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197.]

LIQUEFACTION AND SOLIDIFICATION OF GASES.

417

material influence upon the pressure which its vapour exerts when the results of different experiments with the same liquid are compared at the same temperatures, and before the liquid has wholly assumed the state of vapour; this will be seen by comparing the two columns showing the pressure of ether at temperatures below 187° C. in two different experiments (page 414). It is very probable that the extraordinary discrepancies in the

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estimates of the pressure of liquefied carbonic anhydride given by Faraday, Thilorier, and Addams, are due to this cause. Similar differences, to a less extent, have been observed in the case of sulphurous anhydride, and cyanogen, and some other gases.

Faraday states, as the results of his experiments, that ammonia and sulphuretted hydrogen, when solidified, each furnished a white translucent mass, like fused ammonic nitrate: euchlo

rine gave a transparent orange-coloured crystalline solid. The other liquefied gases which were susceptible of solidification furnished colourless transparent crystalline masses like ice. The specimens of phosphuretted hydrogen, nitrous oxide, and olefiant gas, upon which he operated, although prepared with care, consisted of a mixture of two gases, one considerably more condensible than the other.

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Six gases-viz., oxygen, hydrogen, nitrogen, nitric oxide, carbonic oxide, and marsh gas-have resisted all attempts to liquefy them. Faraday found that oxygen remained gaseous under a pressure of 27 atmospheres, at a temperature of 166° (-110°C.); and a pressure of 58.5 atmospheres at 140° (-96° C.) was equally ineffectual in producing its liquefaction. Nitrogen and nitric oxide resisted a pressure of 50 atmospheres: with carbonic oxide, a pressure equivalent to that of 40 atmospheres; with coal gas, one of 32; and with hydrogen, one of 27 atmospheres, was applied without effecting the liquefaction: in all these experiments the temperature was maintained at-166° (—110° C.). Andrews (Report Brit. Assoc. 1861, 2nd part, 76) has succeeded in applying to these gases still greater pressures than any recorded by Faraday, without producing liquefaction, although a bath of ether and carbonic anhydride was employed: air was reduced to of its original volume, oxygen to 3, hydrogen to 506, carbonic oxide to, and nitric oxide to 。 of its original volume. Hydrogen and carbonic oxide departed less from Boyle's law than oxygen and nitric oxide.

(198) Spheroidal State produced by Heat.-Much attention has of late years been excited by a phenomenon first described in 1756 by Leidenfrost, and which has been made the subject of careful investigation by Boutigny (Ann. Chim. Phys. 1843 [3], 350, and 1844 [3], 16). The following experiments will illustrate its character. If a good conductor, such as a sheet of metal, be heated to between 150° and 200° C., and water be allowed to fall upon its surface, the liquid does not enter into ebullition; but instead of wetting the surface as usual, it rolls about in spheroidal masses in the manner shown in fig. 155; the temperature of such a spheroid never rises to the boiling-point of the liquid.* If the source of heat be

FIG. 155.

*Colley (Pogg. Ann. 1871, cxliii. 125-141) has found that in the case of water, the temperature of the spheroid may vary between 90°56 and 100°34,

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SPHEROIDAL STATE PRODUCED BY HEAT.

419

removed, the temperature will fall, until a point is at length reached when the liquid suddenly begins to boil vehemently, and is dispersed in all directions with a loud hissing noise.

This phenomenon is a complicated result of at least four distinct causes. Of these the most influential is the repulsive force which heat exerts between objects which are closely approximated to each other. A low manifestation of this action has been already noticed when speaking of the effect of a rise of temperature in producing a decrease of capillary attraction (51). When the temperature reaches a certain point, actual repulsion between the particles ensues. Besides this repulsion occasioned by heat, the other causes which may be mentioned as tending to produce the assumption of the spheroidal condition by the liquid, are these-1. The temperature of the plate is so high that it immediately converts any liquid that touches it into vapour, upon which the spheroid rests as on a cushion. 2. This vapour is a bad conductor of heat, and prevents the rapid conduction of heat from the metal to the globule. 3. The evaporation from the entire surface of the liquid carries off the heat as it arrives, and assists in keeping the temperature below the point of ebullition. The drop assumes the spheroidal form as a necessary consequence of the cohesion among the particles of the liquid, and the simultaneous action of gravity on the mass.

Boutigny finds that even if the liquid be boiling, its temperature sinks from 3° to 4° C. below its boiling-point at the moment that it falls on the heated surface, and takes the spheroidal form.

All liquids are capable of assuming this condition; but the temperature to which it is necessary to heat the conducting surface varies with each liquid; the lower the boiling-point of the liquid, the lower also is the required temperature. The exact degree is dependent partly upon the conducting power of the plate, and partly upon the latent heat of the vapour; the temperature of the plate approaches the boiling-point of the liquid more closely as the latent heat is less.

Boutigny considered the temperature of each liquid, when in the spheroidal state, to be as definite as that of its boiling-point. Boutan has, however, shown that these temperatures are liable to slight variations. The following table shows the lowest tempera

according to the size of the drop and the temperature of the vessel containing it; the larger the spheroid, the higher the temperature. The distance between the heated vessel and the spheroid when the latter weighed from 1 to 15 gr., was estimated at o'15 to 0.25mm..

ture of the plate and the temperature of the spheroid for certain liquids, according to Boutigny :—

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Even in vacuo the spheroidal state is observed to occur when the liquid is allowed to fall upon a plate sufficiently heated. Solids in liquefying in hot capsules, pass into this same state, as is well exemplified by throwing a few crystals of iodine upon the heated surface. Provided that the hot surface be a sufficiently good conductor of heat, the nature of the material is unimportant. Silver, platinum, copper, and iron may all be successfully used. Tomlinson has shown that even one liquid may be thrown into the spheroidal form on the surface of another, as water, alcohol, or ether, on the surface of hot oil; but this experiment requires care, otherwise the water sinks in the oil, evaporation from the surface of the drop is prevented, steam is generated with explosion, and the hot oil is scattered about in all directions.

If the hot metal be sufficiently massive, a large body of water may be converted into the spheroidal state. Boutigny has suggested that in certain cases the explosion of steam-boilers may have been due to this cause. It is indeed quite possible, although such an occurrence must be rare, that the water may be all expended in a boiler beneath which a brisk fire is maintained, so that the mass of metal may become intensely heated. On the admission of cold water under such circumstances, it would at first assume the spheroidal state, and as the boiler gradually cooled down, by the introduction of more water, a sudden and uncontrollable burst of vapour would ensue. The safety-valve in such a case would be inadequate to allow the needful escape for the immense quantity of steam which would be instantaneously generated, and au explosion would probably occur.

By tracing the effects above detailed to their extreme consequences, some singular and paradoxical effects have been produced. For example, liquid sulphurous anhydride becomes spheroidal in a red-hot capsule at a temperature of about 14° (-10° C.), or considerably below the freezing-point of water. If a little water be dropped into this spheroid, the temperature of the water is

reduced below its freezing-point, and a mass of ice is

199.]

ATOMIC RELATIONS OF HEAT OF COMBINATION.

421

formed within the glowing crucible. If a bath of solid carbonic anhydride and ether be substituted for the sulphurous anhydride in the red-hot capsule, mercury placed within it in the bowl of a small spoon may be frozen with equal certainty. But perhaps the most marvellous result is the impunity with which the moistened hand may be plunged for an instant into molten lead, or even into cast iron as it issues from the furnace. In these cases the adhering moisture is converted into vapour, which forms an envelope to the skin sufficiently non-conducting to prevent the passage of any injurious quantity of heat during the brief immersion. An ingenious application of this principle has long been employed in the glass-house. In first rudely shaping the large masses of glass which are to be blown into shades, and into cylinders which are afterwards flattened into the heavy sheets technically termed British plate, open hemispherical wooden moulds are used to give the globular form; in order to prevent the wood from being burned, the workman pours a little water into the mould; it protects the wood, but assumes the spheroidal form, and neither touches nor injuriously cools the molten glass.

§ IV. ATOMIC RELATIONS OF HEAT OF COMBINATION.

(199) The quantity of Heat developed by Chemical Action is definite. The last subject to which we shall here advert in connexion with heat, is to the chemist perhaps the most directly interesting of any, on account of its direct quantitative relations to chemical action. Experiment has proved that the amount of heat which each element emits when entering into combination is definite, and has a specific relation to the combining number of each substance. When the same substance is burned with a due supply of oxygen, and with suitable precautions, a given quantity of it always emits the same amount of heat. Thus 1 gramme of hydrogen, when burned in oxygen, always emits heat enough to inelt 43485 grammes of ice; 31 grammes of phosphorus, when burnt to phosphoric anhydride, yields heat sufficient to melt 2248 grammes of ice; and 12 grammes of carbon, when converted into carbonic anhydride, emits heat sufficient to melt 12235 grammes of ice. It would at first sight appear easy to determine by direct experiment the amount of heat which each body emits in the act of combining with an atom of oxygen, and to compare the results thus obtained with a corresponding series of experiments made by combining the same elements with an equivalent proportion of chlorine, of bromine, or of other elements.

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