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order that the liquid may actually enter into ebullition, the space above the liquid must be sufficiently large and cool to allow of the condensation of the steam. In a confined vessel, water may be raised to a higher temperature than would be possible in the open air, but it will not boil. This is the case in the apparatus invented by the celebrated Papin, and called after him Papin's digester. It is a bronze vessel of great strength, covered with a lid secured by a powerful screw. It is employed for raising water to very high temperatures, and thus obtaining effects which would not be possible with water at 100°, such for example as dissolving the gelatine contained in bones.

It is to be observed that the tension of the steam increases rapidly with the temperature, and may finally acquire an enormous power. Thus, at 200°, the pressure is that of 16 atmospheres, that is about 240 pounds on the square inch.

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In order to obviate the risk of explosion, Papin introduced a device for preventing the pressure from exceeding a definite limit. This invention has since been applied to the boilers of steam-engines, and is well known as the safety-valve. It consists of an opening, closed by a conical valve or stopper, which is pressed down by a lever loaded with a weight. Suppose the area of the lower end of the stopper to be 1 square inch, and that the pressure is not to exceed 10 atmospheres, corresponding to a temperature of 180°. The

Fig. 251.-Papin's Digester.

magnitude and position of the weight are so arranged that the pressure on the hole is 10 times 15 pounds. If the tension of the steam exceed 10 atmospheres, the lever will be raised, the steam will escape, and the pressure will thus be relieved.

When the tension of the steam contained in the digester has become considerable, if the lever be raised, so as to permit some steam to escape, it rushes out with a loud noise, and produces a cloud in

once.

the air. On placing the hand in this cloud, scarcely any sensation of heat is experienced, whereas, on performing the same experiment with steam at the ordinary pressure, the hand would certainly be scalded. This apparently paradoxical result is completely in accordance with the principles which have already been stated more than The steam formed at 100°, being at atmospheric pressure, preserves its pressure and temperature on issuing into the air. On the other hand, the steam generated in Papin's digester has a pressure greatly exceeding that of the atmosphere, and accordingly expands rapidly upon its exit, and thus performs work in forcing back the external air. The performance of this work is accompanied by the loss of an equivalent quantity of heat, and the temperature of the jet is consequently considerably lowered.

262. Boiling-point of Saline Solutions. When water holds saline matters in solution, the boiling-point rises as the proportion of saline matter in the water increases. Thus with sea-salt the boiling-point can be raised from 100° to 108°.

When the solution is not saturated, the boiling-point is not fixed, but rises gradually as the mixture becomes concentrated; but at a certain stage the salt begins to be precipitated, and the temperature then remains invariable. This is to be considered the normal boilingpoint of the saturated solution. Supersaturation, however, sometimes occurs, the temperature gradually rising above the normal boilingpoint without any deposition of the salt, until all at once precipitation begins, and the thermometer falls several degrees.

The steam emitted by saline solutions consists of pure water, and it is frequently asserted to have the same temperature as the steam of pure water boiling under the same pressure; but the experiments of Magnus and others have shown that this is not the case. Magnus, for example,1 found that when a solution of chloride of calcium was boiling at 107°, a thermometer in the steam indicated 1054°, and when by concentration the boiling-point had risen to 116°, the thermometer in the steam indicated 111·2°.

These and other observations seem to indicate that the steam emitted by a saline solution when boiling, is in the condition in which the steam of pure boiling water would be, if heated, under atmnospheric pressure, to the temperature of the boiling solution. It can therefore be cooled down to the boiling-point of pure water without undergoing any liquefaction. When cooled to this point, it becomes

1 Poggendorff's Annalen, cxii. p. 415.

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saturated, and precisely resembles the steam of pure water boiling under the same pressure. When saturated steam loses heat, it does not cool, but undergoes partial liquefaction, and it does not become completely liquefied till it has lost as much heat as would have cooled more than a thousand times its weight of superheated1 steam one degree Centigrade.

262 A. Boiling-point of Liquid Mixtures. A mixture of two liquids which have an attraction for each other, and will dissolve each other freely in all proportions-for example, water and alcohol-has a boiling-point intermediate between those of its constituents. But a mechanical mixture of two liquids between which no solvent action takes place--for example, water and sulphide of carbon-has a boilingpoint lower than either of its constituents. If steam of water is passed into liquid sulphide of carbon, or if sulphide of carbon vapour is passed into water, a mixture is obtained which boils at 42.6° C., being four degrees lower than the boiling-point of sulphide of carbon alone. This apparent anomaly is a direct consequence of the laws of vapours stated in § 244; for the boiling-point of such a mixture is the temperature at which the sum of the vapour-tensions of the two independent ingredients is equal to one atmosphere.

263. Influence of Dissolved Air upon the Boiling-point.-The presence of air in the midst of the liquid mass is a necessary condition of regularity of ebullition, and of its production at the normal temperature; this is shown by several convincing experi

ments.

1. Donny's Experiment.—We take a glass tube bent twice, and terminated at one of its extremities by a series of bulbs. The first step is to wash it carefully with alcohol and ether, finally leaving in it some diluted sulphuric acid. These operations are for the purpose of removing the solid particles adhering to the sides, which always detain portions of air. Water is then introduced and boiled long enough to expel the air dissolved in it, and while ebullition is proceeding, the end of the apparatus is hermetically sealed. The other extremity is now plunged in a strong solution of chloride of calcium, which has a very high boiling-point, and the tube is so placed that all the water shall lie in this extremity; it will then be found that the temperature may be raised to 135° without producing ebullition.

1 That is steam heated above the temperature of saturation. Philosophically speaking, superheated steam is merely nonsaturated steam; but the name is never used except where the temperature exceeds the atmospheric boiling-point.

At about this temperature bubbles of steam are seen to be formed, and the entire liquid mass is thrown forward with great violence.

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The bulbs at the end of the tube are intended to diminish the shock thus produced.

2. Dufour's Experiment.-This experiment is still more decisive. A mixture of linseed-oil and oil of cloves, whose respective densities are about 93 and 1.01, is so prepared that, for temperatures near 100°, the density of the whole is nearly that of water. This mixture is placed in a cubical box of sheet-iron, with two holes opposite each other, which are filled with glass, so as to enable the observer to perceive what is passing within. The box is placed in a metallic envelope, which permits of its being heated laterally. When the temperature of 120° has been reached, a large drop of water is allowed to fall into the mixture, which, on reaching the bottom of the box, is partially converted into vapour, and breaks up into a number of smaller drops, some of which take up a position between the two windows, so as to be visible to the observer. The temperature may now be raised to 140°, 150°, or even 180°, without producing evaporation of any of these drops. Now the maximum tension of steam at 180° is equal to 10 atmospheres, and yet we have the remarkable phenomenon of a drop of water remaining liquid at this temperature under no other pressure than that of the external air increased by an inch or two of oil. The reason is that the air necessary to evaporation is not supplied. If the drops be touched with a rod of metal, or, better still, of wood, they are immediately converted into vapour

DUFOUR'S EXPERIMENT.

343

with great violence, accompanied by a peculiar noise. This is explained by the fact that the rods used always carry a certain quantity of condensed air upon their surface, and by means of this air the evaporation is produced. The truth of this explanation is proved by the fact, that when the rods have been used a certain number of times, they lose their power of provoking ebullition, owing, no doubt, to the exhaustion of the air which was adhering to their surfaces.

3. Production of Ebullition by the formation of Bubbles of Gas in the midst of a Liquid.-A retort is carefully washed with sulphuric acid, and then charged with water slightly acidulated, from which the air has been expelled by repeated boiling. The retort communicates with a manometer and with an air-pump. The air is exhausted until a pressure of only 150 millimetres is attained, corresponding to 60° as boiling-point. Dufour has shown that under these conditions the temperature may be gradually raised to 75° without producing ebullition. But if, while things are in this condition, a current of electricity is sent through the liquid by means of two platinum wires previously immersed in it, the bubbles of oxygen and hydrogen which are evolved at the wires immediately produce violent ebullition, and a portion of the liquid is projected explosively, as in Donny's experiment.

From these experiments we may conclude that liquid, when not in contact with gas, has a difficulty in making a beginning of vaporization, and may hence remain in the liquid state even at temperatures at which vaporization would upon the whole involve a fall of potential energy.

That vapour (as well as air) can furnish the means of overcoming this difficulty, is established by the fact noted by Professor G. C. Foster,1 that when a liquid has been boiling for some time in a retort, it sometimes ceases to exhibit the movements characteristic of ebullition, although the amount of vapour evolved at the surface, as measured by the amount of liquid condensed in the receiver, continues undiminished. In these circumstances, it would appear that the superficial layer of liquid, which is in contact with its own vapour, is the only part that is free to vaporize.

The preceding remarks explain the reluctance of water to boil in glass vessels carefully washed, and the peculiar formation, in these circumstances, of large bubbles of steam, causing what is called boil1 Watts's Dictionary of Chemistry, art. "Heat," p. 88.

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