ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXXVII.

(Continued from page 145.)

EVAPORATION AND EBULLITION.

Acceleration of Evaporation.-It has been already observed in
a former lesson, that evaporation is a slow production of vapour
at the surface of a liquid. It is in consequense of spontaneous
evaporation that wet clothes dry in the air, or that an open
vessel full of water is completely emptied of it at the end of a
certain time. It is evaporation which takes place at the sur-
face of seas, lakes, rivers, and the ground, which produces the
vapours that rise in the atmosphere, condense into clouds, and
fall in the form of rain. Four causes have an influence on the
rapidity of the evaporation of a liquid; 1st, the temperature;
2nd, the quantity of the vapour of the same liquid already dif-
fused in the surrounding atmosphere; 3rd, the renewal of the
atmosphere; 4th, the extent of the surface evaporated. As to
the first cause, the increase of temperature accelerates the
evaporation by the excess of the elastic force which it produces
in the vapours. In the case of the second, as the evaporation
of a liquid will be stopped in a space saturated with the same
liquid; and as it only attains its maximum in an air com-
pletely freed from that vapour; it is evident that between
these two extremes, the rapidity of evaporation will vary
according as the surrounding atmosphere is more or less
already charged with the same vapour. As to the third cause,
the renewal of the atmosphere, its effect may be explained on
the same principle. For if the air or gas which envelopes a
liquid be not renewed, it will be quickly saturated, and all
evaporation will then cease. The influence of the fourth cause
is self-evident.

The Laws of Ebullition.-To the rapid production of vapour,
in bubbles of a larger or smaller size, in the same liquid, we
give the name ebullition or boiling. When in a vessel, à liquid,
such as water, is heated at its lower strata, the first bubbles
which make their appearance are only those of the air held in
solution in the water, which are disengaged on the application
of heat. When these are dispelled, then small bubbles of
rapour begin to rise from all the heated points of the sides of
the vessel; these traversing the upper strata of the liquid,
which have a lower temperature, are condensed before they
reach the surface. The formation and condensation of these
first bubbles of vapour are the cause of the singing noise which
commonly precedes ebullition. Lastly, large bubbles begin
to rise and burst at the surface of the liquid, and the continu-
ance of this process constitutes the phenomenon of ebullition,
see fig. 192.
Fig. 192.

Several causes produce a variation in the boiling point of a liquid, viz. 1, the substances held in solution; 2, the nature of the vessel; and 3, the pressure.

1st. When a substance is dissolved in a liquid, and it is either not volatile or less than the liquid in quantity, the ebullition is retarded in proportion to the quantity of the substance held in solution.

Water which boils at 100° Centigrade, boils at the following temperatures, when saturated with the different salts:

Acid solutions present nilar results; but substances held purely in suspension in water, as earthy matters, wood shavings, etc., do not raise the temperature of ebullition. At this point, also, it is important to refer to the experiments of Rudberg, formerly cited under the subject of the Graduation of the Thermometer, experiments in which this philosopher shewed that when the temperature of the ebullition of water is above 100° Centigrade, in consequence of the substances held in solution, the temperature of the vapour which rises from it is still 100o Centigrade, as in the case of pure water, provided the atmospheric pressure be at the standard point, viz. 29-922 inches.

2nd. Gay-Lussac observed that in a glass vessel, water boiled at a higher temperature than in a metallic vessel; a phenomenon which he attributed to the affinity of glass for water. For instance, he found that when distilled water boiled in a brass vessel at the standard temperature and pressure, the water did not enter into the state of ebullition in a glass vessel till the temperature was 101° instead of 100, De pressure being the same; and when the glass vessel was rubbed with concentrated sulphuric acid, or potassa, the temperature of the water rose to 105° and 106 Centigrade detere boiling, Yet a simple fragment of metal placed at the bottom of the vessel was sufficient to restore the temperature of ebullition to 100 Centigrade, and at the same time to dissipate the violent concussions which accompanied the ebullition of the saline or acid solutions in glass vessels. Moreover, in the case of substances held in solution, the temperature of the vapour is not influenced by that which the water assumes iness vessels. At the standard pressure, the temperaturė pour is still 100° Centigrade as in brass vessels. cording to the tables of the elastic force of the vapour nd of steam given in our last lesson, it will be seen 00° Centigrade, the temp、rature at wh. A water

r the standard pressure, the vapour or stem has a recisely equal to this pressure.

nd may be thus state d

bullition only a

A

iple 18 、、、、2rtr {ha

the tension

raise it.

In order to close the apparatus most completely, before the lid is firmly fixed to the vessel, some thin lead plate is inter

Fig. 193.

produce ebullition, must also increase or decrease. In order notwithstanding the elastic force of the steam which tends to to prove that the temperature of ebullition is lowered when the pressure is diminished, we place under the receiver of an air-pump, a vessel containing water at about 30° Centigrade or 86° Fahrenheit, and exhaust the receiver. The liquid will immediately be observed beginning to boil with great rapidity, although in a closed vessel; it is because the vapour is drawn off by the machine as fast as it is produced. In the same manner, in consequence of the diminution of the pressure of the atmosphere at the tops of mountains, water boils below 100° Centigrade. On Mont Blanc, for example, water boils at 84° Centigrade or 1839.2 Fahrenheit. This property has been recently applied in a small apparatus, called the hysometer (from the Greek hysis, water or rain, and metron, measure), which shews the height of the place according to the temperature at which water boils. If the pressure be increased instead of being diminished, ebullition is retarded. Thus, when the pressure is two atmospheres water does not boil till the temperature rises to 121 Centigrade or 2499.8 Fahrenheit. A table of the boiling points of water at different elevations above the level of the sea is here added, in order to impress the mind of the student with the facts now stated.

posed between the edges of the digester and its cover. At the bottom of a cylindrical hole which passes through the cylinder s and the piece o, a small orifice is made in the lid, and is covered by a disk, on which a rod n is made to rest. This rod, which traverses the cylinder s and its base o, presses against the disk by means of a lever a moveable at its extremity a. A weight p, which moves along the lever a a, is employed to act on the rod n with a greater or less force, in proportion as it is nearer to or further from the extremity A, according to the well-known property of the lever. As the load or pressure on the disk can thus be varied, it is regulated so that when the steam in the interior of the vessel has reached a certain tension, say six atmospheres, the disk may be raised, and the steam permitted to escape. In this way the bursting of the apparatus is prevented; and hence the name of this mechanism is derived, viz. the Safety-valve, the beautiful invention of Papin. The digester, being two-thirds filled with water and carefully closed, is heated on a furnace. The water can thus be raised to a temperature far above 100° Centigrade, and the tension of the steam can be carried to a great number of atmospheres, according to the load given to the safety-valve. If the valve be then opened, a jet of steam escapes with a blast and rises to a great height. The water in the vessel, which had not till then entered into the state of ebullit on, now actually boils, and its temperature is reduced to 100° Centigrade.

Confined Vapour-Hitherto we have supposed that the vapours were produced in an indefinite space where they could freely expand. It is only on this condition that ebullition can take place; in a close vessel, the vapours which are roduced not finding any vent, their tension and their density increase as the temperature increases, but the rapid disengagement which constitutes ebullition is impossible. Consequently, while in an open vessel the temperature of a liquid does not exceed that of ebullition, in a close vessel it may rise much beyond that point. The liquid state, however, has a limit, for according to the experiments of M. Cagniard-Latour, if water, alcohol, or ether, be introduced into strong glass tubes, and the latter be hermetically sealed after the air has been expelled by boiling the liquid, it is found that by the application of a sufficient quantity of heat, a period will arrive when each liquid will suddenly disappear, and be transformed into a vapour whose volume will scarcely differ from that of the liquid. Thus, he has found that sulphuric ether is entirely reduced to vapour at 200° Centigrade or 392° Fahrenheit in a space less than that of double of its volume in the liquid state, and that then the tension of the vapour is equiValent to the pressure of thirty-eight atmospheres.

Papin's Digester.-M. Papin, a French physician, who died in 1710, appears to have been the first philosopher who studied the effects of the production of steam in a close vessel. The apparatus called Papin's digester is a bronze cylindrical vessel D, fig. 193, furnished with a cover which can be closely and firmly shut by means of a screw, as shown in the figure,

The apparatus of Papin can be rendered useful in increasing the dissolving power of liquids, by affording the means of raising them to a temperature much higher than their point of ebullition; and this is the reason why it is called the Digester.

The Latent Caloric of Vapour.-According to the second law of ebullition, the temperature of liquids remains stationary during the period of this phenomenon hence we conclude that in vaporisation, as well as in fusion, there is an absorption of a considerable quantity of heat, of which the sole effect is to cause the bodies subjected to its influence to pass from the liquid into the aeriform state; for this quantity of heat does not act on the thermometer, because the vapour which is produced is always at the same temperature as the liquid, or rather at one a little lower than it. There is, therefore, latent caloric, as in the case of fusion, formerly explained, and it is called the caloric of elasticity, or the caloric of vaporisation.

Whatever may be the temperature at which any vapour is produced, there is always the absorption of latent caloric. For example, if we pour on the hand a volatile liquid, such as

ether, we feel a very sensible degree of cold, which proceeds from the caloric of elasticity absorbed by the liquid which is vaporised. Mr. Southern announced the law, that the quantity of heat necessary to vaporise a given weight of water is always the same, whatever may be the temperature at which the vaporisation takes place. This law has not been yet proved experimentally; but it is not in opposition to the theory of vapours as hitherto developed.

The caloric absorbed by vapours may become a source of very intense co'd, capable of solidifying mercury, and even the gases, as has been demonstrated by experiments yet to be noticed. Moreover, the quantity of latent heat absorbed by different liquids during vaporisation is capable of being ascertained by calculation.

LIQUEFACTION OF VAPOURS AND GASES.

Liquefaction of Vapours.-The liquefaction or condensation of vapours is their passage from the neriform to the liquid state. Three causes may operate in condensation; lowering of temperature, compression, and chemical affinity. The two former causes require that the vapours be in the state of saturation, but the latter produces the liquefaction of the most rarefied vapours. Thus, a great number of salts absorb, by condensation, the vapour of water contained in the atmosphere. The vapour which exists in the atmosphere presents, when the temperature is lowered, a particularly curious phenomenon; it does not return immediately to the liquid state, but is transformed into hollow vesicles like soap-bubbles, but extremely small; the water is then said to be in its vesicular state. It is in this state that the vapour of water forms clouds, and that it becomes visible during the of ebullition. At the instant of the condensation of vapours, their latent caloric becomes free, that is, sensible to the thermometer. This is proved by making a current of steam (vapour of water at 100° Cent.) pass into a vessel of water at the ordinary temperature of the atmosphere. The liquid is rapidly heated, and soon reaches the temperature of 100 Cent. It is thus considered that the quantity of het so restored by the condensation of the steam is exactly equal to that which was absorbed in its formation; a fact which appears sufficiently evident.

which contains the liquid to be distilled, and of which the lower part is built in a furnace; 2nd, the head or helm A, which rests upon the cucurbite, and allows the vapour to escape by a lateral neck R; 3rd, the worm c, consisting of a long tin or copper tube, wound in a spiral form, and placed in a vessel filled with cold water; the object of the worm being to condense the vapour by cooling it.

Suppose now that it was required to distil the water of a well or of a river, in order to free it from the salts which it holds in solution, and which are mostly sulphate of lime, carhonate of lime, and chloride of sodium; the cucurbite is twothirds filled with it, and then heated; the water boils, and the steam which arises from it passes through the worm where it is condensed; the water proceeding from this condensation is then delivered into the receiver D. The steam, which is condensed rapidly, heats the water of the vessel which contains the worm; it is therefore necesssary to renew this water continually, otherwise the condensation will not take place. For this purpose, a tube n, constantly supplied with a current of cold water, conducts this water into the lower part of the vessel; whilst the warm water, which is lighter, rises to the

upper part and runs off by a tube m placed at the top of the vessel. The distillation must not be pushed too far, lest the water should contain organic matters which might be decomposed on the hot sides of the cucurbite, and originate volatile products.

Distilled water is perfectly clear, and leaves no residue after its evaporation; but it always contains a little carbonic acid; for this gas existing in all natural waters, is but imperfectly separated from them by distillation. The presence of this gas may be avoided, however, by putting into the cucurbite a certain quantity of lime, which combines with it, and retains it. It is by distillation, in alembics analogous to the preceding, that we extract from wines the alcohol they contain.

Safety-Tubes.-An occurrence which is often produced in the preparation of gases in chemistry, and which is called absorption, consists in this, that when gases are collected over water or over mercury, these liquids enter the apparatus, and render the operation useless. This occurrence arises always from the excess of the pressure of the atmosphere above the pressure or tension of the gas contained in the apparatus. Thus, in fig. 195, suppose a gas, say sulphurous acid, be generating in a flask m, and passing over into a test-glass, A, full of water; so long as the evolution of the gas goes on, its tension is sufficient to overcome the pressure of the atmosphere and of the column of water on; the water of the test-glass cannot rise in the tube, and absorption is impossible. But if

Fig. 95.

the supposition that this liquid has the same density as the
water in the vessel E. Now, as the distance or is less than
the height D H, the air enters by the orifice o before the water
of the vessel rises up to A, and no absorption takes place. The
tube co serves also to prevent explosions. When the pro-
duction of the gas is too rapid, and the tube A D is not suffi-
cient for its generation, the liquid contained in the flask Mм is
forced out and esapes by the tube c, which becomes an issue
for the gas as soon as the level of the liquid falls below the
orifice o.
Another kind of safety-tube, called an S-tube, is represented
in fig. 197. This tube has a bulb, a, containing a certain
quantity of liquid, as well as the branch i d.

Fig. 197.

the tension of the gas decreases, either because its evolution has ceased, or because the flask has been cooled, the exterior pressure becomes the greater, and when the excess of this pressure above the interior pressure surpasses the weight of the column of water co, the water enters the flask, and the operation fails. This occurrence is prevented by means of safety-tubes. These tubes are employed to prevent absorption, by admitting the air into the apparatus, in proportion as the interior pressure decreases. The simplest invention of this kind is an upright tube co, fig. 196, which passes through a cork fixed in the flask M, in which the gas is generated, and immersed a little way in the liquid contained in the flask.

Fig. 196.

When the tension of the gas in the flask a diminishes, the pressure of the atmosphere which acts on the water in the vessel E, tends to force it up the tube D A, to a certain height; but this pressure acting also in the tube or, tends to force down the liquid which is in this tube, to the same extent,

When the tension of the gas in the retort M exceeds the pressure of the atmosphere, the level in the branch id rises higher than that in the bulb a; if the gas has the tension of one atmosphere, the level sinks in the branch id, and as we take care that the height ia is less than bh, as soon as the air admitted by the funnel c enters into the bent part i, it raises the column ia, and enters into the retort before the water of the test-glass rises to b; from this instant, the interior tension is equal to the exterior pressure, and absorption is prevented.

LESSONS IN GEOLOGY.-No. LII.

BY THOS, W. JENKYN, D.D., F.R.G.S., F.G.S., &c.
CHAPTER V.

THE CLASSIFICATION OF ROCKS.
SECTION V.

THE CHALK FORMATION.

You now enter, in the descending order, the secondary rocks, of which the chalk group forms the uppermost or newest strata.

The entire group of rocks, called the Cretaceous formation, is divided into six strata.

1. Maestricht limestone and Faxoe chalk.

2. Upper white chalk, with flints.

3. Lower white chalk, without flints, passing into chalk marl, slightly argillaceous or clayey.

2. Firestone greensand.

[consists of a series or alternations of sands, sandstones, limestones, and clays, with beds of chert, concretions of iron stone, Fuller's earth, and, in some places, fibrous gypsum. Its beds

1. Sand, white, yellowish, or ferruginous, called Shanklin Sands, with concretions of limestone and chert. 2. Land with green particles-silicates of iron. 3. A limestone called the Kentish Rag.

resting on the white chalk with flints, a peculiar bed of calca-
reous rock about 100 feet thick. The fossils found in it are
totally distinct from those of the tertiaries, and a few of them
are found to prevail in bed No. 3, the lower white chalk with-in downward succession are,
out flints; and yet we find in it some races of univalves, shells
which are found only in the tertiary beds. The upper part of
it abounds with corals; and the lower part consists of yellow-
ish limestone, which is extensively quarried for building.
In the Isle of Seeland, Denmark, the sea cliffs present a
yellow limestone resting on bed No. 2, white chalk with
flints. This limestone is formed of corals, and is used for
building stone. Its thickness is unknown. Some portions of
this bed consist of white earthy chalk,

2. The Upper White Chalk, Bed No. 2, is that white earthy limestone, so well known-soft enough for marking and writing, but generally too soft for building stone; and yet the lower it goes the more solid it becomes. It wants only two parts out of a hundred of being entirely carbonate of lime; and, therefore, when burnt, it makes as good lime as the hardest marble, and is used extensively in London and the neighbourhood. Wherever this bed is met with, it is known by being interstratified with flint. These flints are found, sometimes in layers a few inches thick, in continuous black sheets, or more frequently, in nodules, at intervals of two or three feet from each other.

3. The Lower White Chalk, Bed No. 3, is a very deep mass of white chalk, but destitute of flints. The chalk of this bed is harder than that of No. 2. In many places, as about Dover, the chalk of this bed is so solid and firm as to constitute a good building stone, is extensively blasted, and then squared for sea walls and other durable buildings. The abbey of St. Omer, in France, is built of this kind of chalk. In some parts of Yorkshire, and at Havre on the French coast, this bed contains flints.

4. The Firestone Greensand, Bed No. 4, is most frequently called the "Upper Greensand." It had been well if either this bed or No. 6 had been distinguished by some other name than "greensand," for the sake of beginners. As the upper part of this bed forms a coarse calcareous limestone called FIRESTONE, perhaps the term may be allowed as sufficiently distinctive of it.

The bed of chalk without flints passes downward, as you have seen, into a clayey limestone called "chalk marl," and in some places "clunch." Under this clunch are beds of sand abounding with green particles which are silicate of iron, and which give their name to the beds. The sandy particles are united by a calcareous cement, occasionally containing either beds or else nodules of chert and chalcedony.

This formation is very extensive in England. If a man were to travel in somewhat of a zigzag direction from Haldon Hill, west of Exeter, to the neighbourhood of the river Humber in the north of England, he would be in constant company with this bed. In parts of Surrey, as about Reigate, the upper part of it forms a calcareous rock called firestone, and sometimes Merstham stone.

In the cliffs of the Isle of Wight, about Black Gang, this bed is 100 feet thick, having bands of flinty limestone and limy sandstones with nodules of chert. These nodules of chert and chalcedony become more prevalent in the south-western parts of England, as on the Blackdown in Devonshire. At Sidmouth the sea-shore abounds with these pebbles. In the Blackdowns there are quarries which furnish the scythe stones and building materials. This stone, when just exposed in the quarry, is soft and easily tooled and dressed, but it soon hardens by exposure to the atmosphere.

"

The whole of these six or eight beds are called the cretaceous formation. The term 46 than "chalk," for the word "chalk" is limited to the soft cretaceous is more comprehensive white mass of carbonate of lime; but the term "cretaceous comprehends a group of deposits widely dissimilar in litiological character, though agreeing in organic remains, and, on that account, referrible to the same geological epoch, called the "cretaceous." Whether the fossils be imbedded in soft white limestone, or in blue clay, or in loose sand, or in compact stone, they consist of species of the same genera of plants and animals. In the south coast of the Isle of Wight, the entire series of the beds 2, 3, 4, 5, and 6, with the exception of its lowest member, the Kentish rag, are exposed to view.

II. THE ORIGIN OF THE CHALK FORMATION.

1. The chalk, which, to the eye, appears a white soft mass, consists, in reality, of very minute microscopic shell-fish and corals, so exceedingly diminutive that a cubic inch of chalky matter contains, at least, a million of organic remains. Even a blot of whitewash made from chalk exhibits, when examined by a good microscope, a beautiful patchwork of these small shells, which were the calcareous cases of animalcule called foraminifera. All these foraminifera, and still more diminutive infusoria, were once alive in the sea of the cretaceous epoch, and were widely diffused through its waters. When these minute organisms died, their shells were deposited as white mud at the bottom of a very deep sea, and contributed, by accessions for ages, to form the deep beds now called chalk.

Independently of the aid of the microscope, analogy had early suggested to some geologists the probability that chalk, even where every trace of organic structure had disappeared, was the result of the decomposition of shell-fish and coral. First. In the Bermuda Islands there are several lakes or lagoons almost surrounded by reefs of coral, and at the bottom of these lagoons is found a deposit of calcareous mud, soft and white, formed by the death and decomposition of coralline animals, such as the Flustra and Cellepora. Specimens of this mud, when dried, cannot be distinguished from the ancient chalk.

Secondly. Among the coral islands of the Pacific, especially near coral reefs, a soft and white mud is found at the bottom, which has every evidence of having passed through the bodies of the worms that built the coral rocks; and other parts of it through the intestines of fishes. Hence the origin of the coprolites. In the clear waters about those rocks, large shoals of the fish Sparus can be seen feeding quietly on living corals, like cattle on a field of buttercups.

Thirdly. At Faxoe, in Seeland, Denmark, small portions of that remarkable rock consist of white earthy chalk formed by the evident decomposition of corallines.

2. Flints are found sometimes in veins, and sometimes in nodules, from the size of a nut to masses many feet in circumference. The nodules occasionally appear as vertical and diagonal veins, filling up fissures and crevices, and traversing both the chalk rock and the sheets of tabular flint. This fact deserves to be noticed, as it proves that the lower bed of chalk No. 3, had been consolidated and made hard before the superincumbent bed No. 2 had been deposited, and before the streams of siliceous matter had flowed over it.

5. The Gault, Bed No. 5, is a bed of stiff marl, or dark blue clay, with thin layers of red marl, and intermixed with green sand. It is well developed at Folkstone in Kent, and at Black Gang in the Isle of Wight. In the south-east of England this Flints, whether in nodules or in veins, were probably probed is 100 feet thick; and by its organic remains it can be duced by heated water and vapours occasioned by volcanic traced to distant parts of Europe, and to the Alps. This heats below. That the flinty substances were once perfectly formation has, latterly, become famous among agriculturists, soft and fluid, is proved by the sharpness of the moulds and for the extensive beds of phosphates of lime which have been impressions of shells found in them. There would also be in found near Farnham, in Surrey, and elsewhere. These phos- those seas minute animalcula that had siliceous or flinty, phates consist of coprolites or the excrements of fish, and are instead of calcareous, shields or shells, such as sponges, which found most abundant in the gault and the greensand above it. would serve as the base for the aggregation of siliceous matter. 6. The Shanklin Greensand, No. 6, is generally called the 3. The minerals which are found in connexion with the "lower greensand," because of its lying beneath the gault. It chalk formation, such as small pebbles of quartzose sandstone,