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470

Spheroidal State of Liquids.

suddenly converted into vapour with explosive violence. A piece of pure ice free from air was heated in a vessel containing oil, and he found that when the water produced from the ice had reached a temperature of 240°, the wł.ole was converted into vapour with explosion.

The water in the cryophorus, or water-hammer, contains no air, and, if heated, would be suddenly converted with explosion into steam at 240°, destroying the instrument.

676. Spheroidal state.—There is another remarkable condition of water with respect to heat which may be here noticed, namely, the spheroidal state. When water is poured in small quantities at a time, into a clean platinum or porcelain dish, heated to full redness (i.e., above 1000°), it does not boil, and does not produce any visible vapour. The liquid assumes what is called the spheroidal state, and rolls about in a stratum which presents a convexity on all sides like a quantity of mercury in a watch-glass. The water does not appear to touch in any part the red-hot surface of the containing vessel. At this high temperature there appears to be a complete repulsion between the water and the vessel, owing probably to the presence of a layer of vapour between the two. This observation applies not only to water, but to all other liquids which are not too rapidly evaporated. Boutigny, who first pointed out this effect of heat on water, found that the liquid while in this state had a temperature a few degrees below its boiling point. The liquid continues in incessant motion, gradually diminishes in volume, and at last evaporates entirely, leaving only the solid matters which may have been contained in it. If, while the water is in this spheroidal state, the source of heat is suddenly withdrawn, the metal becomes cooled, and at a certain point the water comes in contact with the heated surface, and a large portion of it is suddenly converted into steam with explosive violence. Explosions of steam boilers have been occasionally traced to this cause, when water has been turned into an over-heated boiler. Water already warmed assumes this condition more readily than cold water.

All liquids and all solids which become liquefied by heat may assume a similar condition if the metallic surface be brought to a sufficiently high temperature. Solid iodine thus thrown on platinum melts, and becomes spheroidal, evolving a violet vapour. When the heat is turned off, and the metallic surface cooled, the melted iodine coming in contact with the metal, produces suddenly a copious cloud of dense vapour.

Latent Heat of Vapours.

471

677. Substances differ among themselves in regard to the latent heat of their vapours as much as in their other relations to heat. Thus the latent heat of the vapour or steam of—

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From the less abundant latent heat in these last-mentioned vapours than in that of water, one might at first suppose that there would be a great advantage in using them for steam-engines. Accordingly, numerous experiments have been made, and patents secured, under this idea; but the fact is, that in the same proportion as the latent heat is less, the elasticity of the vapour is less, and therefore no mechanical advantage is obtainable.

678. It has already been stated that many gases-or substances usually met with in the aëriform state-may be reduced to the liquid, or even solid form, by simple pressure, and abstraction of the heat which exists in them while in the aëriform state. Carbonic acid, and other gases, have been treated in this way. Some are liquefied by cooling only, and some by cooling conjoined with pressure. There are a few which resist both cooling and pressure. These are permanent gases; oxygen, hydrogen, and nitrogen are the principal substances of this group. It also became an interesting question whether many of the substances commonly seen as liquids on the face of the earth, where they are bearing the pressure of the atmosphere, would have the form of liquid if that pressure did not exist.

On investigating this subject by experiment, we accordingly find, that ether, alcohol, chloroform, naphtha, benzoline, volatile oils, &c., and even water itself, are known to us here as liquids only because their particles are kept together by the weight and pressure of a superincumbent atmosphere. Any of these substances, relieved by art from such pressure, quickly become vapours or gases, just as carbonic acid gas or any other which has been kept in the state of liquid by great pressure, becomes again a gas on the removal of the pressure.

679. In another page we have explained the dependence of the three forms which any body may assume, viz., of solid, liquid, or gas, on the quantity of heat diffused among the particles: we now see, however, that in order to understand the subject completely,

472

The Phenomena of Boiling.

we must consider also the effect of accidental pressure; for while heat is the power separating the atoms in the changes mentioned, it has to overcome both the mutual attractions of the atoms and the additional force of the atmosphere pressing them together. The combined influence of these forces is fully displayed in the phenomena of boiling and evaporation, which exhibit the progress of the change of a liquid into an aëriform fluid. We now proceed to examine these phenomena.

680. Boiling. If water be placed in a suitable vessel (it may be a glass flask) over a common fire, or over the flame of a lamp, it is gradually heated to a certain degree; and then small bubbles of aëriform matter, viz., water, in the state called steam, are seen forming at the bottom of the vessel, and successively rising to the surface, where they disappear by mixing with the atmosphere; and the operation being continued, the quantity of water diminishes with every bubble, until the whole vanishes as aëriform water or

steam.

This change takes place in water, under common circumstances, at the degree of heat marked 212° on Fahrenheit's thermometer, and called on that account the boiling point of water; at which degree, therefore, the repulsive power or agitation among the particles is just sufficient to overcome both their natural attraction and the compressing force of the atmosphere of fifteen pounds on the square inch. But a less degree of heat suffices if the pressure of the atmosphere be lessened or removed; and a greater degree is required if the pressure be increased. Water on the top of Mont Blanc boils at 180°, because relieved from the pressure of the air which is below the level of the mountain's summit; and at all intermediate heights in descending to the level of the sea, or beyond that into mines, there is a corresponding increase of the boiling temperaturc.

So exactly is this the case, that a good method of ascertaining the heights of different places, is found to be merely by observing the temperature of boiling water at them. To many persons the information here given, that boiling water is not equally hot in all places, will appear extraordinary: but they will now understand the reason, and, further, that even in the same place, at different times, when the barometer is higher or lower than usual, there will be corresponding differences. In the city of Mexico, which is on a table-land 7471 feet above the level of the sea, water boils at 199°. In Quito, at an elevation of 9341 feet, at 195°; on the summit of Mount Etna, at a

The Process of Distillation.

473

height of 10,955 feet, at 192o; and on the summit of Mont Blanc, 15,630 feet, at 182°.

Again, near the bottom of a boiler, the water is hotter than above, because it is bearing an additional pressure, proportioned to the depth, and does not therefore take the form of steam so readily as it would if a little higher up. In very large and deep boilers, therefore, such as are used in great porter breweries, the boiling liquid is much more heated than it can be in smaller vessels ;—a circumstance which probably has an influence on its ultimate quality.

In the close wrought-iron boilers employed for heating houses by the circulation of hot water, the boiling temperature is raised to 250° or 260° according to the columnar pressure of the water on the boiler. This depends on the height of the cistern from which the boiler is supplied. In locomotive engines close boilers under strong pressure are used. The boiling point is here often raised by the proper adjustment of safety valves to from 280° to 290°.

While water under common atmospheric pressure, or when the barometer stands at thirty inches, boils at 2120, other substances, with other relations to heat, have their boiling points higher or lower-ether, for instance, boils at 96°; chloroform at 140°; spirit or alcohol at 174°; fish-oil and tallow at about 600°; mercury and oil of vitriol at 650°. This explains why a burn from boiling oil is so dreaded, and why flesh or fish boiled in water is so different from what is cooked by frying or otherwise in melted fat or in oil.

681. It is in consequence of the different temperatures at which the particles of different substances acquire repulsion enough to rise against the atmospheric resistance, that we are enabled to perform the operation called distilling. If any fermented fluid, for instance, containing alcoholic spirit and water, as wine or beer, be heated up to 180°, the spirit will pass off in the aëriform state, leaving the greater part of the water behind, and it may be cooled to a liquid by condensation in any fit receiver. Distillation is the only means we possess of separating many substances from each other as spirit from wine or any other fermented liquor; various acids from water; pure water itself from the salt of sea-water or other impurity ;-and even the separation of mercury from silver or gold which it has been employed to dissolve from among the rubbish of a mine or riverbottom, is merely a distillation which saves the mercury to be used zgain.

682. We must recall to mind here what has been mentioned in

474

Condensation of Steam.

another part of the work (Art. 669), that a large amount of heat enters into every substance during the change of form from solid to liquid, or from liquid to vapour;—which quantity, from not remaining sensible to the thermometer, has received the name of latent or concealed heat. The whole of this is given out again in the contrary change. In the conversion of water into steam, the heat which thus disappears is about 1000 degrees, or six times as much as is required to raise the cold water to the boiling point: this is proved by the time and fuel expended in boiling any quantity to dryness, and by the fact that a pint of water in the form of steam will combine instantly with six pints of cold water, raising the whole to boiling heat.

But for the fact of latent heat, the conversion of a liquid into an aëriform or gaseous mass would not be the gradual process of boiling which we now see, but a sudden and terrible explosion : for when any quantity of water is raised to the boiling heat, one degree of heat additional would be sufficient to convert the whole into steam. For a similar reason, the thawing of winter snow would always lead to a sudden and frightful inundation; the whole load on a mountain or plain becoming at once converted into a lake bursting from its enclosing barriers. On the other hand, if water in freezing had not to give out gradually its latent heat, after any quantity were once cooled down to the freezing point, the abstraction from it of one degree more would instantly convert the whole into a solid mass. Thus, then, by admirable arrangement effecting most important purposes in nature and art, all changes from solid to liquid and from liquid to vapour, and the reverse changes, are very gradual.

If a little heat be abstracted from steam, a small part of the steam proportioned to the abstraction is immediately condensed into water. What is called steam in common language-as the vapour which becomes visible at a little distance from the spout of a boiling kettle or the top of a tea urn—is not truly steam, but small globules of water already condensed by the cold air and mixed with it.* True steam is as dry and invisible as air itself; but the instant

* These minute globules of water condensed from steam have a spherical form. They decompose light into the prismatic colours. If the condensed vapour is examined as it rises from any metallic surface, as from a teaspoon dipped into hot water, the light of a candle or lamp traversing the globules of steam, will be resolved into the colours of the spectrum. The light is decomposed by diffraction in traversing these globules of condensed vapour

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