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placed in the warm water,-a portion of caloric is abstracted : we express the effect, by saying that we feel cold.

Q. Are these the uniform causes of these sensations ?

A. Yes: to every substance which feels cold, a portion of caloric is given ; from every substance that gives the sensation of heat, a portion of caloric is received.

Q. What is caloric?

A. The ideal meaning of the term is heat ; and here it would seem to express the cause, as well as the effect produced. But what the substance is, of which the word is the sign, it is difficult to say. By some, caloric is taken as a real and a distinct substance, everywhere diffused ; with others the term stands for some specific and peculiar vibration of the particles of bodies, or the effect supposed to be produced by this.

Q. Have both the parties attempted to support their opinions by facts ?

A. They have: the one by various chemical operations, and the effects produced by combination, condensation, &c.; as the mixing sulphuric acid and water, for example, in a glass phial, by which heat will be evolved. The latter appeal to the effects of friction; as by two pieces of wood, the wheel of a coach, the drag on that wheel, &c.

Q. Seeing that in this case, as in a thousand others, we must believe facts, without being able to comprehend the agent and the agency, can you tell me any uses of caloric?

A. They are many: “ the verdure of the earth, the lustre of the waters, and the freshness of the air, are connected with heat. Without the presence and effects of heat, the earth would be an impenetrable rock, incapable of supporting animal or vegetable life; the waters would be for ever deprived of their fluidity and motion, and the air of its elasticity and utility together.” Caloric appears to be the great instrument employed to control the operation of attraction. By the latter, all substances tend most closely to each other, and would by cohesion form solid bodies: by the former this tendency to adhesion is so counteracted, that

have the state, the consistence, and the proper solidity or fluidity of the bodies around us.

Q. Would caloric by increase cause solid bodies to become fiuid?

A. Yes; and in some cases so completely dissolve them as to render them invisible.

Q. Is it a general law, that bodies become either more solid or fluid as their caloric is diminished or increased ?

A. It is; and generally bodies are expanded by caloric. Q. How may I assure myself of this?

A. Begin with air: take a blaulder; let it contain a small portion of air; tie the neck very close, so that no air can escape ; then place the bladder near a fire, and the air will so expand as to burst it. Pass to liquids : take a Florence flask; put therein water up to the neck; colour it with port wine, red ink, or what else, that it may the more easily be seen; then place the flask in a vessel of hot water, and the expansion will cause the water in the flask to rise up in the neck. Look to solid substances: take an iron rod ; fit it exactly to any ring, mould, or cavity; make the rod very hot, and it will not then pass. The wheelwright, to make his wheel as compact as possible, takes his iron while red hot and in a state of expansion, quickly places it on the wheel, then turns it in the water, that by the contraction of the iron the parts may be brought more closely together. The difference in temperature in tropical regions and ours, and in our own in summer and winter, causes the variation of clocks and watches: by heat the pendulum and spring dilate, by cold they contract ; this causes the vibrations to increase or diminish in a given portion of time. Glass also, and especially when thick, if suddenly exposed to heat, the surface immediately expands, while the inner part remains comparatively unaffected; the expansion is unequal, and the glass cracks. The same effect is produced by suddenly cooling some hot bodies, as throwing cold water on hot iron; the contraction is sudden on the surface, and unequal as to the whole, and it cracks. To this contraction, being first heated, granitic rocks are said to yield and split, in any

Q. In what way does caloric so expand bodies ?

A. The caloric is supposed to be so introduced as to be wedge-like in its effect among the particles of bodies: by this they are removed to a greater distance from each other, they occupy more space, become larger ; if the expansion is unequal, they crack or split.

Q. As caloric is so widely diffused, does it exist every where in the same state ?

A. Caloric is said to exist in two states.
Q. What are they?
A. A state of liberty, and a state of combination.
Q. Be so kind as to explain what you mean.

A. Caloric in combination, or, as it is sometimes termed, latent heat, does not immediately affect the sense of feeling: it is not indicated by the thermometer; it exists as a constituent part of a body, but it may be brought to the state of sensible heat:-while caloric at liberty is the heat that is felt; and in degree is indicated by the thermometer. This is generally denominated free caloric.

Q. What is the operation, and what are the advantages, of free caloric?

A. The particles thereof are supposed to pass continually in every possible direction, and by equal diffusion tend to equalize the temperature of surrounding bodies.

Q. Make this plain by some example.

A. If a heated body be placed in contact with one that is cold, the former parts with a portion of its caloric to the latter, until they both become of an equal, but, compared with what they were before, an intermediate temperature. If a cold body be brought into a warm room, it will take a portion of the caloric which the room contains. This may be felt, if first hot iron be brought into a room, and afterwards ice. What we term fire would be an example of this, if combustion did not continue; the room and the cinders would become equal in temperature.

Q. How does caloric thus tend to equalize the temperature of surrounding objects ?

A. By its radiation from all bodies.

A. Yes ; this is taken to be the fact.
Q. How may I know this?

A. Place the hand on a piece of marble; it feels cold, and takes a portion of caloric from it. But place a piece of ice on the marble, and the part in immediate contact dissolves, by receiving more caloric from the marble than it has given.

Q. Do all surfaces radiate caloric in an equal degree?

A. They do not : black radiates much more than white, and glass more than a polished metallic surface.

Q. By what means can this be ascertained ?

A. Take a cubical tin canister ; blacken one of its sides, cover the second with white paper, on the third place a piece of glass, and leave the fourth surface unaltered. Fill the canister with boiling water; and it may be found by mirrors and a thermometer, that the black side radiates the most heat, and the paper, glass, and tin surface, less in succession.

Q. But generally do different surfaces thus radiate caloric? A. It is supposed that they do.

Q. How then can they continue all to radiate caloric ? Will not the one radiate the whole before the other?

A. No; it is found that the bodies which radiate caloric in the greatest degree, absorb it also in the same proportion.

Q. Can this be made apparent by any example ?

A. Yes: take pieces of cloth of the same dimensions, as black, red, &c., to white; place these differently-coloured pieces on snow, when the sun shines; it will be found that the black piece will soon begin to sink therein, the red next, but the white will remain on the surface. The former absorbed heat, and the snow dissolved ; the latter reflected heat, and the snow continued unaltered.

Q. What is the difference between the radiation and the reflection of caloric?

A. The caloric radiated, is that which properly belongs to the body: the caloric reflected, is that which falls on the surface from other bodies.

Q. The tin surface radiated the least heat; but if I touch

;

Q. In what way does caloric so expand bodies ?

A. The caloric is supposed to be so introduced as to be wedge-like in its effect among the particles of bodies: by this they are removed to a greater distance from each other, they occupy more space, become larger ; if the expansion is unequal, they crack or split.

Q. As caloric is so widely diffused, does it exist every where in the same state ?

A. Caloric is said to exist in two states.
Q. What are they?
A. A state of liberty, and a state of combination.
Q. Be so kind as to explain what you mean.

A. Caloric in combination, or, as it is sometimes termed, latent heat, does not immediately affect the sense of feeling : it is not indicated by the thermometer ; it exists as a constituent part of a body, but it may be brought to the state of sensible heat:-while caloric at liberty is the heat that is felt; and in degree is indicated by the thermometer. This is generally denominated free caloric.

Q. What is the operation, and what are the advantages, of free caloric?

4. The particles thereof are supposed to pass continually in every possible direction, and by equal diffusion tend to equalize the temperature of surrounding bodies.

Q. Make this plain by some example.

A. If a heated body be placed in contact with one that is cold, the former parts with a portion of its calorie to the latter, until they both become of an equal, but, compared with what they were before, an intermediate temperature. If a cold body be brought into a warm room, it will take a portion of the caloric which the room contains. This may be felt, if first hot iron be brought into a room, and afterwards ice. What we term fire would be an example of this, if combustion did not continue; the room and the cinders would become equal in temperature.

Q. How does caloric thus tend to equalize the temperature of surrounding objects ?

A. By its radiation from all bodies.

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