CHAPTER XXVI.

EBULLITION.

254. Ebullition.-When a liquid contained in an open vessel is subjected to a continual increase of temperature, it is gradually changed into vapour, which is dissipated in the surrounding atmosphere. This action is at first confined to the surface; but after a certain time

bubbles of vapour are formed in the interior of the liquid, which rise to the top, and set the entire mass in motion with more or less vehemence, accompanied by a characteristic noise; this is what is meant by ebullition or boiling.

If we observe the gradual progress of this phenomenon,—for example, in a glass vessel containing water, we shall perceive that, after a certain time, very minute bubbles are given off; these are bubbles of dissolved air. Soon after, at the bottom of the vessel, and at those parts of the sides which are most immediately exposed to the action of the fire, larger bubbles of vapour are formed, which decrease in volume as they ascend, and disappear before reaching the surface. This stage is accompanied by a peculiar sound, indicative of approaching ebullition, and the liquid is said to be singing. The sound is probably caused by the collapsing of the bubbles as they are condensed by the colder water through which they pass. Finally, the bubbles increase in number, growing larger as they ascend, until they burst at the surface, which is thus kept in a state of agitation; the liquid is then said to boil.

255. Laws of Ebullition.-1. At the ordinary pressure, ebullition commences at a temperature which (roughly speaking) is definite for each liquid.

This law is analogous to that of fusion (§ 225). It follows from this that the boiling-point of any liquid is a specific element, serving to determine its nature.

The following table gives the boiling-points of several liquids at the pressure of 760 millimetres:

2. The temperature, in ordinary circumstances, remains constant during ebullition. If a thermometer be introduced into the glass vessel of Fig. 245, the temperature will be observed to rise gradually during the different stages preceding ebullition; but, when active. ebullition has once commenced, no further variation of temperature will be observed. This phenomenon points to the same conclusion as the cold produced by evaporation.

Since, notwithstanding the continuous action of the fire, the temperature remains constant, the conclusion is inevitable, that all the heat produced is employed in doing the work necessary to change the liquid into vapour. The constancy of temperature during ebullition explains the fact that vessels of pewter, tin, or any other easily fusible metal, may be safely exposed to the action of even a very hot fire, provided that they contain water, since the liquid remains at a temperature of about 100°, and its contact prevents the vessel from over-heating.

3. The elastic force of the vapour given off during ebullition is equal to the pressure of the external air.

This important proposition may be experimentally demonstrated in the following manner:

We take a bent tube A, open at the longer extremity, and closed at the shorter. The short branch is filled with mercury, all but a small space containing water; in the long branch the mercury stands. a little higher than the bend. Water is now boiled in a glass vessel, and, during ebullition, the bent tube is plunged into the steam. The water occupying the upper part of the short branch is partially converted into steam, the mercury falls, and it assumes the same

level in both branches. Thus the pressure exerted by the atmosphere at the open extremity of the tube is exactly equal to that exerted by the vapour formed by water in ebullition.

256. Theory of Ebullition.This latter circumstance supplies the true physical definition of ebullition. A liquid is in ebullition when it gives off vapour of

the same tension as the atmosphere above it.

The necessity of this equality of tension is easily explained. If a bubble of vapour exists in the interior of a liquid as at m (Fig. 247), it is subject to a pressure exceeding atmospheric by the weight of the liquid above it. As the bubble rises, the latter element of pressure becomes less, and the tension of the vapour composing the bubble accordingly diminishes, until it is reduced to

atmospheric pressure on reaching the surface.

The boiling-point of a liquid is therefore necessarily fixed, since it is the temperature at which the tension of the vapour at saturation is equal to that of the atmosphere. It must be remarked, however, that this temperature varies in the different layers of the liquid, and that it increases with the depth below the surface. Accordingly, in determining the second fixed point of the thermometer, we have stated that the instrument should be plunged into the steam, and not into the water.

257. Effect of Pressure upon the Boiling-point.-It evidently follows from the foregoing considerations that the boiling-point of a liquid must vary with the pressure on the surface; and experiment shows that this is the case. Water, for instance, boils at 100° under the external pressure of 760 millimetres; but if the pressure decreases, ebullition occurs at a lower temperature. Under the receiver of an air-pump, water may be made to boil at any tem

Fig. 247.

FRANKLIN'S EXPERIMENT.

337

perature between 0° and 100°. In Carré's apparatus (Fig. 239) the water in the glass bottle is observed to enter into active ebullition a few moments before the appearance of the ice. The reason, therefore, why boiling water has come to be associated in our minds with a fixed temperature is that the variations of atmospheric pressure are comparatively small.

At Paris, for instance, the external pressure varies between 720 and 790 millimetres (28.3 and 311 inches), and the boiling-point, in consequence, varies from 98.5° to 101·1°.

258. Franklin's Experiment.-The boiling of water at a temperature lower than 100° may be shown by the following experiment::

A little water is boiled in a flask for a sufficient time to expel most of the air contained in it. The

flask is then removed from the source of heat, and is at the same time securely corked. To render the exclusion of air still more certain, it may be inverted with the corked end immersed in water which has been boiled. Ebullition ceases almost immediately; but if cold water be now poured over the vessel, or, better still, if ice be applied to it, the liquid again begins to boil, and continues to do so for a considerable time. This fact may easily be explained: the contact of the cold water or the ice lowers the temperature and tension of the steam which presses upon the surface of the liquid, and the decrease of tension causes the renewal of ebullition.

A Jahandier

CIAPL

260. Determination of Heights by Boiling-point.-Just as we can determine the boiling-point of water when the external pressure is given, so, if the boiling-point be known, we can determine the external pressure. In either case we have simply to refer to a table of maximum tensions of aqueous vapour at different temperatures.

As the barometer is essentially unsuitable for portability, Wollaston proposed to substitute the observation of boiling-points as a means

of determining pressures. For this purpose he employed a thermometer with a large bulb and with a scale extending only a few degrees above and below 100°. He called this instrument the barometric thermometer.

Regnault has constructed a small instrument for the same purpose, which he calls the hypsometer. It consists of a little boiler heated

by a spirit-lamp, and terminating in a telescope tube with an opening at the side through which the steam escapes. A thermometer dips into the steam, and projects through the top of the tube so as to allow the temperature of ebullition to be read.

This temperature at once gives the atmospheric pressure by reference to a table of vapour-tensions, and the subsequent computations for determining the height are the same as when the barometer is employed (§ 112).

When only an approximate result is desired, it may be assumed that the height above sea-level is sensibly proportional to the difference between the observed boilingpoint and 100° C., and Soret's formula1 may be employed, viz.:

h=295 (100−t),

where h is expressed in metres and t in degrees Centigrade.

Thus, at Quito, where the boiling-point of water is about 90·1°, the height above sealevel would be 9.9 x 295-2920 metres, which agrees nearly with the true height 2808 metres.

At Madrid, at the mean pressure, the boiling-point is 97·8°, which gives 2.2 × 295-649 metres; the actual height being 610 metres.

261. Papin's Digester.-While a decrease of pressure lowers the boiling-point, an increase of pressure raises it. Accordingly, by putting the boiler in communication with a reservoir containing air at the pressure of several atmospheres, we can raise the boiling-point to 110°, 115°, or 120°; a result often of great utility in the arts. But in 1If h be expressed in feet, and t in degrees Fahrenheit, the formula becomes

h=538 (212-t).