The solidifying temperature of the bodies above tallow, in the table, is usually called their freezing or congealing point; and of tallow, and the bodies below it, the fusing or melting point. Now, though these temperatures be stated, opposite to some of the articles, to fractions of a thermometric degree, it must be observed, that various circumstances modify the concreting point of the liquids, through several degrees; but the liquefying points of the same bodies, when once solidified, are uniform and fixed to the preceding temperatures.

Water, all crystallisable solutions, and the

three metals, cast-iron, bismuth, and antimeng expand considerably in volume, at the instant of solidification. The greatest obstacles cannot resist the exertion of this expansive force. Thus. glass bottles, trunks of trees, iron and lead pipes, even mountain rocks, are burst by the dilatation of the water in their cavities, when it is converted into ice. In the same way our pavements are raised in winter. Major Williams of Quebec burst bombs, which were filled with water and plugged up, by exposing them to a freezing cold. The beneficial operation of this cause is exemplified in the comminution or loosening the texture of dense clay soils, by the winter's frost, whereby the delicate fibres of plants can easily penetrate them.

There is an important circumstance occurs in the preceding experiments on the sudden congelation of a body kept liquid below its usual congealing temperature, to which we must now advert. The mass, at the moment its crystallisation commences, rises in temperature to the term marked in the preceding table, whatever number of degrees it may have previously sunk below it. Suppose a globe of water suspended in an atmosphere at 21° Fahrenheit; the liquid will cool and remain stationary at this temperature, till vibration of the vessel, or contact of a spicula of ice, determines its concretion, when it instantly becomes 11° hotter than the surrounding medium. We owe the explanation of this fact, and its extension to many analogous chemical phenomena, to the sagacity of Dr. Black. His truly philosophical mind was particularly struck by the slowness with which a mass of ice liquefies when placed in a genial atmosphere. A lump of ice at 22° freely suspended in a room heated at 50°, which will rise to 32° in five minutes, will take 14 times 5, or seventy minutes to melt into water, whose temperature will be only 32°. Dr. Black suspended in an apartment two glass globules of the same size alongside of each other, one of which was filled with ice at 32°, the other with water at 33°. In half an hour the water had risen to 40°; but it took ten hours and a half to liquefy the ice, and heat the resulting water to 40°. Both these experiments concur, therefore, in showing that the fusion of ice is accompanied with the expenditure of 140° of calorific energy, which have no effect on the thermometer. For the first experiment tells us that 10° of heat entered the ice in the space of five minutes, and yet fourteen times that period passed in its liquefaction. The second experiment shows that 7° of heat entered the globes in half an hour; but twenty-one half hours were required for the fusion of the ice, and for heating of its water to 40°. If from the product of 7 into 21 147, we subtract the 7° which the water was above 33°, we have 140° as before. But the most simple and decisive experiment is to mingle a pound of ice in small fragments with a pound of water at 172°. Its liquefaction is instantly accomplished, but the temperature of the mixture is only 32°. Therefore, 140° of heat seem to have disappeared. Had we mixed a pound of ice-cold water with a pound of water at 172°, the resulting temperature would have been 102°, proving that the 70°

which had left the hotter portion, were manifestly transferred to that which was cooler. The converse of the preceding experiments may also be demonstrated; for on suspending a flask of water, at 35° for example, in an atmosphere at 20°, if it cool to 32° in three minutes, it will take 140 minutes to be converted into ice of 32°; because the heat, emanating at the rate of 1° per minute, it will require that time for 140° to escape. The latter experiment, however, from the inferior conducting power of ice, and the uncertainty when all is frozen, is not susceptible of the precision which the one immediately preceding admits. The tenth of 140 is obviously 14; and hence we may infer, that when a certain quantity of water, cooled to 22°, or 10° below 32°, is suddenly caused to congeal, 1-14th of the weight will become solid. We thus understand how the thaw which supervenes after an intense frost, should so slowly melt the wreaths of snow and beds of ice, a phenomenon observable in these latitudes from the origin of time, but whose explanation was reserved for Dr. Black. Indeed, had the transition of water from its solid into its liquid state not been accompanied by this great change in its relation to heat, every thaw would have occasioned a frightful inundation, and a single night's frost would have solidified our rivers and lakes. Neither animal nor vegetable life could have subsisted under such sudden and violent transitions.

Drs. Irvine, father and son, to both of whom the science of heat is deeply indebted, investigated the proportion of caloric disengaged by several other bodies in their passage from the liquid to the solid state, and obtained the following results:

The quantities in the second column are the degrees by which the temperatures of each of the bodies, in its solid state, would have been raised by the heat disengaged during its concretion. An exception must be made for wax and spermaceti; which are supposed to be in the fluid state, when indicating the above elevation.

Dr. Black supposed that the new relation to heat which solids acquire by liquefaction was derived from the absorption, and intimate combination, of a portion of that fluid, which, thus employing all its repulsive energies in subduing the stubborn force of cohesion, ceased to have any thermometric tension, or to be perceptible to our senses. He termed this supposed quantity of caloric, their latent heat; a term very convenient and proper, while we regard it simply

as expressing the relation which the calorific agent bears to the same body in its fluid and solid states. To the presence of a certain portion of latent or combined heat in solids, Dr. Black ascribed their peculiar degrees of softness, toughness, malleability. Thus we know that the condensation of a metal by the hammer, or under the die, never fails to render it brittle, while, at the same time, heat is disengaged. Berthollett subjected equal pieces of copper and silver to repeated strokes of a fly press. The elevation of their temperature, which was considerable by the first blow, diminished greatly at each succeeding one, and became insensible whenever the condensation of volume ceased. The copper suffered greatest condensation, and evolved most heat. Here the analogy of a sponge, yielding its water to pressure, has been employed to illustrate the materiality of heat supposed deducible from these experiments. But the phenomenon may be referred to the intestine actions between the ultimate particles which must accompany the violent dislocation of their attracting poles. The cohesiveness of the metal is greatly impaired.

The equilibrium between the attractive and repulsive forces, which constitutes the liquid condition of bodies, is totally subverted by a definite elevation of temperature, when the external compressing forces do not vary. The transition from the liquid state into that of elastic fluidity, is usually accompanied with certain explosive movements, termed ebullition. The peculiar temperature at which different liquids undergo this change is therefore called their boiling point; and the resulting elastic fluid is termed a vapor, to distinguish it from a gas, a substance permanently elastic, and not condensable as vapors are, by moderate degrees of refrigeration. It is evident that when the attractive forces, however feeble in a liquid, are supplanted by strong repulsive powers, the distances between the particles must be greatly enlarged. Thus a cubic inch of water at 40° becomes a cubic inch and 1-25th on the verge of 212°, and at 212° it is converted into 1694 cubic inches of steam. The existence of this steam indicates a balance between its elastic force and the pressure of the atmosphere. If the latter be increased beyond its average quantity, by natural or artificial means, then the elasticity of the steam will be partially overcome, and a portion of it will return to the liquid condition And conversely, if the pressure of the air be less than its mean quantity, liquids will assume elastic fluidity by a less intensity of calorific repulsion, or at a lower thermometric tension, Professor Robison performed a set of ingenious experiments, which appear to prove, thai when the atmospheric pressure is wholly withdrawn, that is, in vacuo, liquids become elastic fluids 124° below their usual boiling points. Hence, water in vacuo will boil and distil over at 212°— 124 88° Fahrenheit. This principle was long ago employed by the celebrated Watt, in his researches on the steam-engine, and has been recently applied in a very ingenious way by Mr. Tritton in his patent still (Phil. Mag. vol. 51),

and Mr. Barry, in his evaporator for vegetable extracts (Med. Chir. Trans. vol. 10).

On the same principle of the boiling varying with the atmospheric pressure, the Rev. Mr. Wollaston has constructed his beautiful thermometric barometer for measuring heights. He finds that a difference of 1° in the boiling point of water is occasioned by a difference of 0.589 of an inch on the barometer. This corresponds to nearly 520 feet of difference of elevation. By using the judicious directions which he has given, the elevation of a place may thus be rigorously determined, and with great convenience. The whole apparatus weighing twenty ounces, packs in a cylindrical tin case, two inches diameter, and ten inches long.

When a vessel containing water is placed over a flame, a hissing sound or simmering is soon perceived. This is ascribed to the vibrations occasioned by the successive vaporisation and condensation of the particles in immediate contact with the bottom of the vessel. The sound becomes louder as the liquid is heated, and terminates in ebullition. The temperature becomes now of a sudden stationary when the vessel is open, however rapidly it rose before, and whatever force of fire be applied. Dr. Black set a tin cup full of water at 50°, on a red hot iron plate. In four minutes it reached the boiling point, and in twenty minutes it was all boiled off. From 50° to 212o, the elevation is 162°; which interval, divided by 4, gives 40° of heat, which entered the tin cup per minute. Hence twenty minutes, or 5 × 4 multiplied into 40 810, will represent the quantity of heat that passed into the boiling water to convert it into a vapor. But the temperature of this is still only 212°. Hence, according to Black, these 810° have been expended solely in giving elastic tension, or, according to Irvine, in supplying the vastly increased capacity of the aeriform state; and therefore they may be denominated latent heat, being insensible to the thermometer. The more exact experiments of Mr. Matt have shown, that whatever period be assigned for the heating of a mass of water from 50° to 212°, six times this period is requisite with a uniform heat for its total vaporisation. But 6 x 162972 the latent heat of steam; a result which accords with Dr. Ure's experiments made in a different way. Every attentive operator must have observed the greater explosive violence and apparent difficulty of the ebullition of water exposed to a similar heat in glass, than in metallic vessels. M. Gay Lussac has studied this subject with his characteristic sagacity. He discovered that water boiling in a glass vessel has a temperature of 214.2°, and in a tin vessel contiguous to it, of only 212°. A few particles of pounded glass thrown into the former vessel, reduced the thermometer plunged in it to 212 6, and iron filings to 212°. When the flame is withdrawn for a few seconds from under a glass vessel of boiling water, the ebullition will recommence on throwing in a pinch of iron filings.

The following is a tabular view of the boiling points by Fahrenheit's scale of the most important liquids, under a mean barometrical pressure of thirty inches :

656

Do.

Do.

Do. Do.

Do. Do. Do. Do.

Do.

Phosphorus Sulphur Linseed oil

Mercury (Dulong, 662°)

These liquids emit vapors, which, at their respective boiling points, balance a pressure of the atmosphere equivalent to thirty vertical inches of mercury. But at inferior temperatures they yield vapors of inferior elastic power.

Mr. W. Creighton of Soho communicated, in March 1819, to the Philosophical Magazine, the following ingenious formula for aqueous vapor. Let the degrees of Fahrenheit x 85 D, and the corresponding force of steam in inches of mercury- 0.09 I. Then log. D-2-22679 × 6 = log. I.

He then gives a satisfactory tabular view of the near correspondences between the results of his formula, and by experiments.

M. Gay Lussac, by determining experimentally the volume of vapor which a given volume of liquid can produce at 212°, has happily solved the very difficult problem of the specific gravity of vapors. He took a spherule of thin glass, with a short capillary stem, and of a known weight. He filled it with the peculiar liquid, hermetically sealed the orifice, and weighed it. Deducting from its whole weight the known weight of the spherule, he knew the weight, and from its specific gravity the bulk of the liquid. He filled a tall graduated glass receiver, capable of holding about three pints, with mercury, inverted it in a basin, and let up the spherule. The receiver was now

surrounded by a bottomless cylinder, which rested at its lower edge in the mercury of the basin. The interval between the two cylinders was filled with water. Heat was applied by means of a convenient bath, till the water and the included mercury assumed the temperature of 2129. The expansible liquid had ere this burst the spherule, expanded into vapor, and depressed the mercury. The height of the quicksilver column in the graduated cylinder above the level of the basin, being observed, it was easy to calculate the volume of the incumbent vapor. The quantity of liquid used was always so small, that the whole of it was converted into vapor.

The following exhibits the specific gravities as determined by the above method:

The above specific gravities are estimated under a barometric pressure of 29.92 inches.

It appears, that a volume of water at 40° forms 1694 volumes of steam at 212°. The subsequent increase of the volume of steam, and of other vapors, out of the contact of their respective liquids, we formerly stated to be in the ratio of the expansion of gases, forming an addition to their volume of three-eighths for every 180° Fahrenheit. We can now infer, both from this expansion of one measure into 1694, and from the table of the elastic forces of steam, the explosive violence of this agent at still higher temperatures, and the danger to be apprehended from the introduction of water into the close moulds in which melted metal is to be poured. Hence, also, the formidable accidents which have happened, from a litttle water falling into heated oils. The little glass spherules, called candle bombs, exhibit the force of steam in a very striking manner; but the risk of particles of glass being driven into the eye, should cause their employment to be confined to prudent experimenters. Mr. Watt estimated the volumes of steam resulting from a volume of water at 1800; and in round numbers at 1728; numbers differing little from the above determination of M. Gay Lussac. Desagulier's estimate of 14,000 was therefore extravagant.

It has been already mentioned, that the caloric of fluidity in steam surpasses that of an equal weight of boiling water by about 972°. This quantity, or the latent heat of steam, as it is called, is most conveniently determined, by transmitting a certain weight of it into a given weight of water, at a known temperature, and, from the observed elevation of temperature in

the liquid, deducing the heat evolved during condensation. Dr. Black, Mr. Watt, Lavoisier, count Rumford, Clement, and Desormes, as well as Dr. Ure have published observations on the subject.

Aqueous vapor of an elastic force balancing the atmospheric pressure,' says Dr. Ure, has a specific gravity compared to air, by the accurate experiments of M. Gay Lussac, of 10 to 16. For facility of comparison, let us call the steam of water unity, or 100; then the specific gravity of the vapor of pure ether is 400, while the specific gravity of the vapor of absolute alcohol is 2.60. But the vapor of ether, whose boiling point is not 100°, but 112°, like the above ether, contains some alcohol; hence we must accordingly diminish a little the specific gravity number of its vapor. It will then become, instead of 4:00, 3:55. Alcohol of 0.825 specific gravity contains much water; specific gravity of its vapor 2:30. That of water, as before unity, 1.00. The interstitial spaces in these vapors will therefore be inversely as these numbers, or

T

230

for ether, for alcohol, for water. Hence, of latent heat, existing in ethereal vapor, will occupy a proportional space, be equally condensed, or possess the same tension with in alcoholic, and in aqueous vapor. A small modification will no doubt be introduced by the difference of the thermometric tensions, or sensible heats, under the same elastic force. Common steam, for example, may be considered as deriving its total elastic energy from the latent heat multiplied into the specific gravity + the thermometric tension.

'Hence the elastic force of the vapors of water, ether, and alcohol, are as follows:-

Three equations, which yield, according to my general proposition, equal quantities. When the elastic forces of vapors are doubled, or when they sustain a double pressure, their interstices are proportionably diminished. We may consider them now as in the condition of vapors possessed of greater specific gravities. Hence the second portion of heat introduced to give double the elastic force, need not be equal to the first, in order to produce the double tension.

This view accords with the experiments of Mr. Watt, alluded to in the beginning of the memoir. He found that the latent heat of steam is less when it is produced under a greater pressure, or in a more dense state; and greater when it is produced under a less pressure, or in a less dense state. Berthollet thinks this fact so unaccountable, that he has been willing to discard it altogether. Whether the view I have just opened, of the relation subsisting between the elastic force, density, and latent heat of different vapors, harmonise with chemical phenomena in general, I leave others to determine. It certainly agrees with that unaccountable fact. Whatever be the fate of the general law now respectfully offered, the statement of Mr. Watt may he implicitly received under the sanction of his acknowledged sagacity and candor.' Ure's Researches on Heat, pp. 54 & 55.

M. Clement inserted a pipe, connected with a steam boiler capable of bearing high pressure, into a given quantity of water at a certain temperature, contained in a bucket. He now turned the stop-cock on the pipe, and allowed a certain quantity of steam, at 212° Fahrenheit, to enter. He then noted the increase of temperature which the water had received. He repeated the experiment; only the steam in the boiler and that which issued through the pipe had been heated till its elasticity was double of that at 212°. As soon as the water in the bucket indicated, by its increase of volume, that the same quantity of steam had been condensed as in the first experiment, he shut the stop-cock and measured the temperature of the bucket. He found it to be the same as before. A third experiment with steam having an elastic force equal to three atmospheres, was next subjected to examination; and he found the same result. Hence he inferred, that equal weights of steam, incumbent over water, at whatsoever temperature, contain the same quantity of heat; or, in popular language, that the total heat of steam is a constant quantity: for, in proportion as the sensible heat augments, the latent or specific heat diminishes. On this proposition he has founded a luminous theory of steam engines, which, we hope, he will soon present to the world in his promised Traité de Chaleur.

As it is the vastly greater relation to heat which steam possesses above water that makes the boiling point of that liquid so perfectly stationary in open vessels, over the strongest fires, we may imagine that other vapors, which have a

smaller latent heat, may not be capable, by their formation, of keeping the ebullition of their respective liquids at a uniform temperature. This variation of the boiling point has been observed actually to happen with oil of turpentine, petroleum, and sulphuric acid. When these liquids are heated briskly in apothecaries' phials, they rise 20° or 30° above the ordinary point at which they boil in hemispherical capsules. Hence, also, their vapors, being generated with little heat, are apt to rise with explosive violence. Oil of turpentine varies, moreover, in its boiling point, according to its freshness and limpidity. It is needless therefore to raise an argument on a couple of degrees of difference. But, in Dr. Murray's, and in all our other chemical systems, published prior to 1817, 560° was assigned as the boiling point of this volatile oil. Mr. Dalton's must be excepted, for he says, 'several authors have it, that oil of turpentine boils at 560°'

Mr. Darwin has explained the production of snow on the tops of the highest mountains, by the precipitation of vapor from the rarefied air which ascends from plains and valleys. The Andes,' says Sir H. Davy, 'placed almost under the line, rise 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 from below, if they ascend, become cooled in consequence of expansion; and the cold air, if by any force of the blast it is driven downwards, is condensed, and rendered warmer as it descends.'

Evaporation and rarefaction, the grand means employed by nature to temper the excessive heats of the torrid zone, operate very powerfully among mountains and seas. But the level sands are devoured by unmitigated heat. In milder climates, the fervors of the solstitial sun are assuaged by the vapors copiously raised from every river and field, while the wintry cold is moderated by the condensation of atmospheric vapors in the form of snow. The equilibrium of animal temperature is maintained by the copious discharge of vapor from the lungs and the skin. The suppression of this exhalation is a common cause of many formidable diseases. Among these, fever takes the lead. The ardor of the body, in this case of suppressed perspiration, sometimes exceeds the standard of health by six or seven degrees. The direct and natural means of allaying this morbid temperature were first systematically enjoined by Dr. Currie of Liverpool. He showed that the dashing or affusion of cold water on the skin of a fever patient, has most sanatory effects when the heat is steadily about 98°, and when there is no sensation of chillness, and no moisture on the surface. Topical refrigeration is elegantly procured, by applying a piece of muslin or tissue paper to any part of the skin, and moistening it with ether, carburet of sulphur, or alcohol. By pouring a succession of drops of ether on the surface of a thin glass tube containing water, a cylinder of ice may be formed at Midsummer.

The most convenient plan which the chemist can employ, to free a gas from vapor, is to pass