Fig. 17.

Let B ([fig. 17.]) be a strong spherical vessel of brass, supported on a stand S, under which is placed a large spirit lamp L, or other means of heating it. In the top of this vessel are three apertures, in two of which are screwed a [Pg110] thermometer T, the bulb of which enters the hollow brass sphere, and a stop-cock C, which may be closed or opened at pleasure, to confine the steam, or allow it to escape. In the third aperture at the top, is screwed a long barometer tube, open at both ends. The lower end of this tube extends nearly to the bottom of the spherical vessel B. In the bottom of this vessel is placed a quantity of mercury, the surface of which rises to some height above the lower end of the tube A. Over the mercury is poured a quantity of water, so as to half fill the vessel B. Matters being thus arranged, the screws are made tight, so as to confine the water, and the lamp is allowed to act on the vessel; the temperature of the water is raised, and steam is produced, which, being confined within the vessel, exerts its pressure on the surface of the water, and resists its ebullition. The pressure of the steam acting on the surface of the water is communicated to the surface of the mercury, and it forces a portion of the mercury into the tube A, which presently rises above the point where the tube is screwed into the top of the vessel B. As the action of the lamp continues, the thermometer T exhibits a gradually increasing temperature; while the column of mercury in A shows the force with which the steam presses on the surface of the water in B,—this column being balanced by the pressure of the steam. Thus, the temperature and pressure of the steam at the same moment may always be observed by inspecting the thermometer T and the tube A. When the column in the tube A has risen to the height of 30 inches above the level of the mercury in the vessel B, then the pressure of the steam will be equivalent to double the pressure of the atmosphere, because, the tube A being open at the top, the atmosphere presses on the [Pg111] surface of the mercury in it. The thermometer T will be observed gradually to rise until it attains the temperature of 212°; but it will not stop there, as it would do if immersed in water boiled in an open vessel. It will, on the other hand, continue to rise; and when the column of mercury in A has attained the height of 30 inches, the thermometer T will have risen to 251°,—being 39° above the ordinary boiling point.

During the whole of this process, the surface of the water being submitted to a constantly increasing pressure, its ebullition is prevented, and it continues to receive heat without boiling. That it is the increased pressure which resists its ebullition, and causes it to receive a temperature above 212°, may be easily shown. Let the stop-cock C be opened; immediately the steam in B, having a pressure considerably greater than that of the atmosphere, will rush out, and will continue to issue from C, until its pressure is balanced by the atmosphere. At the same time the column of mercury in A will be observed rapidly to fall, and to sink below the orifice by which it is inserted in the vessel B. The thermometer T will also fall until it attains the temperature of 212°. At that point, however, it will remain stationary; and the water will now be distinctly heard to be in a state of rapid ebullition. If the stop-cock C be once more closed, the thermometer will begin to rise, and the column of mercury ascending in A will be again visible.

If, instead of a stop-cock being at C, the aperture were made to communicate with a valve, like the safety-valve of a steam engine, loaded with a certain weight,—say at the rate of 15 lbs. on the square inch,—then the thermometer T, and the mercury in the tube A, would not rise indefinitely as before. The thermometer would continue to rise till it attained the temperature of 251°; and the mercury in the tube A would rise to the height of 30 inches. At this limit the resistance of the valve would be balanced by the pressure of the steam; and as fast as the water would have a tendency to produce steam of a higher pressure, the valve would be raised and the steam suffered to escape; the thermometer T and the column of mercury in A remaining stationary during this process. If the valve were loaded more heavily, the phenomena would be [Pg112] the same, only that the mercury in T and A would become stationary at certain heights. But, on the other hand, if the valve were loaded at a less pressure than 15 lbs. on the square inch, then the mercury in the two tubes would become stationary at lower points.

(59.)

This may be easily accomplished by the aid of an air pump. Let water at the temperature of 200° be placed in a glass vessel under the receiver of an air pump, and let the air be gradually withdrawn. After a few strokes of the pump, the water will boil; and if the mercurial gauge of the pump be observed, it will be found that its altitude will be about 231⁄2 inches. Thus the pressure to which the water is submitted has been reduced from the ordinary pressure of the atmosphere expressed by the column of 30 inches of mercury, to a diminished pressure expressed by 231⁄2 inches; and we find that the temperature at which the water boils has been lowered from 212° to 200°. Let the same experiment be repeated with water at the temperature of 180°, and it will be found that a further rarefaction of the air is necessary, but the water will at length boil. If the gauge of the pump be now observed, it will be found to stand at about fifteen inches, showing, that at the temperature of 180° water will boil under half the ordinary pressure of the atmosphere. These experiments may be varied and repeated; and it will be always found, that, as the pressure is diminished or increased, the temperature at which the water will boil will be also diminished or increased.

(60.)

(61.)

(62.)

Barometer.
inches		Boiling
Point.
26 204°·91
26·5205°·79
27 206°·67
27·5207°·55
28 208°·43
28·5209°·31
29 210°·19
29·5211°·07
30 212°
30·5212°·88
31 213°·76