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vaporization and therefore no further cooling can take place) and the more it is set in motion.

When vaporization takes place in the open air below the boiling point, the maximum of cooling is produced, as soon as the quantity of heat rendered latent by the process becomes equal to that which the air gives up in order to place itself in equilibrio, as regards pressure and temperature, with the gas in course of formation, + the quantity supplied from without (this however may be neglected when the difference does not exceed a few degrees). Gay-Lussac's formula for calculating the lowest temperature attainable in this manner will be found in the Ann. Chim. Phys. 21, 82.-If air dried by chloride of calcium under a pressure of 076° metre be directed on a thermometer bulb surrounded with baptist, and the baptist be moistened with water, the following degrees of cold will be produced:

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When the external pressure to which the air is subjected amounts to only 0.65 metre, a cooling of 10:5° takes place at 12.5°, and at 0.5 metre it amounts to 12o at the same temperature. According to the preceding table, dry atmospheric air would cause the freezing of water at 8°; but since common air contains a large quantity of vaponr of water, the freezing of water usually commences at about 2°: it is only on high mountains, where the air is dryer and rarer, that congelation takes place at higher temperatures. (Gay-Lussac.)—A wet finger exposed to the air becomes coldest on the side from which the wind blows.-Alkaraza, a porous water-jug in which the water which exudes through the pores evaporates and thus cools that which remains.— The production of ice in India (Crell. Chem. J. 1, 197) is according to Wells (Schw. 22, 187) and D. Scott (Edinb. J. of Sc. 8, 216; also Schw. 52, 372) an effect not so much of cooling by evaporation, as of radiation towards empty space the temperature of which is very low.

The Psychrometer of August (Pogg. 5, 69 and 335; 14, 139) is an instrument for determining the hygrometric state of the atmosphere. Two small thermometers a, b, are placed near one another, one of them 6 being covered with wet muslin : its temperature becomes lower than that of a, in proportion as the air is drier and more water evaporates. The difference of the temperatures indicated by a and 6 being multiplied by 2, and the product subtracted from the temperature denoted by a, the remainder shows the temperature at which the aqueous vapour in the air will condense.

If a thin glass bulb containing water be surrounded with cotton, ether dropped on the cotton and a stream of air directed on it, the water will freeze in a few minutes. If the bulb of a mercurial thermometer be wrapped up in cotton, then liquid sulphurous acid dropped on the cotton and the instrument moved about in the air, the mercury gradually sinks to 36°, then falls suddenly into the bulb and freezes. (Bussy, J. Pharm. 10, 202; also Schw. 41, 451.) Half an ounce of mercury placed in a watch-glass and covered with liquid sulphurous acid, freezes when blown upon with a pair of bellows. (Wach, Schw. 50, 24.) Solid carbonic acid moved about in the air produces a degree of cold amounting to -72.2°; if it be mixed with ether the temperature falls to - 76.7°; on blowing on the mixture it is further reduced to - 79o. Mercury immersed in solid carbonic acid in the open air freezes in a few seconds; in a mixture of solid carbonic acid and ether, it freezes instantly. Liquid carbonic acid enclosed in a tube may also be solidified by immersion in such a mixture; the frozen portion sinks to the bottom of that which still remains liquid, till the whole becomes frozen into a compact mass like that usually formed by solid carbonic acid. (Mitchell.)

When a body is heated in the air to its boiling point, its temperature cannot be raised higher by any further addition of heat, because all the heat which afterwards enters the body combines with it to form a gas and thus becomes latent. The vaporizing body remains constantly at the boiling temperature, and the vapour produced has likewise the same temperature, provided no more heat be subsequently added to it. This is the principle of the Water-bath, Balneum Maris s. Mariae, a covered vessel in which water is heated to the boiling point: the cover has apertures in it, into which a number of vessels are inserted so as to dip either into the water or into the steam; they are thus exposed to a uniform temperature not exceeding 100°. If a lower temperature be required, spirit of wine of various strengths may be used; higher temperatures may be obtained by means of solutions of chloride of calcium of various degrees of concentration. Variations in the temperature of a body heated to ebullition may

be produced by the following causes.

1. By change of atmospheric pressure. Par, inches..

26.5 27

27.5 28 28.5 29 Boiling point 98.366° 98.775° 99.183° 99.592°

100•408° 100·87° Hence the difference for each inch is about 0.817° (Graham.)

2. If heat be applied to the lower part of a liquid, such as water, contained in a deep vessel, the heat must rise to a higher degroe at this lower part before bubbles of vapour can be developed, -because their elastic force has to overcome, not only the atmospheric pressure, but likewise that of the superincumbent column of liquid. Under a column of water 10 metres high (= 1 atmosphere) water cannot boil below 122° ; but in passing up through the liquid, the vapour expands and becomes cooler, so that when it eseapes at the surface it has a temperature of 100°. (Gay-Lussac.)

3. Since the formation of gas in a mass of liquid takes place principally at the edges of any solid bodies which may be contained in it, it follows that, when no such edges are present, the heat must accumulate in somewhat greater quantity before the bubbles can form; hence the boiling point of a liquid, e. g. water, is lower by some tenths of a degree, 1ļo at most, and the boiling goes on more uniformly and less by fits and starts, when the surface of the vessel is uneven, or when metallic powders, filings, or wire are introduced, or when the liquid is agitated. (Achard, Schw. 27, 27;—Gay-Lussac, Ann. Chim. 82, 174; Ann. Chim. Phys. 7, 307:-Muncke, Gill. 57, 215.) Water boils in metallio vessels at

26

100

100°, in glass vessels at 101.33°; but in the latter case, the vapour which rises has a temperature of only 100°, the higher boiling point being probably due to a grenter adhesion of glass to water. (Rudberg, Pogg. 40, 49.). Ether of 0.755 sp. gr. which boiled at 44:4° C. in a glass flask, required a heat of 65.5°, and sometimes even 79°, to make it boil in a glass tube closed at the bottom and immersed in hot water in the latter case it could not move so freely); metallic filings or wire, fragments of glass, or pounded glass lowered the boiling point to 51°; if the liquid had ceased to boil at this temperature, the ebullition was reproduced on the introduction of shavings or splinters of wood, (these however must likewise have acted by means of the air which escaped from their pores. Gm.) Similar results were obtained with alcohol and water. (Bostock, Ann. Phil. 25, 196.) Watery liquids heated to the boiling point and then removed from the fire, exhibit fresh ebullition on the introduction of solid bodies, only when a permanent gas likewise comes into play,because the mixture of watery vapour and permanent gas sustains the pressure of the atmosphere at a temperature below 100°. Pure boiled water evolves no bubbles of vapour on the introduction of platinum wire, but nevertheless boils up: so also does pure water which still contains air, on the immersion of a platinum wire which has been freed from adhering air by previous boiling. The same phenomenon is exhibited by water freed from air on the introduction of a metal wire having air stiil adhering to it; and more violently on the introduction of a piece of wood; also by water containing from jöbo to too of sulphuric or other acids, on the immersion of iron or other metals which develope gas by the action of the acid, or on the introduction of calcspar. (Schönbein, Pogg. 40, 391.)

4. Many foreign bodies produce precisely the opposite effect on the boiling point of a liquid: they cause an accumulation of heat in the liquid, and thus raise the temperature far above the boiling point,-till on a sudden, this excess of heat is expended in the formation of vapont, which not only causes a rising and projection of the liquid, but also occasions violent noisy agitation and often fracture of the containing vessel. Upon this, the liquid goes on boiling but with continually dimi. nishing force; and at length comes to rest and remains tranquil, till a fresh accumulation of heat causes the same action to be repeated. This jumping ebullition occurs in water containing sulphate of potash in greater quantity than it is able to dissolve, in oil of vitriol which contains even a small quantity of sulphate of lead in the state of powder, and in nitric acid containing crystals of nitrate of silver. The presence of fat oil, according to Scrymgeon, raises the boiling point of water some degrees; and, according to Magnus, a layer of volatile oil on the surface of water causes a rise of temperature of 10° above the boiling point, accompanied by violent, percussive ebullition, which however passes into quiet boiling as soon as the layer of oil becomes broken by bubbles of vapour. Water holding salts in solution-e.g. neutral tartrate of potash, and more especially caustic potash-has a great tendency to become overcharged with heat, so that the liquid flows over when stirred. The presence of metal filings, wire, or cuttings tends to prevent irregular ebullition, inasmuch as metallic points facilitate the formation of bubbles; but according to Legrand (Ann. Chim. Phys. 59, 426), platinum is not nearly so efficacious as zinc or iron, metals in fact which decompose water, although the zinc placed in saline solutions, under such circumstances, is not sensibly diminished but only slightly tarnished,

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When a volatile body has its boiling point raised by combination with a less volatile substance, the vapour at the moment of its formation likewise possesses the higher temperature, but quickly cools down to the ordinary boiling point of the body: e. g. vapour of water evolved from boiling saline solutions. (Gay-Lussac, Ann. Chim. Phys. 20, 324; 70, 420.)

- Pambour, Jahresb. 19, 55.) According to Faraday (Ann. Chim. Phys. 20, 325) and Rudberg (Pogg. 34, 257) the vapour of water evolved from boiling saline solutions, even from a solution of chloride of calcium which boils at 150°, has always the temperature of 100°.

When a volatile liquid is situated at the bottom of a less volatile one with which it does not mix, its boiling point is somewhat raised in proportion to the pressure exerted by the upper liquid; but the temperature of the vapour bubbles which rise from the lower through the upper liquid is below the boiling point of the more volatile body, because its vaponr becomes mixed to a certain extent with that of the upper liquid (the temperature of which is somewhat higher than that of the lower) --so that the mixed vapour, by virtue of the sum of its tensions, is able to balance the pressure of the air at a somewhat lower temperature. Thus, the boiling point of sulphuret of carbon situated below a layer of water is 47°, and the temperature of the mixed vapour 43.5°; the temperature of water boiling under oil of turpentine is 102°, and that of the mixed vapour 94.5o. A volatile liquid, situated above a less volatile one, e. g. water above mercury, boils at the same temperature as when alone. (Magnus, Pogg. 38, 481.) Comp. Liebig (Pogg. 24, 277)-Gay-Lussac (Ann. Chim. Pys. 49, 393; 50, 111).

Leidenfrost's Experiment. When a volatile liquid is dropped on a surface heated to redness or a little below, it does not adhere to the surface, but swims upon it in a globular form: neither does it become heated to its boiling point; but diminishes slowly in consequence of the formation of vapour, which escapes between the heated surface and the liquid, and likewise when the quantity of liquid is considerable—escapes in bubbles through it. When the temperature of the surface falls to a certain point, the liquid comes in contact with it, boils with violence, and is rapidly converted into vapour. The surface may consist of platinum, silver, copper, iron, and other metals, or of glass or porcelain; but the lower its conducting power, the more strongly must it be heated. The experiment succeeds with sulphurous acid, ether, alcohol, water, oil of vitriol, and mercury; the higher the boiling-point of the liquid, the more strongly must the surface be heated. A small platinum crucible, strongly ignited over an Argand spirit-lamp, may be gradually filled with water to the brim. Melted slags falling in drops upon water swim for some time on the surface in a state of incandescence, and then sink with a hissing noise. (Faraday.) Alcohol dropped on an almost boiling mixture of alcohol and oil of vitriol, swims upon it in globules. Potash and its salts, likewise charcoal powder or ink, added to the water, interfere with the phenomenon, according to Pouillet. The same appearance is presented, according to Delatre & Boutigny, by sulphurous acid, water, and ether, even in

If, on the other hand, a heated glass flask containing ether in this peculiar floating condition be stopped with the finger, and the formation of vapour thereby prevented, the phenomenon will be arrested. (Desmarest.) Liquids in this state are not resolved into permanent gases, but evaporate unchanged. On connecting the metallic surface with one pole of a simple galvanic circle, and the liquid with the other, it is found, according to Poggendorff, that no electric current passes,- and consequently, the liquid and the heated surface are not in contact. --The heat which radiates from the heated surface undoubtedly produces, before the surface and the liquid can come into immediate contact, a quantity of vapour sufficient to support the liquid; and this vapour being continually renewed prevents the liyuid from touching the surface, and thus intercepts the more rapid transmission of heat by conduction. Compare Eller (Hist. de l'Acad. de Berl. 1746, 42), who appears to have been the first to observe the effect; Leidenfrost (De aquae comm. qualitatibus. Duisb. 1756); Klaproth (Scher. J. 7, 646); Muncke (Pogg. 19, 514); Faraday (Qu. J. of Sc. N. S. 3, 221); Buff (Pogg. 25, 591; also Ann. Pharm. 2, 220); Lechevallier (J. Pharm. 16, 666); Baudrimont (Ann. Chim. Phys. 61, 319); Laurent (Ann. Chim. Phys. 62, 327); Boutigny (Pogg. 51, 130); Ensmann (Pogg. 51, 444); Ritter (J. pr. Chem. 10, 108); Desmarest (Í. Pharm. 26, 746); Poggendorff (Pogg. 52, 538).]

The statement of Jaquemyns (Pogg. 37, 467), that the bottom of a metallic vessel may be touched with impunity as long as water is boiling in it, deserves verification.

When a volatile body is exposed to a high temperature in a narrow space, either vacuous or filled with air, and enclosed by solid walls, the vapour produced from it acquires a continually increasing elasticity, and thus raises the temperature of the body more and more,—so that at length a degree of heat is attained at which the body, under the ordinary temperature of the air, would be instantly converted into vapour. (Papin's Digester.)

Faraday has succeeded in freezing mercury in a red-hot platinum crucible, by putting into the crucible, first ether, then solid carbonic acid, and then—when the whole is in the spheroidal state-dipping into it a small metal spoon containing between 400 and 500 grains of mercury. The mercury freezes in two or three seconds. (V. Ann. Chim. Phys. 19, 383.) The film of vapour which intervenes between the red-hot metal and the semi-fluid mass of ether and carbonic acid, completely prevents the communication of heat from the crucible to the mercury.-In a similar manner, water may be frozen in a red-hot crucible by the agency of liquid sulphurous acid.

d. Increase of Volume. The expansion of a body on passing into the gaseous state varies according to its nature, and likewise according to temperature and external pressure.

One measure of water at 0° yields, under a pressure of 0.76 metre, 170w me sures of vapour at 100, according to Gay-Lussac, and 1728 according to Dalton: 1728 is the cube of 12; hence, according to Dalton's view, the atoms of water in the gaseous state must be removed from one another 12 times as far as in the liquid state; if, however, there be any foundation for this supposition, it must be true for all external pressures. (On the expansion of certain other bodies by conversion into gas, comp. pp. 57, 3, and 73, 1.)

From the following table, Gay-Lussac concludes (Ann. Chim. Phys. 2, 130) that bodies, in assuming the gaseous form, expand so much the less in proportion as they suffer a greater amount of contraction in cooling from their boiling points :

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