the other, so as to know both. Suppose the column of mercury has risen nearly 15 inches, or even 60, then we know that the pressure which the steam has is equal to half an atmosphere, as indicated by the mercurial tube, over and above the actual pressure of the atmosphere, so that the whole pressure exactly amounts to an atmosphere and a half. But the thermometer will also have risen, and now will point out a temperature of 230 degrees Fahrenheit-water at that temperature, when converted into vapour, having a force equal to an atmosphere and a half, according to the usual mode of expression. If the heat be still continued, the further ascent both of the mercury in the barometer tube and the mercury in the thermometer will be observed; and when the former stands at 30 inches, the latter will indicate exactly 250 degrees, as may be seen in the diagram. But as 30 inches is equal to one atmosphere, and as the tube was open admitting the pressure of the air, the vapour of water was able to overcome the resistance of two atmospheres; or water under a pressure of two atmo spheres boils at a temperature of 250 degrees Fahrenheit, and the vapour possesses that strength in elastic force.

Suppose the thermometer (T) now stands at 250 degrees (1), and the stop-cock (6) be suddenly turned, an immense volume of steam, formed under the high pres sure, suddenly escapes; the mercury in the tube (1) falls rapidly, and the thermometer also equally descends, until it attains the temperature of 212 degrees. The mercury will fall down to the level it had immediately under the water, and steam will now be produced as under ordinary circumstances. The moment, however, the stop. cock is shut (the heat still being kept steadily applied), the thermometer will begin to rise, and the column of mercury begin to ascend.

mit has a barometer tube, E F, fixed in it, open at both ends, but at the same time perfectly air-tight, so as to prevent all communication between the interior of the vessel and the external atmosphere. The upper extremity of the tube is immediately in contact with the atmosphere, while the lower is very near the bottom of the vessel. In the lower part of the vessel there is a quantity of mercury (m), into which the under extremity of the barometer tube dips. At one side of the vessel an aperture receives a thermometer (T), which is securely fixed, so as to be perfectly air-tight, and introduced obliquely, so that the bulb rests a little above the middle height of the vessel. The other or third aperture (b) is provided with a stop-cock, which admits of being opened or closed at pleasure. The vessel is now to be supplied with water (w), filling it to the middle, and heat is to be applied by a furnace below. It is apparent that if the heat be applied and continued while the stop-cock (b) is open, the air will fill the upper portion of the boiler. and the

The application of the heat may be continued, in a good stout vessel, up to a greater elevation than what is now described, causing the production of steam of a still higher pressure, and, of consequence, greater elastic force, the barometer and thermometer mutually reflecting each other. It is in this manner that the high-pressure steam, as it is ordinarily called, is generated; but in proportion as the higher the temperature is at which it is produced, the greater is the danger to be apprehended from the bursting of the boiler, unless proper precautions are adopted.

The accompanying table gives the correspondence observed between the temperature at which the water boils, the density of the steam generated, and the force it possesses in inches of mercury and atmospheres:—

By this table we observe that the elastic force of the vapour produced from water rises in a rapid ratio above the ordinary temperature of boiling. If, for example, we take the temperature of water at 350 degrees, the specific gravity of the vapour produced, air at 60 degrees being 1, will be 3-6; and it would have a force equal to maintain a column of mercury 270 inches high, or 22 feet 6 inches, if no atmosphere pressed on the mercury; and 240 inches of mercury, if the atmosphere pressed on the fluid in the tube; the total sum of pressure on the square inch being then equal to 132-3 pounds, or corresponding exactly with the weight of nine atmospheres.

Tables have also been drawn up from experiments, illustrating the force of vapour from water at temperatures below the ordinary point of boiling, as in the subjoined :

From all these tables, it is apparent that there is an invariable correspondence always observed between the force of the vapour of steam and the temperature at which it is generated. Hence the one may be given as the rule of the other. For instance, if it is required to know the force with which the steam is working in any machine, the thermometer, which is preserved in a case air-tight, and introduced into the boiler where the steam is generated, will indicate the temperature of the water, or of the steam (for they are always the same; that is, at whatever temperature water boils to afford steam, the steam so produced is of the same temperature). On ascertaining, then, the temperature by a reference to the table, we find the corresponding force of the elastic vapour (the steam). An example will be sufficient to show this most clearly. When the thermometer stands at 212 degrees, and steam escapes from the water, we know it is then able to support a column of mercury 30 inches high; and a column of mercury 30 inches high is equivalent to the pressure of 1 atmosphere. The steam, then, is of the kind called low pressure. If, however, the temperature indicated be 250 degrees, then opposite in the table we find 29.4 pounds pressure on the square inch, and 60 inches of mercury; but as 29.4 pounds is double the weight of the atmosphere on the square inch, and also as 60 inches of mercury is double the height of the column which the air will support, the steam must then be acting with a force equal to 2 atmospheres.

The force with which steam acts increases proportionally much greater than the temperature at which it is generated. If, for example, the pressure be equivalent to 1 atmosphere at 212 degrees, at 250 degrees it will be equal to 2 atmospheres; that is, in the addition of heat equal to 38 degrees of Fahrenheit above 212 degrees; and at 293-7, which is little more than the difference between 212 and 250 degrees, which gives only an increase of 1 atmosphere, the pressure is equal in all to 4 atmospheres, or double that above 250 degrees; and so on, as will be seen in the preceding table.

Mr. Tredgold gives the following rule to ascertain the elastic force of the vapour of water, in inches of mer

cury, at any given temperature of Fahrenheit's ther mometer. To the given temperature 100 is to be added, and the sum divided by 177. The quotient is to be raised to the sixth power, which is the force required. If, for example, the temperature be 307 degrees; to this 100 added gives 407. This, divided by 177, gives 2-3, of which the sixth power is nearly 148, the elasticity of the vapour, in inches of mercury, almost equivalent to 5 atmospheres. This rule, it is to be observed, only refers to the vapour produced from pure water; when it is mixed with a considerable proportion of saline matter, as in the case of sea-water, a different divisor must be adopted, which is to be regulated by the temperature at which the water boils, for the point of boiling varies with the amount of salt in the water. Water saturated with common salt contains about portions of that matter, and its boiling point is about 226 degrees. The divisor to be used in this case is 185 instead of 177, and the elastic force of the steam will then be found not to exceed 113 inches.

The existence of any body in the aëriform state is only a contingent condition of matter; some, called goses, have naturally no tendency to pass into the fluid or solid form; others, however, called vapours, are maintained in the gaseous state by the influence of heat-and on withdrawing it, speedily resume their ordinary condition. Steam belongs to this class of bodies, and on being cooled, immediately condenses or returns to the fluid state. The white cloud produced on steam escaping from the safety. valves of boilers, or from high-pressure engines, is not steam, in the strict acceptation of the word, for steam is invisible, but the water formed by the condensation of the steam in consequence of the cold air with which it now mixes. The extent to which the water expands is variously estimated; but it seems to be very nearly that 1 cubic inch of water becomes 1 cubic foot of steam, or the space occupied by 1 cubic inch of water, when converted into steam, is nearly 1700 times greater-correctly as 1 to 1696.

In the state of vapours, the vapour may be in two distinct and very different conditions; it may be immediately in contact with the water whence it is formed, or it may be in a vessel distinct and separate from all connec tion with the water. In either condition it is a distinct aëriform body, and possesses all these properties peculiar to that class of bodies, it being always understood that the heat is maintained sufficiently high to preserve it in this particular condition, to wit, of vapour. Aeriform bodies, and consequently water, when in the aëriform condition, have a property quite peculiar, denominated their elasticity. This essentially consists in a disposition of all the particles, whereby they have a tendency to recede outwards or fly from the centre, so that they spread themselves out into a more extended area. If, for instance, we have a bladder partially filled with air under the receiver of an air-pump, and then exhaust the air, it will be found, that proportionally as the air is removed from the interior of the receiver, the bladder expands, and finally it will swell, and even be burst, by the expansive force of the air within. Aëriform bodies have a tendency, accordingly, to expand indefinitely, were there not causes which counteract this disposition.

The first of these is the pressure to which they are subject, and the second is the attraction of gravitation, by which all particles of matter are drawn down towards a centre, and which is incessant action. A similar power is also exercised by the application of cold, which diminishes the repulsive tendency. As there is a constant force counteracting this disposition to expand, the elasticity of a gas or vapour is in the exact ratio of this counteracting force. Gases, as they are capable of expansion, so they may also be condensed or diminished in bulk. But in this condensed state, as they then occupy a less space, there necessarily must be an increase

of the density or specific gravity. Thus, if the space occupied by any gaseous body be equal to 100 cubic inches, and the 100 cubic inches weigh 31 grains; on compressing these to one-half, so that they only occupy 50 cubic inches, each cubic inch will obviously contain double the amount of matter it previously had, and therefore, whatever was previously the weight of the cubic inch, it will now be double. But with this increase of density there is an increase of elasticity; for as the elasticity of a gas is directly proportionate to the force which compresses it, and as this force has diminished the bulk by one-half, hence, as the density is doubled, the elasticity is increased in the same ratio. The elastic force of a gas, therefore, is directly in proportion to its density, and in the inverse proportion of its bulk.

On removing the pressure, then, it seems that gaseous matter extends through space, so as to fill up what otherwise might seem a vacuum.

Incidental to the formation of steam, it may be observed that there is a great quantity of heat which disappears on the vapour being formed, and which cannot be discovered by the thermometer, but is again given out when the vapour returns to the state of water. This invariably takes place, and always in a definite proportion.

The most singular and most important practical fact connected with this property is, that whatever be the temperature at which the water is boiled to form steam, the sum of that temperature, and the number of degrees of latent caloric (as the heat which disappears is technicaily named, from the Latin word lateo, to lie hid), is always the same. Suppose the water boils at 212 degrees, and the quantity of latent caloric absorbed be equal to 1000 degrees, the sum of these will be exactly 1212. But if the water boil at 112 degrees (under diminished pressure), the latent caloric will then be 1100, to make up the aggregate sum 1212 degrees; and in like manner, if, under increased pressure, the water be made to boil at 312 degrees, the quantity of latent caloric will only be 930 degrees. Hence steam formed at a low pressure, or at the ordinary temperature of the air, does not require a different amount of fuel that it may undergo this change, than the same vapour generated at 100 degrees | higher, or any other temperature; for the sum of the Latent and sensible heat is always the same-1212 degrees, as measured by the thermometer of Fahrenheit. To convert, accordingly, a given weight of water into steam, the same amount of fuel is required at all temperatures.

The condensation of steam by water may be easily shown by taking a flask with a small quantity of water in it, and, exposing it to a temperature sufficient to produce ebullition, steam will rapidly be formed, and all the atmospheric air expelled. A cork (previously ascertained to fit accurately) is then introduced into the neck of the flask, which is at the same time withdrawn from the fire. The flask, now full of the vapour of water, is introduced into a vessel of cold water with the neck inverted; on the cork being withdrawn, the cold water immediately absorbs the elastic vapour, and is forced in by the pressure of the atmosphere, so as completely to fill the vessel, if it contained nothing but the vapour of steam. The same phenomenon may be observed by acting in a similar manner with the vapour of ammonia, or of muriatic acid (spirits of salt). The application of this additional property of steam, and the mode of bringing it into play, will be specially detailed under the description of the steam-engine. It is here merely cursorily noticed, lest we might seem to overlook one of the most important properties of this fluid.

It is owing to this important property, namely, the great degree to which it can be condensed by cold water, that the production of a vacuum is accomplished, and the steam-engine rendered complete in almost all its parts.

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The chief properties of water, then, as converted into steam or vapour, may be briefly enumerated :-Expan sion-the matter in this new condition of vapour occupying about 1700 times the space it occupied as water; the disappearance of a great amount of caloric, which bears always a definite proportion to the temperature at which the water passes into steam; the exertion or dis play of a definite elastic power, bearing a fixed ratio to the temperature at which it is generated; the natural return of the gaseous fluid to the state of water, either on gradually withdrawing the heat, or on suddenly bringing it in contact with cold water.

Accessory to the consideration of water and its various properties, physical as well as chemical, is the history of the different matters which are employed to give out heat, and to convert it into steam. The consumpt of coal or fuel, of whatever kind it may be, constitutes one of the most serious obstacles in the extension of the steam-engine, and especially in its application to long voyages. The great object is to produce the greatest amount of heat at the least possible expense of fuel. Charcoal, or the substance carbon, is, properly speaking, the principal ingredient in the combustible matters which are usually taken to produce heat. It constitutes the main bulk of coal, of anthracite, a species of coal chiefly found in America. It is found also in great quantities in the matter of saw-dust, tar, &c.

During the process of combustion, the quantity of heat which is disengaged can be precisely determined, as, for instance, by ascertaining how much of a given amount of combustible matter is required to raise the temperature of water from 32 degrees to the boiling point (212 degrees). In a series of experiments made on this subject, Despretz obtained the following results, which are here arranged in a tabular form :

The importance of this subject is sufficiently obvious when we consider the immense number of steam-engine incessantly at work, and the enormous annual consump of coals. In long voyages in steam-vessels, the greater part of the cargo is necessarily composed of coal instead of merchandise, and thereby one of the chief objects of steaming is virtually defeated.

It is here to be carefully noted, that to raise water to the boiling point, and to convert water into steam, do not imply the same thing, though they both imply the applicatior. of heat steadily to the fluid matter. This arises from the great quantity of latent caloric which the steam requires, and which amounts by calculation, as well as by careful experiment, nearly to 1000 degrees of Fahrenheit; that is to say, if it takes a given time, with an equal and