very short time elastic fluid will be formed, in such quantities, as to expel the water from the vessel: on suffering the glass to cool, the elastic matter will be condensed, and will become again fluid.

If a globule of mercury be held in a spoon of platina, over the flame of a lamp, it will be vividly agitated, and will rapidly diminish. This is owing to its becoming elastic, and flying off in gas: and by a very low temperature, which may be artificially produced by mixing together very cold snow and a salt called muriate of lime, mercury may be congealed into the solid form.

Different bodies change their states at very different temperatures. Thus mercury, which is a solid at about 40° below Fahrenheit, boils at about 660°; sulphur, which becomes fluid at 218°, boils at 570°; ether boils at 98°. The temperatures at which the common metals become gaseous, are generally very high, and most of them incapable of being produced by common means. Iron, manganese, platina, and some other metals, which can scarcely be fused in the best furnaces, are readily melted by electricity; and by the Voltaic apparatus a degree of heat is attained, in which platina not only fuses with readiness, but seems even to evaporate.

With respect to the conversion of solids, fluids, or gases, into ethereal substances, the proofs are not of the same distinct nature as those belonging to their conversion into each other. When the temperature of a body is raised to a certain extent, it becomes luminous; and heated bodies not only affect other bodies by direct contact, but likewise exert an influence on them at a distance, which is ascribed to what is usually called radiant heat. One solution of this phenomenon is, that particles are thrown off from heated bodies with great velocity, which, by acting on our organs, produce the sensations of heat or light, and that their motion, communicated to the particles of other bodies, has the power of expanding them; thus if heat, or the force of repulsion, be so increased in an elastic fluid, as to overcome the force of cohesion and gravitation, these particles would move in right lines through free space; and we know of no other effects they could produce, than those of heat and light. It is perhaps in favour of this opinion, that all the different elastic fluids expand equally, when their temperatures are equally raised; and from observations made on the eclipses of Jupiter's satellites, and from other phenomena presented by the heavenly bodies, it appears that the motions of light are equable.

It may, however, be said, that the radiant matters emitted by bodies in ignition, are specific substances, and that common matter is not susceptible of assuming this form; or it may be contended, that the phenomena of radiation do, in fact, depend upon motions communicated to subtile matter every where existing in space.

9. The temperatures at which bodies change their states from fluids to solids, though in general definite, are influenced by a few circumstances, such as motion and pressure. Water, kept perfectly at rest, may sometimes be cooled to 22°, without congelation; but if at a temperature below 329 it be agitated, ice instantly forms. A saturated solution of Glauber's salt, introduced whilst warm into a bottle, from which the pressure of the atmosphere is excluded, remains liquid after cooling, but if the atmosphere be suffered to act upon it, it instantly crystallizes. The boiling point of fluids is still less fixed, than the point of fusion of solids, and is immediately dependent upon pressure. Thus ether will boil readily at the freezing point of water, in the exhausted receiver of an air-pump; and it appears from the researches of Professor Robinson, that in a vacuum all liquids boil about 1459 lower than in the open air. Under pressure, liquids may be heated to a high degree; water in a Papin's digester, may have its temperature raised to 300°, but at the moment the pressure is removed, elastic matter is disengaged with great violence.

10. A peculiar distinction has been made by some authors between permanent elastic fluids, and elastic fluids which are condensible by pressure or cold; but these substances differ only in the degree of the point of vaporization; and steam at 500 degrees of Fahrenheit, there is every reason to believe, would be equally incondensible with air at a range of temperature such as we can command below our common temperatures: and some gases that are permanent under all common circumstances, as ammonia, are condensible by intense cold aided by pressure.

All bodies that boil at moderate temperatures, seem to evaporate, so as to produce a certain quantity of elastic matter in the common state of the atmosphere; and this quantity is greater in proportion as the temperature is high. According to Mr. Dalton, the force of vapour increases in geometrical progression to the temperature, but the ratio differs in different fluids. It is certain

that as the temperature approaches near the point of ebullition, in liquids, the strength of the vapour, i. e. the quantity that would rise in free space, rapidly increases.

In hot, dry weather, it is obvious that there must be much more vapour in the atmosphere than in cold wet weather; and the largest quantity exists in summer and in the tropical climates, when moisture is most needed for the purposes of life; and it appears to be the aqueous vapour in the atmosphere, which, when condensed by the mixture of cold with hot air, or by other agencies. occasioning a change of its temperature, is the cause of dew, mists, rain, and ultimately of springs and rivers.

11. When solids are converted into fluids, or fluids into gases, there is always a loss of heat of temperature, and vice versâ, when gases are converted into fluids, or fluids into solids, there is an increase of heat of temperature, and in this case it is said that latent heat is absorbed or given out. Thus if equal weights of snow at 32° and of water at 172° be mixed together, the whole of the snow is melted, but the temperature of the mixture is found to be 32°, so that 140° degrees of heat are lost. Again, if water be heated in a Papin's digester to 300 degrees, and the valve be raised, a quantity of steam instantly rises, which has the temperature of 212°, and the temperature of the water in the digester is found to be the same, so that a great quantity of heat of temperature is lost in converting the water into steam.

If, when the air is at 20°, a quantity of water be exposed to it in a tall glass, the water gradually cools down to 22° without freezing, but if it be shaken, so as to be converted into ice, the temperature of the ice is found to be at 32°, so that the degree of heat is raised during the act of freezing.

If one part of steam or aqueous gas, at 212o, be mixed with six parts by weight of water at 62°, the whole of the steam will be condensed, and the temperature of the fluid will be about 212°, so that there is an immense increase of the heat of temperature, and 900° may be considered as taken from the steam, and as added to the water.

All the phenomena of these changes may be referred to a simple general law, of which Dr. Black was the discoverer, and which has been most ably illustrated by the researches of Wilke, Watt, Irvine, and Crawford, namely, " that whenever a body changes

its form, its relations to temperature are likewise changed, either increased or diminished;" and many important operations, both artificial and natural, depend upon this law. The knowledge of it, for instance, led Mr. Watt to make his great improvement of the steam-engine, by which the steam is condensed out of the cylinder in which its force is efficient, and fresh gaseous matter introduced without any chance of a loss of its elasticity.

One of the most perfect modes of heating large rooms, and of procuring a uniform temperature for the purposes of manufacture, is by the condensation of steam. By the cold produced in consequence of the evaporation of water in hot climates, congelation is effected; and in the nights in Bengal, when the temperature is not below fifty, by the exposure of water in earthenware pans upon moistened bamboos, thin cakes of ice are formed, which are heaped together and preserved under-ground by being kept in contact with bad conductors of heat. The cold produced by evaporation, is likewise the cause of the formation of ice in Mr. Leslie's elegant experiment, in which sulphuric acid is placed in a vessel upon the plate of an air-pump, and water in another vessel raised above it; the surfaces both of the acid and the water being considerable. When an exhaustion is made, the sulphuric acid rapidly absorbs the vapour rising from the water; fresh vapour is immediately formed, and in a few minutes, if the circumstances are favourable, speculæ of ice are seen to form on the surface of the water.

When aqueous vapour is condensed into fluid in the atmosphere, heat is produced; and the formation of rain, hail, and snow, tends to mitigate the severity of the winter. In the summer, evaporation is constantly tending to cool' the surface. The melting of the polar ice moderates the heat that would arise in the northern regions from the constant presence of the sun during the polar sumAnd the evolution of heat during the congelation of water, prevents too great a degree of cold, and renders the transitions of temperature more slow and gradual.

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12. When the forms of bodies are changed by mechanical means, or when mechanical forms are made to act upon them, there is usually a change of temperature. A piece of caotchouc extended and suffered to contract rapidly by mechanical means, becomes hot; a nail is easily made red-hot by a few well directed blows of the hammer; and by the friction of solids, considerable in

crease of temperature is produced; thus the axle-trees of carriages sometimes inflame.

By strong pressure, fluids even are made luminous, as has been lately shewn by M. Dessaignes.

When an elastic fluid is compressed by mechanical means, its temperature is raised; and when the compressing forces are great and rapidly applied, the effect is such as to cause the ignition of bodies. A machine for setting fire to the tinder of the agaric, by the compression of air, has been for some time in use.

When air is made to expand by removing compressing forces, a diminution of temperature is occasioned. Thus the mercury in the thermometer sinks at the time of the rarefaction of air, by exhausting the receiver of an air-pump.

In the common language of chemistry, it may be said that the capacity of elastic fluids for heat is diminished by compression, and increased by rarefaction; and it is probable that when the volumes of elastic fluids are changed by change of temperature, there is likewise a change of capacity, and on these ideas, it is easy to account for the correspendence between the diminution of the temperature of the atmosphere and its heights; for if it be conceived that the capacity of air rarefied by heat, increases as it ascends, the heat of temperature which was the cause of its ascent, must, at a certain elevation, become heat of capacity: and the higher and more rarefied the air, the more it is removed from the source of heat, and the greater its power of diminishing temperature.

A very curious phenomenon is produced during the action of the fountain of Hiero at Schemnitz in Hungary; the air in the machine is compressed by a column of water, 260 feet high, and when a stop-cock is opened so as to suffer air to escape, its sudden rarefaction produces a degree of cold which not only precipitates aqueous vapours, but causes it to congeal in a shower of snow, and the pipe from which the air issues, becomes covered with icicles. Dr. Darwin has ingeniously explained the production of snow on the tops of the highest mountains by the precipitation of vapour from the rarefied air which ascends from plains and vallies. The chain of the Andes, placed almost under the line, rises in the midst of burning sands: about the middle height is a pleasant and mild climate: the summits are covered with unchanging snows; and these ranges of temperature are always distinct; the hot winds