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ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XLII.

(Continued from page 229.)

CALORIMETRY.

THE object of Calorimetry is to measure the quantity of heat which bodies give out or absorb when their temperature is raised or lowered by a given number of degrees, or when they change their state. We cannot measure the absolute quantity of heat lost or gained by a body, but only the relative quantity; that is, the ratio between this absolute quantity and that which a body gives out or absorbs in producing a given effect. Among the French philosophers, the unit of heat, which is called caloric, is assumed to be the quantity of heat necessary to raise the temperature of a kilogramme of water from 0° to 1o Centigrade. Among ourselves, the unit of heat has been assumed to be the quantity of heat necessary to raise one pound of water from the boiling point to 1° Fahrenheit above that point, that is, from 32° to 33° Fahrenheit. In what follows, we shall adhere to the French unit, as being the most convenient, especially in reference to the Centigrade thermometer; and as the experiments on calorimetry have been chiefly conducted by French philosophers.

Specific Caloric.-The specific caloric, or as it has been termed the caloric capacity of a body, is the quantity of beat which it absorbs when its temperature is raised from 0° to 1° Certigrade, as compared with that which a weight of water equal to that of the body would absorb in the same circumstances; this is in fact the same thing as taking the specific caloric of water for unity. It is easily proved that all bodies have not the same calorific capacity. If we mix, for instance, a kilogramme of mercury at 100° Centigrade with a kilogramme of water at 0° Centigrade, we shall find that the temperature of the mixture is only about 3° Centigrade. This shows that though the mercury is cooled down through 97° Centigrade, the quantity of heat which it has lost only heats the same weight of water up to 3o Centigrade. The water, therefore, which is equal in weight to the mercury, absorbs about 33 times more heat than the mercury in the production of the same degree of temperature.

Three methods have been employed in the determination of the specific caloric of bodies: the method of melting ice; the method of mixtures; and the method of the reduction of temperature, in the latter of which the specific caloric of a body is calculated, according to the time which it requires to cool it down from a given number of degrees. We proceed to show how the quantity of heat absorbed by a body is determined, when its mass and specific caloric are given, and its temperature is raised a certain number of degrees. Let m denote the weight of a body in kilogrammes, c its specific caloric, and its temperature. The quantity of heat necessary to raise a kilogramme of water from 0° to 1° Centigrade being assumed as unity, it will require m of these units to raise a weight of m kilogrammes of water from 0° to 1° Centigrade; and to raise the latter from 0° to to Centigrade, it will require t times as much, viz. mt. This being the quantity of heat necessary to raise m kilogrammes of water from 0 to Centigrade, its specific caloric being 1, it is evident that for a body of the same weight whose specific caloric is c, it will require e times m t, or mtc. Whence it follows, that when a body is heated from 0° to to Centigrade, the quantity of heat which it absorbs may be represented by the product of these three quantities-its weight, its temperature in degrees, and its specific caloric. A similar expression may be easily formed for the quantity of heat, according to Fahrenheit's scale, the pound being the unit of weight.

Method of Mixtures. In order to calculate the specific caloric of a solid or a liquid body by the method of mixtures, we weigh it and raise it to a known temperature, which is determined, when the body is a solid, by keeping it for a certain time in a current of vapour at 100 Centigrade; we then immerse it in a mass of cold water of which the weight and the temperature are also known. From the quantity of heat which the body imparts to the water, we at once deduce its specific caloric. Thus, let м represent the weight of the body;

⚫ A kilogramme is about 2-2 pounds Avoirdupo's.

VOL. V.

T, its temperature at the moment when it is immersed in the liquid, and c its specific caloric. Also, let m be the weight of the cold water, and t its temperature. Next, let m' be the weight of the vessel which contains the water, c' its specific caloric, and t its temperature, which is evidently the same as that of the water. The vessel employed is generally a small cylinder made of silver or brass, with thin and polished sides. As soon as the hot body is immersed in the liquid, the temperature of the latter is raised, and if represents the highest temperature which it reaches, it is plain that the body is cooled has consequently lost a quantity of heat which is measured down by a number of degrees denoted by (T0), and that it by the expression Mc (T0). The water and the vessel, on the contrary, are heated up by a number of degrees denoted heat denoted by m (0-1) and m'e' (0-1), because the by (0-1), and they have absorbed respectively quantities of specific caloric of water is unity. Now, the quantity of heat given out by the hot body is evidently equal to the sum of the quantities of the heat absorbed by the water and the vessel; therefore, we have the equation Me (T0) = m (0 − t) + when the specific calorice of the vessel is known. If it be m' c′ (0 — t) (A); whence, it is easy to deduce the value of c unknown, we must first find it by immersing in the water a body of the same material as the vessel, and thereby obtaining the specific caloric required. The preceding equation then takes the form

Mc' (T0) = m (0 — t) + m' c′ (0 — t) (B); that is, the equation only contains the unknown quantity e'. The specific caloric of the vessel being thus determined, the equation (A) is resolved by making (t) in the second member a common factor, and it becomes M C (T — 0) = (m + m'e') (0-1) (C); whence, by dividing both sides by м (T-6),

we have

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(m + m'c') (0 — t,) M (T- 0) Or, by putting me, in which μ denotes the weight of the water which would absorb the sanie quantity of heat as the vessel, the formula (D) may be written thus, (m + μ) (0. t) M (T 0)

c=

(E.)

In this formula, the value of μ is expressed by saying that the vessel reduced into water.

In order to give the method of mixtures that degree of precision which it requires, the heat absorbed by the glass and the mercury must be taken into account. In order to estimate the loss of heat arising from radiation in the preceding process, a primary experiment is made with the body whose specific caloric is sought, with the view of ascertaining approximately the number of degrees by which the temperature of the water and of the vessel must be raised. Supposing, for example, that this number was 10° Centigrade, we cool down the water and the vessel to half this number, oelow the temperature of the surrounding air; we then proceed to the actual experiment required. The temperature of the water being then raised sensibly by 10° Centigrade, it follows that the vessel whose temperature was at first 5° Centigrade below that of the surrounding air, is at the end of the experiment 6o Centigrade above it. Compensation has, therefore, taken place between the loss and the gain of heat which arose from radiation during the experiment. M. Regnault has calculated, by the method of mixtures and by that of the reduction of temperature, the specific caloric of a great number of bodies. The following are the numbers which he obtained, by the former method, for those bodies which are most frequently required in the arts, for the temperature between 0 and 100° Centigrade. TABLE OF THE MEAN SPECIFIC CALORIC OF SUBSTANCES, ACCORDING TO M. REGNAULT. Substances.

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Specific Caloric.
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0.42590

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0.24111

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with ice as shown in the figure. The water which escapes by
the stop-cock D. is then collected, and when its daw 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 giver out by the
body м, while it was cooling down from 3 to 02 Cantigrade;
that is, to mt c, as already shown; for it must be evident that
in cooling down from 9 to 0°, a body will give out exactly
the quantity of heat which was absorbed in heating it from Ga
up to to Centigrade. Hence, we have the equation, mtc=
79 r
in i

The numbers given in this table represent the mean specinc caloric of bodies, between 1° and 100° Centigrade. It appears, however, from the labours of MM. Dulong and Petit on heat, that the specific caloric increases with the temperature of the bodies. That of metals, for example, is greater between 100° and 200 Centigrade than between 0 and 100° Centigrade, and greater still between 2000 and 300° Centigrade. Thus, to raise the temperature of a bedy from 2009 to 250 Centigrade, requires more heat than to raise it from 100° to 160° Čentigrade; and again, more from 100 to 150° Centigrade than from C° to 50° Centigrade.

Method of Melting Ice.-This method is founded on the principle of the latent caloric absorbed by melting ice, a quantity of heat which, as we shall soon have occasion to remark, is about 79 units for kilogramme of ice. The apparatas employed in this method was invented by MM. Lavoisier and Laplace, and is denominated the calorimeter of ice. The exterior view of this apparatus is represented in fig. 221, and a vertical section in fig. 222.

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79 P; and c

If the specific caloric be calculated by

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 meited, the weight P cannot be determined exactly, Moreaver, the exterior air which enters into the calorimeter by the stop-cocks, increases the quantity of melting ice. These inconveniences are partly remedied by substi tuting 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 hor 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 cooled down to zero, it is withdrawn, as well as the water, from the melted ice; and the weight of the latter being found, it is only necessary to apply the formula given above.

Specific Calario 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 1° 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 1° 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 constant volume and a variable pressure. The specific caloric of bodies at a constant volume is always less for the same gas, than it is at a constant pressure

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The specific caloric of gases, as compared with water, were determined in 1812, by MM. Delaroche and Berard. in doing this, iney measured the quantity of neat 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 aii. by comparing the quanti ties of neat given out by equal volumes of gas and air to the same weight of water, at the same temperature and atmos pheric 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.

It is formed of three concentric cylinders in tin plate. In the central one is placed the body M, 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 meiting 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

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.

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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. aud 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 6 which the quid reaches, we can thence deduce from formula (C) the temperature r 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 bedies 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 inethod 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 of this body, and after having taken from it the temperature we pour it into a mass of water whose weight m and Lem perature t are known. This being done, et c represent the specific caloric of lead; a 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 (t); the mass of lead in cooling down from tof, has given out, in one part, a quantity of heat denoted ye(); and in another, at the moment of solidification, it disengages a quantity of heat represented by Mr. eve, therefore. the equation (r) + Mîm (8 We hence,

x=

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239 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 puit of weight of differ ent liquids, we adopt as evident, the principle that a vapour which is liquified, gives out a quantity ot 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 calcric of the gases in relation to that of warer. The apparatus employed in this kind of research is exhibited in fig. 223.

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is indicated by a thermometer; it then passes into a worm, ss, The vapour is generated in a retort, c, where its temperature 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 cf the vapour which has passed through the worm, placed in the vessel M, indicate the height to which the temThe thermometers perature of the water bas been raised. Now, let denote the weight of the vapour which was condensed, r its temperature when it entered the worn, and x ita caloric of vaporisation. Also let m be the weight of the water in which the worm is immersed, including that of the vessel v and of the wor reduced to water, the initial temperature of the water, and its final temperature. In order to measure the heat giver out by the vapour, we observe that at the commencement of the experiment the water produced by the condensation comes ont at the temperature Cent. while at the end of it, i 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 (t). The weigh. M of the vapour has, therefore, given out a quantity of heat denoted by M ; but at the moment of its Caloric of Melting Ice.-The knowledge of the caloric of the liquefactior, it disengaged a quantity of heat represented by melting of ice.is interesting on account of its useful applica-M: moreover, the heat absorbed or the cold water, the tions. It is also determined by the method of mixtures. Thus, worm, and the versel, is (ei). We hare. therefore, the let и denote a mass of ice a: 0 Centigrad, and m a mass o Farm water at 19 Centigrade sufficient to melt all the ice. equation мz+ { - } (¿ - |-· α) } == in (@t, whence the Let the ice be thrown into the water, and as soon as it is all value of may be found, M. Desprez has ascertained by melted, let the final temperature of the mixture be noted. In this means, for the caloric of elasticity in the vapour of water represents this temperature, the water being cooled down at 100° Cent., that is, seam, the number 610; in other words, from to Centigrade to 0. has given out a quantity of heat repre- a gramme of water at 100 Cent, absorbs in its vaporisation sented by m(0); and if a represents the caloric of the melt- the quantity of hear acessary to raise 540 grammes of water ing of ice, it has absorbed, in order to melt, a quantity of heat fram 0° to 12 Cens,; or, which is the same thing, the quanti enoted by Ma; but it is heated throughout, after the melting, of heat necessary to rise gramme from 0 to 540 Cent, and its teraperature rises from 0° to 9° Centigrade, it has there As 100° Cent are equel to 180° Fahs, we here this proportion fore absorbed a quantity of heat denoted me. We have, there- to express the same quantity in degrees of Fahrenhei's the.. fore, the equation + M0m (t0): whence we can mometer, viz., 100: 180:: 540972. herefore 972° Fah deduce the value of x. By this process, and at the same time expressee what is called the lavent eat of stean, according to avoiding with the greatest care all sources of error. MM. La M. Despretz. The latent heat of steam is generally reckone Provostaye and Desains found that the calcric of the melting in rounc numbers at 1000 Fabr. 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 thing, kilogramme of water from 0° to 19° CentiLatent Caloric of Vaporisation. We have seen that liquids, when converted into vapour, make latent a very considerable

the same

grade.

- J

SOURCES OF HEAT.

The different sources of heat are the following:-1st, the
mechanical sources. viz
triccion perenssion, and pressure;
molecular action, change of state, an exceety; 3rd, the
2nd, the physical sources, viz. ser adiation, terrestrial hea,
chemical sources, viz. combination and combustion; 4th, the

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

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,

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.

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 annuelly 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 38100000 part of the solar heat.

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

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. 225.

Fig. 226.

this opening with a square of glass, and then expose it to the sun, the rays will immediately strike against it. These rays will not all penetrate into the interior of the box, but the greater part of them will pass through the glass and tend to 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.

the mean temperature at the surface to be 10° Centigrade or 50° Fahrenheit, a depth cc, of about 200 feet will give a temperature of 12° Centigrade or 53°-6 Fahrenheit; a depth A A, of about 1,500 feet will give a temperature of 25° Centigrade

Fig. 227.

[graphic]

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° Centi- Heat of Molecular Phenomena.-The phenomena of molecular grade for every 30 metres deeper that the shaft is sunk; that action, such as imbibition and absorption, capillary action, etc., 5,1° Fahrenheit for every 56 feet. This law of the increase are in general accompanied by the development of heat. M. of temperature underground has been verified at great depths Pouillet has found that whenever a liquid is poured on a pulin mines and artesian wells. By boring underground to the verised solid, there is an elevation of temperature which varies depth of 3,828 yards, the temperature of the stratum has been according to the nature of the substances. With non-organic found 100° Centigrade, or that of the boiling point of water. matter, such as the metals, the oxides, and the earths, the The existence of the central heat is confirmed by that of ther-rise in the temperature is from two to three-tenths of a degree; mal springs and volcanoes. As already observed, the depth of but with organic matter, such as sponge, farina, starch, the invariable stratum is not the same at all points on the liquorice-root, dried membranes, etc., the increase in tempearth's surface. At Paris, it is 293 yards, and the temperature rature varies from one to ten degrees. The absorption of gases at this depth is constantly the same, namely, 11.8 Centigrade by solid bodies presents the same phenomenon. M. Dobeor 539-24 Fahrenheit. Fram the preceding data, we can calcu- reiner found that if powdered platinum, such as may be late approximately to what depth it will be necessary to sink obtained in the state of a chemical precipitate, under the name shaft in order to obtain water of a certain degree of heat; of Platinum black, be placed in oxygen, this metal will absorb and if this water were once brought to the surface, it would about 250 times its bulk of that gas, and the temperature will easy to employ it in heating apartments and a variety of then be raised so high as to produce intense combustion. biber uses, by passing it through pipes to a limited distance. Spongy platinum, which is obtained by precipitating the chlo is the following diagram, fig. 227, there is a representation of ride of platinum with sal-ammoniac (chloride of ammonium), section of the interior of the earth to the depth of more produces the same effect. A jet of hydrogen thrown upon it 1 3,200 feet, showing the interior strata and three artesian takes fite by the disengagement of the heat due to the absorp surface water of three different temperatures. sus terminating at different depths, and sending up to the tion. On this principle is constructed the hydrogen lamp. Supposing This apparatus is composed of two glass vessels, fig. 228.

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.

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