increase of density without giving out a portion of its latent caloric*.

You speak of latent caloric; is there any difference in the nature of

caloric?

No: we have reason to believe that caloric is always uniform in its nature; but this term is necessary, because there exist in all bodies two portions of caloric, very distinct from each other. How are these two portions of caloric distinguished?

The one is called sensible heat, or free caloric; the other latent heat, or combined caloric ‡.

when it combines with muriatic acid gas; and this is so considerable, that both take a solid form. Berthollet.

Sulphuric acid and water experience this condensation by mixture. If four parts of the former be mixed with one of the latter, the mixed fluids will quickly acquire a temperature higher than that of boiling water. is necessary to be cautious in making this experiment.

It

If iron filings and sulphur be mixed into a paste with water, a sulphuret of iron will be formed, which decomposes the water and absorbs oxygen so rapidly that the mixture takes fire, even though it be buried under the ground. Encyclopædia.

Mixture does not uniformly produce heat. The mixture of some substances produces an intense cold. But the cause of both effects is easily explained. Whenever substances become more condensed by mixture, heat is evolved; when they expand, cold is produced. The mixture of muriate of lime and ice produces the greatest degree of cold yet known.

How the same substance may exist in a body in two distinct states, may easily be explained by a piece of common bread which has been dipped in water. This bread will contain two portions of water very distinct; one of them was in a state of combination, and formed a constituent part of the bread; the other is only interposed between the particles of the bread, and may again be forced out by pressure.

The difference which there is in the effects of caloric, in the two states in which it exists, may be shown by a variety of experiments. A bar of wrought iron at the lowest temperature we are ever accustomed to use it, contains a large portion of latent caloric; and if it be briskly hammered for some time on an anvil, it will become red hot by the action of the caloric which was in a latent state; and which by the hammering is forced out and exhibited in the form of sensible heat. While chymically combined with the iron, it only tended to give it malleability and ductility; but when converted to free caloric it operates with as much activity as though it had never existed in a latent state.

If a little sulphuric acid be mixed with about an ounce of nitrous acid, and the mixture be poured into oil of turpentine, the whole will burst into flame. This is owing to the compound having less capacity for caloric than these separate fluids; consequently a part of their combined caloric is liberated, and produces the inflammation.

The reverse of this may be shown by hanging a pan of snow over a large The snow will receive a large accession of caloric from the fire witheing at all sensibly warmer. The caloric as it enters becomes chymiombined, and the fire will not in the least alter its temperature, till ole becomes fluid.

What do you mean by free caloric?

Free caloric is the matter of heat disengaged from other bodies, or, if united, not chymically united with them*.

What is latent caloric ?

Latent caloric is that portion of the matter of heat which makes no addition to the temperature of the bodies in which it exist. What substances contain latent caloric?

Caloric in a latent state exists in all substances that we are acquainted with‡.

Do all substances contain the same quantity of latent caloric?

No: caloric combines with different substances in different proportions.

What language do chymists make use of to express the difference in this respect?

One body is said to have a greater capacity for caloric than another.

Is this capacity for caloric uniformly the same in the same bodies?

* Some writers have called the matter of heat when in this state interposed caloric.

We owe to Dr. Black the discovery of latent heat. The train of thought and series of experiments which convinced him of this fact may be seen amply detailed in the preface to his lectures, 34 and following pages. "By this discovery we now see (as his editor expresses it) heat susceptible of fixation-of being accumulated in bodies, and, as it were, laid by till we have occasion for it; and are as certain of getting the stored-up heat, as we are certain of getting out of our drawers the things we laid up in them." -Black's Lectures, by Robison. He might have added, that whenever caloric quits its latent state, how long soever it may have lan do, mant and inactive, it always resumes its proper qualities and character, and affects the thermometer and the sense of feeling as if it had never been latent.

Caloric pervades all bodies; this is not the case with any other substance we know of-not even light.

Caloric lies hid in every thing around us. The Creator knew the continual need we should have of this substance, and he has endowed it accordingly with the property of taking up its resting-place and of combining with all matter, however diversified may be its nature or properties. Caloric is a substance which we are ever in want of; it is therefore deposited on every side, and is ready for every exigency. Various and multiplied as are the means which have been chosen for the promotion of the general good, they are all wise and beneficent, all fully adequate to the end for which they were designed.

§ Caloric as it penetrates bodies frequently forms a chymical combination with them, and becomes essential to their composition. This is always the case when a solid is converted to a liquid, or when a liquid passes to a gaseous state. But if caloric be superadded to a body when it is in a state of saturation, it merely traverses its surface, and passes from it, in the form of sensible heat, to some of the adjacent bodies.

The propriety of this term may be shown to a pupil by dipping a lock of wool and a piece of sponge in water, and directing him to observe how much more water the sponge is capable of taking up than the wool. sponge may be said to have a greater capacity for water than wool has.

Hence

Yes: the same bodies have at all times the same capacity for caloric, unless some change takes place in the state of those bodies.

Can you adduce an instance of a change of this kind?

When liquid substances become solid, they lose in a great measure their capacity for caloric; and, when solid bodies become liquid, their capacity for caloric is proportionately increased+. How does this property of bodies operate?

Whenever a body has its capacity for caloric thus increasedţ,

* The nature of the combination of bodies with caloric was first placed in a clear light by Dr. Black. He discovered that all matter is subject to the following law, viz., that "whenever a body changes its state, it either combines with, or separates from, caloric."

This subject is treated with great perspicuity by Dr. Thomson in the first vol. of his System of Chymistry, to which I refer those who wish to investigate it.

If muriate of ammonia be dissolved in hot water, the temperature of the water will be found to be much lowered by the solution of the salt. When the salt takes again a solid form by crystallization, it will part with the caloric which it combined with in the act of solution, and a rise of temperature will be the consequence.

If when the air is at 22° we expose to it a quantity of water in a tall glass, with a thermometer in it and covered, the water gradually cools down to 12° without freezing, though 10° below the freezing point. Things being in this situation, if the water be shaken, part of it instantly freezes into a spongy mass, and the temperature of the whole instantly rises to the freezing point; so that the water has acquired 10° of caloric in an instant. Now whence came these 10° ? Is it not evident that it must come from that part of the water which was frozen, and consequently that water in the act of freezin gives out caloric?-Dr. Black. Water in a solid state has less capacity for caloric, than it has when in state of fluidity.

The freezing of water, and the cooling of melted lead, may be adduced as familiar examples of the former; and the absorption of caloric in the meltin of salts will sufficiently exemplify the latter. By the solution of some salts water may be deprived of so large a portion of its caloric as to be frozen in the midst of summer. An account of several cheap and powerfel frigorific mixtures may be seen in the Philos. Trans. for 1787, 1788, 1789. See also Watson's Chymical Essays, vol. iii. 139. The heat which is given out during the slaking of quick-lime, escapes from the water in consequence of its changing from a liquid to a solid form by its union with the lime. The same effect is produced in making butter. When the cream changes from a fluid to a solid, a considerable degree of heat is produced.

Oxygenized muriatic acid gas becomes a liquid at a temperature somewhat below 40°, and at 32° forms solid crystals. Ammoniacal gas condenses inte a liquid at-45°. Dr. Thomson, vol. i. 377.

When water is poured upon dry pulverized plaster of Paris, in order to form cornices for rooms, great heat is produced by the mixture. This is owing to the water giving out its caloric of fluidity as it becomes solidified in the plaster.

Whenever caloric becomes active it produces heat; whenever it passes into tate it produces cold.

Ference in the capacity which different bodies have for caloric, is

it requires a larger portion of the matter of heat to raise it to a given temperature, than another body does which has a less capacity for caloric.

Can you exemplify this curious property of matter?

If equal qualities, by weight, of water and mercury, cooled down to the same point, be afterwards separately heated to the heat of boiling water, the water will be found to have required more than three times the quantity of caloric that the mercury did to bring it to that temperature*.

What term is made use of to denote the quantity of caloric thus required?

The portion of caloric necessary to raise a body to any given temperature is called the specific caloric of that body.

Is there any method of ascertaining the specific caloric of different bodies, and comparing the relative capacity of each for caloric?

An instrument called a calorimeter is used for this purpose. The substances to be tried are heated to the same temperature,

owing to one substance having a chymical affinity for caloric superior to that of another. See this fully explained by Mr. William Henry in the fifth volume of the Manchester Memoirs.

I

*This property may be shown inore readily by the following experiments: -Take 1 lb. of water at 100°, and mix it with 1 lb. of water heated to 200°, the mixture will be found to give the exact mean temperature of 150°; but 1 lb. of mercury at 100°, and 1 lb. of water at 200°, will produce a heat much higher than the mean temperature; which shows that mercury has not so great a capacity for caloric as water has; consequently it raises its temperature instead of chymically combining with it.

A metal plunged into an equal weight of water of a higher temperature, gains more degrees of thermometric heat than the water loses; and this takes place, in different proportions, for each species of metal. Berthollet.

Whenever two different kinds of substances of different temperatures are mixed, the capacity of each for caloric may be known by observing the temperature of the mixture; for the capacity of each will be in the inverse ratio. of the change of temperature. But it is necessary, in order to justify this calculation, as Fourcroy has remarked, that the bodies themselves should not act chymically upon each other; and it is also necessary to prevent a portion of their caloric from being carried off by the vessels in which the experiment is made.

† This term is always used in a comparative sense, expressive of the relative portions of caloric contained in equal weights or measures of different bodies at the same temperature, or the comparative quantity of caloric which can produce the same effect. Thus, if the specific caloric of mercury be said to be 1, that of water may be said to be 3, as noted in an experiment just related.

This instrument was first suggested by M. Laplace, and contrived by Lavoisier. A drawing of the machine, with an accurate description of it, may be seen in Lavoisier's Elements. Though this instrument be capable of measuring what is called the specific caloric of bodies, no method has yet been discovered of ascertaining the absolute quantity which bodies contain. It is therefore unknown at what point a thermometer would stand, if it were plunged into a substance entirely deprived of caloric.

and then placed in this machine surrounded with ice*. By observing how much ice each of them melts in cooling down to a given point, the specific caloric which each of them contained is determined+.

What do call the instrument which is in common use to measure you the temperature of bodies?

It is called a thermometert. It consists of a glass tube contain ing a portion of mercury, with a graduated plate annexed to it The tube is hermetically sealed, to preserve it from the pressure of the atmosphere§.

Do you understand how a thermometer is affected by the temperatur: of bodies?

When a thermometer is brought in contact with any substance, the mercury expands or contracts till it acquires the same tem perature; and the height at which the mercury then stands in the tube, indicates the exact temperature of the substance to which it has been applied ]].

*Ice has the property of absorbing all the caloric with which it comes in contact, without communicating any part of it to the surrounding bodies till the whole of the ice is melted; therefore the caloric specific of bedies may eas ly be calculated by its means.

Si Isaac Newton talks of boiling water being three times as hot as the blood in the human body. He imagined the freezing point to be the real zero, below which there was no heat. Later experiments have shown that substances may be cooled many degrees below the freezing point on Fahrenheit's scale. At Kamtschatka the atmosphere has been known to be 40 de grees colder than the zero of our thermometers.

The thermometer was invented about the beginning of the 17th centu ry; but it was improved, and rendered useful, by Mr. Boyle and Sir Isaa Newton. 1

Thermometers are made by putting mercury into small glass tubes with bulbs, and heating these bulbs till the mercury boils. This ebullition ex hausts the tubes of air, and they are hermetically sealed while the mercury is boiling; which preserves the vacuum. They are afterwards graduated by

a correct scale.

For very delicate experiments air thermometers are used, in which, as the air is expanded or contracted, a coloured liquor is made to fall or rise, which marks the degree of expansion, and consequently the variation of temperature. They are called thermoscopes.

§ Mercury, though a metal, has so great an attraction for caloric, that i absorbs sufficient to keep it in a fluid state in the common heat of the atmos phere. Owing to this affinity for caloric it expands very readily by every addition of the matter of heat. It is also equally affected by equal incre ments of heat at every temperature between its freezing and boiling points Hence it is the most proper substance for thermometers.

To measure the de rees of heat in high temperature, Mr. Wedgwood com trived a very useful instrument which he called a pyrometer, a description c which may be seen in the 72d volume of the Philosophical Transactions Since Mr. Wedgwood's death, the method of making the pieces of clay f these pyrometers has been lost.

||Fahrenheit's thermometer is universally used in this kingdom. In it th