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Fig. 20.

The pneumatic trough consists of a vessel containing water, fig. 19, across which, at the depth of from 5 to 8 centimetres from the top, a ledge or shelf is placed; the

jars destined to receive pla .„ the gas are filled with

water, and placed with their mouths downwards upon the shelf, which is kept about an inch (2 or 3 centimetres) under water; into these jars the gas is allowed to bubble up, and it may be transferred from one jar to another by an inverted pouring. When a jar has been filled, or partially filled with gas, it may be readily removed from place to place by sliding under its open mouth, while still immersed in water, B plate or shallow tray, containing water, on which it may be lifted out of the pneumatic trough as at B. (30) The Gas-holder.—When large quantities of gas are to be stored up, a different apparatus, the gas-holder, is employed, and in this instrument also, advantage is taken of the pressure of the atmosphere. The gasholder is represented in fig. 20. It consists of a cylinder, n, surmounted by a tray, A, for holding water; this tray communicates with the cylinder by means of two pipes provided with stop-cocks; one of these pipes, f, proceeds nearly to the bottom of the cylinder, B, and is open at both extremities; the other pipe, e, only just enters the top of the lower cavity: at the lower part of the cylinder is a short wide pipe, c, passing obliquely upwards, and furnished with a plug, by which it can be closed at pleasure. A third stop-cock is introduced at the upper part of the cylinder at g, to which a flexible tube may be attached for the convenience of transferring the gas. Now suppose the gas-holder to be full of atmospheric air, and to be wanted for use; the pipe, c, at the bottom is closed, water is poured into the tray, and both stop-cocks in the vertical pipes are opened: the water descends through the longer pipe, f, whilst the air escapes in bubbles through the shorter one, e: when B is completely full, the stop-cocks are closed, and the plug at the bottom removed; no water escapes, owing to the pressure

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of the atmosphere upon the surface of the liquid in the wide tube, c, the water being retained just as in the ordinary bird-fountain. The neck of the retort, B, or other vessel for producing the gas, is introduced completely within the cylinder, and the water is displaced by the gas which rises and accumulates in the upper part, whilst the water runs off into a vessel placed below. The progress of the experiment may be watched by means of the glass tube, d, which is open both at top and bottom into the cylinder, B; the level of the water within the instrument is thus always exhibited. In order to use the gas stored up, the plug is replaced at c, and the stop-cock in the long pipe opened to allow the column of water to exert its pressure on the gas, which escapes on cautiously turning the stop-cock, e, and may either be received in a jar placed in the tray over the short tube, e, or it may be conveyed away through a flexible tube attached to the stop-cock, g.

Fig. 21.

(40) Water dissolves all gases;

some in small quantities, and others

with very great avidity; the latter

of course cannot be collected" over

water. Indeed, in all cases where

great accuracy is requisite, some

other liquid must be substituted

in the trough and jars for water.

Mercury is the fluid which offers

fewest inconveniences, and it is

usually employed for this purpose in a trough of earthenware,

iron or wood, the form of which is seen in fig. 21.

(41) Correction of Gases for Pressure.—The foregoing mode of collecting gases over mercury leads us to consider a correction of great importance in cases where an accurate measurement of the volume of a gas is requisite. In all cases, a portion of air or gas which communicates with the atmosphere either through the walls of a flexible bag or bladder, or that is confined over water or mercury, is subject to the pressure of the atmosphere, transmitted to it either through the flexible material, or through the interposed portion of liquid. If, in the pneumatic trough, the liquid within and without the jar stand at the same level, the pressure upon the included gas will be exactly that due to the atmosphere at the time; if, however, the liquid within stand higher than that in the bath, the gas will be subjected to a pressure less than that of the atmosphere at the time, by the amount necessary to support the column of liquid above the outer level of that in the bath.

Observation has shown that the pressure of the atmosphere at the same spot is liable, from different causes, to continual variation. The average pressure at the sea-level is equivalent to that due to


a column of mercury 76omm- (or 29*93 inches) in height; but in this climate it is sometimes so much diminished as to support a column of only about 7iomm-; at other times the pressure will be equivalent to nearly 78omm• of mercury. Now the same quantity of gas will, under these different circumstances, sometimes occupy a volume considerably greater, at others considerably less, than the average.

It is necessary, therefore, in all experiments upon the density or volume of gases, to observe the height of the barometric column, as this gives the pressure to which the gas is at the same time subjected. This, however, is true only when the liquid in the bath, and that in the jar, are on the same level. In practice it is rarely possible to make them rigidly so. The liquid generally stands highest in the jar. Supposing the gas to have been collected over mercury, in order to allow for the dilatation occasioned by this inequality of level, the difference of the two levels must be accurately measured, and the measurement so obtained must be subtracted from the height of the mercurial column in the barometer at the time. A similar correction is required if the gas be standing over water, but it is smaller in amount, a column of water of 13596 centimetres in height being equivalent to 1 centimetre of mercury. When the necessary measurements have been made, a simple calculation shows the volume that any gas would have occupied, assuming it to have been measured under a barometric pressure of 760 millimeters*

Suppose that having measured 50 cubic centimetres of oxygen standing over mercury, the level of the metal in the jar being a5mm* higher than that in the bath, the barometer at the time standing at 740mm-, it is desired to ascertain what volume the gas would occupy under a pressure of 76omm- By

* In this country the standard or normal pressure to which gases are corrected, has been generally that of a column of mercury 30 inches in height; the French standard being that of a column of mercury 760 millimetres (or 29922 English inches) in height: consequently 100 cubic inches, measured under the English standard pressure of 30 inches, would, under the French standard, fill a space of 100263 cubic inches.

Strictly speaking, however, the observations should be reduced to the pressure of a column of mercury 29'92 2 inches in height at 320 F. Such a column, owing to the expansion of mercury by heat, would be increased g-^j- of its length, at the mean temperature of 6o° F., and consequently would then measure 3C006 inches: and under this pressure 100 cubic inches of any gas, measured at a barometric pressure of 30 inches, would be reduced to 99*98 cubic inches, a. difference so trifling that it may almost always be neglected.


Boyle's law (27) the volume of a gas is inversely as the pressure. Therefore—

Standard pressure. Observed pressure. Observed vol. Corrected vol.

-6omm. . (7i5mm-°r| .. co. . \m (=47-039

749 —25 J ° [cub. centim.)

Or, putting the whole into a general form:— If V be the corrected volume of the gas, V the observed volume, H the standard height of the barometer, H' the observed height at the time of the experiment, h the difference of level in the mercurial bath; then

V {W - h)

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In estimating the density from the volume of a gas, it is necessary to make a further correction for the temperature (144), as well as for the state of moisture or dryness which it may possess at the time.

(42) Density of the Atmosphere at Different Heights.—A remarkable consequence of the law of elasticity in gases is exhibited in the increasing rarefaction of the atmosphere in ascending from the surface of the earth. The air is subject to a pressure which decreases gradually with the progressive elevation above the sea-level. This will be evident if we consider the atmosphere to be composed of a series of layers or strata: the lowest layer supports the pressure of the entire superincumbent mass; the one next above this supports the pressure of all but the lowest; the third that of all but the two lower ones, and so on in succession. In consequence of Boyle's law—viz., that the volume of a gas is inversely as the pressure, it is found that if the air be examined at a series of heights, increasing according to the terms of an arithmetical progression, the density of the air decreases according to the terms of a geometrical progression. In the following table the heights above the surface are taken in arithmetical progression, increasing regularly by distances of 3"5 miles; the volume of equal weights of air at these successive heights increases in geometrical progression, the volume being doubled for each step in the ascent; while the density, and the corresponding height of the barometer, decrease in the same geometric ratio, being at each successive elevation exactly half what they were at the preceding one :—

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I —







1 n


The annexed diagram (fig. 22), slightly altered from one in Tomlinson's Treatise on Pneumatics, is supposed to represent a vertical section of the atmosphere; the left-hand column shows the height in miles above or below the sea-level; the right-hand column the corresponding heights of the barometer in inches; A indicates the altitude of the highest peaks of the Himalaya; B, the altitude of 7016 metres, or 23,019 feet, the height attained in a balloon by Gay-Lussac (17 Sept., 1804);* c, Dolcoath mine,Cornwall, 260 fathoms, or 475*48 metres; D, the deepest sea sounding yet obtained, 7706 fathoms, or 8919 miles, (Capt. Denham) H.M. ship Herald, Oct. 20, 1852, lat. 360 49' S., long. 370 6' W.f


* The height reached by Mr. Glaisher and Mr. Coxwell in their celebrated balloon ascent on Sept. 5, 1862, was about 37,000 feet (11,277 metres), or T 13 7 miles.

t This very deep sounding, however, according to subsequent careful observations by American navigators, appears to be greatly in excess of the truth: the line was probably dragged by strong currents so as to have deceived the observer. The deepest soundings which appear to be worthy of confidence were obtained to the southward of the great banks of Newfoundland, and do not exceed 4825 miles, or about 7620 metres (25,000 feet). The deepest sounding taken in the dredging expedition of 1869 was 14,610 feet, or 2767 miles (4453 metres), on July 22, in lat 470 38' N., long. 120 8' W. The deepest

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