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It is formed of three concentric cylinders in tin plate. In the central one is placed the body м, fig. B, whose specific heat is required; the other two compartments are filled with pounded ice. The ice of the compartment A, is intended to be melted by the hot body; and that of the compartment B, is merely intended to prevent the radiation of the caloric from the air surrounding the apparatus. Two stop-cocks D and E, are used only to allow the water which proceeds from the melting ice to escape. In order to find the specific caloric of a solid body by means of this calorimeter, we find first the weight of this body m in kilogrammes, we then raise it to a known temperature t by keeping it for some time in a hot bath of water or of oil, or in a current of steam; we then place it quickly in the middle cylinder, instantly put on the lids, and cover them

with ice as shown in the figure. The water which escapes by
the stop-cock D, is then collected, and when its flow is stopped,
its weight P is found in kilogrammes; this weight evidently
shows that of the melted ice. Now, since a kilogramme of ice,
when melting, absorbs 79 units of heat, P kilogrammes will
have absorbed P times 79 units. But this quantity of heat is
necessarily equal to that which has been given out by the
body м, while it was cooling down from to to 0° Centigrade;
that is, to mtc, as already shown; for it must be evident that
in cooling down from to to 0°, a body will give out exactly
the quantity of heat which was absorbed in heating it from 0°
up to to Centigrade. Hence, we have the equation, mtc=
79 P
79 P; and c
If the specific caloric be calculated by
mt
the preceding process, account must be taken of the heat given
out by the vessel in which the liquid is contained.

The method of the ice-calorimeter is affected with several causes of error. The principal is that of a part of the water proceeding from the melting ice remaining attached to that which has not been melted, the weight P cannot be determined exactly. Moreover, the exterior air which enters into the calorimeter by the stop-cocks, increases the quantity of melting ice. These inconveniences are partly remedied by substituting ice-wells for the calorimeter. The name ice-well is applied to a hole made in a piece of solid ice by means of a hot iron, in which we immerse the body whose specific caloric is sought, after having heated it to a known temperature; the edges of the hole are smoothed with the hot iron, and the hole itself is covered with a piece of ice carefully smoothed in the same way, and made exactly to fit it. When the body is the melted ice; and the weight of the latter being found, it is cooled down to zero, it is withdrawn, as well as the water, from only necessary to apply the formula given above.

Specific Calorie of Gases.-The specific caloric of gases is referred to that of water or of air; in the former case, it represents the quantity of heat necessary to raise a given weight of gas by 10 Centigrade, as compared with that which would be necessary to raise the same weight of water by the same quantity; in the second case, the quantity of heat necessary to raise a given volume of gas by 19 Centigrade, as compared with that which would be necessary to raise the same volume of air by the same quantity. In the latter way of considering the specific caloric of gases, we can throughout suppose them at a constant pressure and a variable volume; or, even at a The specific constant volume and a variable pressure. caloric of bodies at a constant volume is always less for the -same gas, than it is at a constant pressure.

The specific caloric of gases, as compared with water, were determined in 1812, by MM. Delaroche and Berard. In doing this, they measured the quantity of heat given out by a known weight of gas to a known weight of water, the former being made to pass through a worm placed in the liquid. They then deduced the specific caloric of the gas by a calculation analogous to that which has been given for the method of mixtures. They also determined the specific caloric of gases, at a constant pressure, in relation to air, by comparing the quantities of heat given out by equal volumes of gas and air to the same weight of water, at the same temperature and atmospheric pressure, during the whole of the experiments. Since these experiments, MM. De la Rive and Marcet, in 1835, applied the method of the reduction of temperature to the determination of the same quantities.

Lastly, the specific caloric of gases, at a constant volume, always with relation to air, has been calculated by M. Dulong by means of the formula employed to determine the velocity of the progagation of sound in different gases. The following table of the specific caloric of different gases is taken from Peschell's Physics :

TABLE OF THE SPECIFIC CALORIC OF GASES.
Specific Calorie,

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with air; with water.

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Application of Specific Caloric.-The knowledge of the specific caloric of bodies affords the means of measuring approximately the most elevated temperatures. Thus, if we place in a medium whose temperature is required, a mass of difficultly fusible metal, as a cylinder of platinum, and allow it to remain so long as to acquire the temperature of the medium; then, if we immerse it in water whose weight and temperature are known, and observe the highest temperature which the liquid reaches, we can thence deduce from formula (C) the temperature T to which the mass of platinum has been raised. Yet the temperature thus obtained will be only approximate; for we have seen that the specific caloric increases with the temperature, and as we do not know that of the platinum a the elevated temperature to which it has been brought in the supposed experiment, we can only substitute for c in the formula an approximate value.

Latent Caloric of Fusion.-We have seen that when bodies pass from the solid to the liquid state, there is an absorption of a quantity of latent heat; and we proceed to show how to measure the quantity of heat absorbed by the unit of weight. This question is resolved by the method of mixtures, on the evident principle that when a body is solidified, it disengages a quantity of heat exactly equal to that which it absorbed during fusion. To take an example: suppose it were required to determine the caloric of fusion in lead. We melt a weight M of this body, and after having taken from it the temperature T, we pour it into a mass of water whose weight m and temperature t are known. This being done, let e represent the specific caloric of lead; the caloric of fusion, that is, the quantity of heat absorbed by the unit of weight in melting, or, which is the same thing, that which is restored at the moment of solidification; and the final temperature of the water heated by the lead. The mass of water being heated from t to 0 degrees, it has absorbed a quantity of heat represented by m (0-1); the mass of lead in cooling down from r to 0, has given out, in one part, a quantity of heat denoted by Me (T-9); and in another, at the moment of solidification, it disengages a quantity of heat represented by мx. We have, therefore, the equation мe (T0) + мx = m (0―t); whence,

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Caloric of Melting Ice.-The knowledge of the caloric of the melting of ice.is interesting on account of its useful applications. It is also determined by the method of mixtures. Thus, let м denote a mass of ice at 0° Centigrade, and m a mass of warm water at to Centigrade sufficient to melt all the ice. Let the ice be thrown into the water, and as soon as it is all melted, let the final temperature of the mixture be noted. If O represents this temperature, the water being cooled down from to Centigrade to 0, has given out a quantity of heat represented by m (t0); and if x represents the calorie of the melting of ice, it has absorbed, in order to melt, a quantity of heat denoted by Mr; but it is heated throughout, after the melting, and its temperature rises from 0° to 0° Centigrade; it has there fore absorbed a quantity of heat denoted м0. We have, therefore, the equation мx+м0m (t0); whence we can deduce the value of x. By this process, and at the same time avoiding with the greatest care all sources of error, MM. La Provostaye and Desains found that the caloric of the melting of ice is 79; that is, a kilogramme of melted ice absorbs, in the state of latent caloric, the quantity of heat necessary to raise 79 kilogrammes of water from 0° to 1° Centigrade, or which is the same thing, 1 kilogramme of water from 0° to 79° Centigrade.

Latent Caloric of Vaporisation.-We have seen that liquids, when converted into vapour, make latent a very considerable

quantity of heat, which is denominated the caloric of elasticity, or the caloric of vaporisation. In order to determine the quantity of heat absorbed then by the unit of weight of different liquids, we adopt as evident, the principle that a vapour which is liquified, gives out a quantity of caloric precisely equal to that which it had absorbed in vaporisation. The method employed in this case is the same as that resorted to in the determination of the specific caloric of the gases in relation to that of water. The apparatus employed in this kind of research is exhibited in fig. 223.

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The vapour is generated in a retort, c, where its temperature is indicated by a thermometer; it then passes into a worm, ss, immersed in cold water. Here it is condensed, and gives out, to the worm and the water in the vessel M, its latent caloric. The water which is produced by the condensation is collected in a vessel, A, and its weight shows the weight of the vapour which has passed through the worm. The thermometers placed in the vessel M, indicate the height to which the temperature of the water has been raised. Now, let м denote the weight of the vapour which was condensed, T its temperature when it entered the worm, and its caloric of vaporisation. Also let m be the weight of the water in which the worm is immersed, including that of the vessel and of the worm reduced to water, t the initial temperature of the water, and its final temperature. In order to measure the heat given out by the vapour, we observe that at the commencement of the experiment the water produced by the condensation comes out at the temperature Cent.; while at the end of it, it comes out at 0° Cent.; whence it follows, that during the whole experiment it comes out at a mean temperature between these, that is, at the temperature of (+0). The weight м of the vapour has, therefore, given out a quantity of heat denoted by M { T-(t0); but at the moment of its liquefaction, it disengaged a quantity of heat represented by M: moreover, the heat absorbed by the cold water, the worm, and the vessel, is m (0-t). We have, therefore, the equation M+MT- (+0)}= m (0-1); whence the value of x may be found. M. Despretz has ascertained by this means, for the caloric of elasticity in the vapour of water at 100° Cent., that is, steam, the number 540; in other words, a gramme of water at 100 Cent. absorbs in its vaporisation the quantity of heat necessary to raise 540 grammes of water from 0° to 1° Cent.; or, which is the same thing, the quantity of heat necessary to raise 1 gramme from 0° to 540° Cent. As 100° Cent. are equal to 180° Fahr., we have this proportion to express the same quantity in degrees of Fahrenheit's thermometer, viz., 100: 180: 540: 972; therefore 972° Fahr. expresses what is called the latent heat of steam, according to M. Despretz. The latent heat of steam is generally reckoned in round numbers at 1000° Fahr.

}

H

SOURCES OF HEAT.

The different sources of heat are the following:-1st, the mechanical sources, viz. friction, percussion, and pressure; 2nd, the physical sources, viz. solar radiation, terrestrial heat, molecular action, change of state, and electricity; 3rd, the chemical sources, viz. combination and combustion; 4th, the

physiological sources, viz. the causes of the production of heat in living beings.

Mechanical Sources.-The friction of two bodies against each other develops a quantity of heat which increases with the pressure and velocity with which they are are rubbed. For example, the axle-boxes of carriage wheels, by their friction against the axle, are frequently heated so much as to take fire. Sir H. Davy partly melted two pieces of ice by rubbing them together in an atmosphere below the freezing point. By boring a mass of bronze under water, Count Rumford found that in order to obtain 250 grammes (about half a pound and an ounce) of filings, the heat developed by the friction was sufficient to raise 25 kilogrammes (about 55 pounds) of water from 0° to 100° Cent. In the tinder-box apparatus, it is by the friction of the steel against the flint that the metallic particles, which are detached, are so heated as to take fire in the air. The heat disengaged by friction is attributed to a vibratory motion thus communicated to the particles of bodies.

It has been supposed that a cast-iron stove could be made so as to heat the whole of the air of an apartment by the single operation of a motion of rotation. This ingenious process has not only been proposed, but even put in practice in some part of America; but it is evident that this could only be useful where moving power was abundant and of very little value, as in certain mountainous regions, where waterfalls are very considerable, and where, free from the action of frost by their velocity and temperature, they can be found at every step. The following is a representation of a fire-place heated by the friction of a mill-stone, and answering the purpose of cooking victuals and warming the house. See fig. 224.

Fig. 224.

When a body is compressed in such a manner that its density is increased, its temperature rises with the diminution of its volume. This phenomenon, which is scarcely sensible in liquids, is manifested in solids to a considerable degree; and in gases, which are extremely compressible, the disengagement of heat is still greater. The powerful development of heat which is produced by the compression of a gas, is shown in the Tachopyrion, or Fire-syringe. This instrument is composed of a thick glass tube or brass cylinder, in which a piston, covered with leather, moves so as to be air-tight. See fig. 225.

At the bottom of the piston is a small cavity, in which is put

a small piece of amadou or tinder. The tube now being full of air, the piston is quickly and forcibly driven to the bottom by the hand; the compressed air then instantly inflames the amadou, which will be seen burning, if the piston be instantly and rapidly withdrawn from the tube. The inflammation of the tinder in this experiment implies at least a temperature of 300° Cent. or 572° Fahr. At the instant of compression, it produces a very sensible light, which was at first ascribed to the high temperature to which the air has been carried; but it has been discovered to arise from the combustion of the oil with which the piston is greased.

It is by the elevation of temperature which it generates, that pressure is sufficient to produce the combination, and consequently the detonation of a mixture of oxygen and hydrogen. The heat disengaged by compression is explained by the closer bringing together of the particles, which causes a certain portion of latent heat to pass into the state of sensible heat. Percussion is also a source of heat, as may be proved by hammering a malleable metal on an anvil. The heat thus disengaged arises not only from the closer bringing together of the particles, but also from a vibratory motion; for lead, which is not increased in density by percussion, is a metal which does not admit of being thus heated.

Physical Sources.-Of all the sources of heat known to us, the most intense is the sun. Of the cause of the heat given out by the sun, we are ignorant: some consider it as a flaming mass liable to immense eruptions; while others say that it is composed of strata which chemically re-act on each other, like the couples in the voltaic pile, and that thus electric currents are generated which are the sources of the solar light and heat. On either hypothesis, the incandescence of the sun would have its limit. Experiments have been made in order to measure the quantity of heat annually emitted by the sun. M. Pouillet, by means of an apparatus which he calls the pyrheliometer (sun-fire-measure), has calculated that if the quantity of heat which the earth receives from the sun in the course of a year, were entirely employed in melting ice, it would be capable of melting a stratum of it round the globe of about 34 yards in depth. Now, according to the surface which the earth presents to the radiation of the sun, and according to the distance between them, the earth receives only 2381000000 part of the solar heat.

If we had the sun always at our disposal, however feeble his rays might become at certain times of the year, we could still, by means of very simple artifices, draw from it a sufficient quantity of heat for the purpose of heating our apartments. Thin and transparent bodies, particularly squares of glass, possess with regard to the solar rays a very singular property which cannot be too extensively known. For instance, if we take a box, see fig. 226, having one of its sides open, close Fig. 226.

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Fig. 225.

Fig. 227.
B

this opening with a square of glass, and then expose it to the the mean temperature at the surface to be 10 Centigrade or sun, the rays will immediately strike against it. These rays 50° Fahrenheit, a depth o c, of about 200 feet will give a temwill not all penetrate into the interior of the box, but the perature of 12° Centigrade or 53°-6 Fahrenheit; a depth ▲ A, greater part of them will pass through the glass and tend to of about 1,500 feet will give a temperature of 25° Centigrade heat the interior. If the opening were not closed by a square of glass, the rays having once reached the interior would go out as freely as they entered, and apart from the influence which they might have on the sides, the temperature of the interior of the box would be the same as that of the exterior. But things happen otherwise in the case of the glass square. The calorific rays have no longer the same facility in going out which they had in going in. The square performs the office of a valve which only opens inwardly. If there be only one square for the rays to pass through, a considerable number of them manage to escape; but if there be a number of squares in succession for the rays to pass through, the more will they be prevented from escaping, and a greater number of rays will remain prisoners. This process will be continually taking place with new rays, and the longer the machine is placed in the sun, the more will they collect, and the more will the heat increase inside. Moreover, the stronger the heat becomes, the greater will be the number of squares necessary to preserve it. But with a sufficient number of squares we can, in a small stove, develop a heat strong enough to cook eggs or to prepare beef-tea. The construction of hot-houses is founded on the observation of these phenomena, the knowledge of which remounts to a remote epoch, but the explanation of which was reserved for modern physics.

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Terrestrial or Central Heat.-The temperature of the interior of the earth is in winter always higher than the temperature of the surface. If we take, for example, the air which has penetrated into caves or still deeper cavities, and make it ascend by proper channels into the interior of houses, we shall be able in fact to mitigate the rigour of the cold, although in a very limited manner. In some mills driven by water, for the purpose of preventing the moving power from freezing, and the motion of the wheels from being stopped, it is usual to pass a stream of water through the earth before it reaches the sluice; this water is heated in its subterranean passage, and prevents the cold water with which it is mixed from freezing in the water-course which supplies the mill. This mode of warming is the most economical that can be imagined; but unfortunately its applications are of too limited an extent. It includes, however, the germ of a principle which should create an immense revolution in our means of warming buildings. It is well known that the farther we dig into the interior of the earth, the more is the temperature found to be elevated. The terrestrial globe, in fact, possesses a heat of its own, which is denominated the central heat. At a depth not very great below the surface, and which varies with the country where the shaft is sunk, we meet with a stratum of earth whose temperature remains the same in all seasons of the year; whence we conclude that the solar heat only penetrates underground to a certain determinate depth. Then, below this stratum, which is denominated the invariable stratum, it is found that the temperature increases at a mean, by 1° Centigrade for every 30 metres deeper that the shaft is sunk; that is, 1° Fahrenheit for every 56 feet. This law of the increase of temperature underground has been verified at great depths in mines and artesian wells. By boring underground to the depth of 3,828 yards, the temperature of the stratum has been found 100 Centigrade, or that of the boiling point of water. The existence of the central heat is confirmed by that of thermal springs and volcanoes. As already observed, the depth of the invariable stratum is not the same at all points on the earth's surface. At Paris, it is 29 yards, and the temperature at this depth is constantly the same, namely, 110.8 Centigrade or 53° 24 Fahrenheit. Fram the preceding data, we can calcu late approximately to what depth it will be necessary to sink a shaft in order to obtain water of a certain degree of heat; and if this water were once brought to the surface, it would be easy to employ it in heating apartments and a variety of other uses, by passing it through pipes to a limited distance. In the following diagram, fig. 227, there is a representation of a section of the interior of the earth to the depth of more than 3,200 feet, showing the interior strata and three artesian wells terminating at different depths, and sending up to the surface water of three different temperatures. Supposing

or 97° Fahrenheit; and a depth B B, of about 3,000 feet will give a temperature of 38° Centigrade or 1000-4 Fahrenheit. In the artesian wells of Grenelle, at the depth of 1,798 feet, the temperature is 27°-8 Centigrade or 82° Fahrenheit.

Various hypotheses have been framed in order to account for the central heat of the globe. That most generally adopted by philosophers and geologists is, that the earth existed at first in a liquid state, by the action of an elevated temperature, and that by radiation the surface was gradually solidified so as to form a solid crust, which is in reality only about 45 miles in thickness, the central mass being still in a liquid state. As to the process of cooling, this can only be extremely slow, on account of the weak conducting power of the terrestrial strata. It is on this account, also, that the central heat does not appear to raise the temperature of the surface of the ground by more than one thirty-sixth part of a degree Centigrade, or onetwentieth of a degree Fahrenheit.

Heat of Molecular Phenomena.-The phenomena of molecular action, such as imbibition and absorption, capillary action, etc., are in general accompanied by the development of heat. M. Pouillet has found that whenever a liquid is poured on a pulverised solid, there is an elevation of temperature which varies according to the nature of the substances. With non-organic matter, such as the metals, the oxides, and the earths, the rise in the temperature is from two to three-tenths of a degree; but with organic matter, such as sponge, farina, starch, liquorice-root, dried membranes, etc., the increase in temperature varies from one to ten degrees. The absorption of gases by solid bodies presents the same phenomenon. M. Dobereiner found that if powdered platinum, such as may be obtained in the state of a chemical precipitate, under the name of Platinum black, be placed in oxygen, this metal will absorb about 250 times its bulk of that gas, and the temperature will then be raised so high as to produce intense combustion. Spongy platinum, which is obtained by precipitating the chloride of platinum with sal-ammoniac (chloride of ammonium), produces the same effect. A jet of hydrogen thrown upon it takes fire by the disengagement of the heat due to the absorption. On this principle is constructed the hydrogen lamp. This apparatus is composed of two glass vessels, fig. 228.

The upper vessel, A, is inserted in the lower vessel B, by means of a ground tubular neck, which is rendered air-tight. At the end of this neck is a mass of zinc, z, immersed in water charged with sulphuric acid. The reaction of the water on the acid and the metal, produces a disengagement of hydrogen, which at first finding no means of escape, drives the water of Fig. 228.

B

Z

Lower Greensand of the chalk formation, you come to a group of rocks called the oolite (pronounced óo-o-lite). The name of this sytem of rocks is derived from two Greek words-wov, 00-on, an egg, and Xilog, lithos, a stone-eggy-stone, or the egg-rock. The rock is called oolite on the ground that, where it was first especially examined, the stone consisted of diminutive egg-like grains, much resembling the roe of a fish; and hence called, sometimes, the roe-stone. Each of these egglike grains has within it a microscopical fragment of sand, or worn coral, as a nucleus, around which, as the grain was rolled along in a stream of limy water, layers of calcareous matter gathered around it, and when it became too heavy for the water, it sank into the calcareous bottom, and formed what is now called oolite.

The oolite group of rocks is sometimes called the Jurassic
System, from the fact that they form the great mass of the
Jura mountains, which separate the north-east of France from
Switzerland. But when the system is called Jurassic, it com-
prehends the lias, on which the oolites rest.

The oolitic system is generally divided into three great
groups, called the Upper, the Middle, and the Lower.
UPPER.. Kimmeridge Clay.
A. Portland Stone and Sand.

MIDDLE.

LOWER.

c. Coral Rag.

D. Oxford Clay.

E. Cornbrash.

F. Forest Marble.

G. Great Oolite, or Bath Stone.
H. Stonesfield Slate.

1. Fullers' Earth.

J. Interior Oolite, or Cheltenham Stone.

the vessel в into the vessel A, until the zinc is no longer immersed in it; the cork of the upper vessel is employed laterally so as to allow the air to escape as the water ascends. A short copper tube, H, fixed on the side of the vessel B, carries a All the oolitic strata develop themselves as you travel from small conical piece, E, having an orifice, above which, in a London to Bath. On that route you find that the different capsule, D, is placed spongy platinum. Now, as soon as the clays and limestones have given rise to high escarpments and stop-cock which closes the copper tube is opened, the hydro-wide valleys. Between each valley covered with clay, the gen is disengaged and burns in contact with the platinum. limestone beds, whether they be of chalk or oolite, form hills Great care must be taken not to present the platinum to the and mountains, which terminate abruptly towards the west, current of hydrogen, until this gas has expelled all the air while from underneath them the clays are seen to rise. This which is in the vessel B, otherwise there would be a strong is represented in fig. 6. detonation arising from the combination of the oxygen and hydrogen contained in the vessel B. The heat produced by the changes in the state of a body have been already investigated under the heads of "Solidification" and "Liquefaction," in a former Lesson; and as to the heat developed by electricity, this must form part of our separate chapter in Physics, under that title.

LESSONS IN GEOLOGY.-No. LIV.
BY THOS. W. JENKYN, D.D., F.R.G.S., F.G.S., &c.
CHAPTER V.

THE CLASSIFICATION OF ROCKS.
SECTION VII.

THE OOLITES.

IMMEDIATELY underneath the Purbeck beds of the Wealden, or, where there is no Wealden, immediately underneath the

I. LITHOLOGICAL CHARACTER OF THE OOLITE.
I. THE UPPER OOLITE.

A. PORTLAND STONE.-The Portland stone is well known as supplying a valuable building material, which is especially adapted to ornamental architecture. Large quarries of it have been opened at Purbeck, in Dorsetshire, and at Fonthill and Tisbury, in Wiltshire. This bed has, in reality, three seams or layers. 1. The uppermost, which is of a yellowish colour, is called by the workmen the cap, and is burned for lime. 2. The middle, which supplies the very best building 3. The lower, which contains the casts of shells, and is, on that account, not so fit for being tooled.

stone.

B. KIMMERIDGE CLAY.-Kimmeridge is the name of a village in a small bay of the isle of Purbeck, where this clay is best developed. The clay is slaty in texture, blue and yellowish in colour, and consists of calcareous or limy matter abounding with vegetable and animal remains.

Some beds of this clay are very much like peat, and so

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