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(334) Compound Nature of the Atmosphere.-The chemical researches of the philosophers of the last century are especially remarkable on account of the important information which they afforded upon the nature of the atmosphere. Indeed, the knowledge thus obtained may be regarded as the starting-point of the brilliant chemical discoveries which have since succeeded each other with such rapidity. These researches have abundantly proved that the air is far from being, as it was once supposed to be, an elementary body. It has been found, on the contrary, to be a mixture of several substances, some of which are elementary, others compound.

The most remarkable and abundant of the constituents of the air are the elementary bodies, oxygen and nitrogen; and of its compound ingredients, the most important are aqueous vapour and carbonic acid, or rather carbonic anhydride, as it must be called, in order to preserve consistency in our nomenclature.

The most direct proofs of the compound character of the

COMPOUND NATURE OF THE ATMOSPHERE.

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atmosphere are afforded by examining the effects produced upon it by burning bodies. Bodies, as is well known, cannot burn without the free access of air. On placing a lighted taper under the receiver of the air-pump and exhausting the air, the flame becomes extinguished. A limited quantity of air will support combustion for but a limited period: a lighted taper floating on water under an inverted bell-glass, the edge of which is plunged beneath the water, soon begins to burn dimly, and at length becomes extinct. But the taper ceases to burn long before the air is all spent; the receiver still contains a large quantity of a gaseous body in which a candle will not burn. The results obtained by burning a candle in a limited portion of air are, however, rather complicated, because the products which are formed by the burning body rise in the form of gas, and become mixed with the remaining portion of air. Lavoisier contrived to obviate this inconvenience by acting upon the air with a substance which produced a solid body as the result of the chemical action, so that it left the air unmixed with any gas which rose from the burning body. The material which he employed to decompose the air was metallic mercury, a substance which acts very slowly, and which does not burn in the ordinary sense of the term. The experiment may be performed as follows:

Into the bulb of a flask or retort (A, fig. 266), provided with a neck of considerable

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FIG. 266.

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cury, into a jar partially filled with atmospheric air. The bulk of this portion of air is then accurately observed, and the temperature and barometric pressure at the time of the observation are recorded. Heat is now applied to the flask, and maintained steadily at a point just below that required to make

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PROPERTIES OF OXYGEN.

the mercury boil. If this temperature be continued for three or four consecutive days, the air inclosed both in the flask and in the jar will participate in the action. The mercury in the flask will gradually become covered with red scales, and the air, which at first expanded from the action of the heat, and depressed the level of the mercury in the jar, will slowly decrease in bulk until fresh scales no longer continue to be formed. When this point is reached, the source of heat may be removed, and the remaining air, when cold, will be found to measure about one-fifth less than it did at the commencement. If a portion of this residual air be decanted into another jar, it will be found to be unfit for the support of animal life; a mouse or other small animal introduced into it speedily dies, and the flame of a candle is instantly extinguished. The gas which has been thus obtained is an elementary body, nearly in a state of purity, termed nitrogen (339). In this experiment the heated mercury has been slowly effecting the removal of the oxygen from the air.

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§ I. OXYGEN. = 16; Atomic Vol. ; Molecule in free state, 00; (0=8) Specific Gravity, 1*1056.* ᎾᎾ

(335) If the red scales which are formed upon mercury when, as in the foregoing experiment, it is heated in a confined portion of air, be introduced into a small glass retort and exposed to a strong heat, they will gradually disappear; drops of mercury will become condensed in the cool part of the retort, and a gas will be disengaged, which may be collected over water. If the experiment be performed with accuracy the quantity of the gas obtained will be exactly equal in volume to the bulk absorbed from the air by the

* In the former editions of this work, in accordance with general usage, the symbol for oxygen has been taken as O= 8, and the volume occupied by 8 parts by weight of oxygen has been employed as the unit of gaseous volume. The arguments of Gerhardt, and the progress of research, however, seem conclusively to indicate that the number for the atomic weight of oxygen should be doubled, if that of hydrogen remain unaltered. Consequently an extensive change in notation, and indeed in the interpretation of chemical phenomena generally, becomes necessary.

If the atomic weight of oxygen be represented as = 16, the molecule of free oxygen will be (00); the atomic weight of hydrogen being (H=1), the molecule of free hydrogen will be (HH), occupying the same volume as a molecule of oxygen; and the molecular weight of water will be H20=18, instead of HO=9. The molecular volume of compounds will, unless specifically stated to be otherwise, be represented uniformly by 2 volumes instead of by the anomalous method of representing some compounds by 2-, others by 4-volume formulæ, as heretofore practised, in the various equations by which chemical reactions are represented.

Molecular formule will always be employed where it is practicable. Such molecular formula will indicate quantities of each compound the volumes

mercury.

PROPERTIES OF OXYGEN.

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To this gas the name of Oxygen (generator of acids, from ove sour, and yɛvváw to produce) has been given. It is an elementary body, and from the abundance in which it occurs, the number and the variety of its compounds, and the necessary part which it performs in the maintenance of life, it must be regarded as the most remarkable and important of the simple bodies.

FIG. 267.

Properties.—Oxygen possesses extremely powerful chemical attractions for other elementary substances; one element only (fluorine) being known with which it does not combine. Owing to the intensity with which many of these combinations take place, oxygen gas possesses the power of supporting combustion in an eminent degree. If a splinter of wood with a glowing spark on any part of it be plunged into the gas, the wood will instantly burst into flame, and will burn with extraordinary brilliancy. Many bodies which burn tranquilly in air often deflagrate with violence in oxygen. Phosphorus burns in it with a brilliancy which is painful to the eye, and in like manner sulphur and charcoal, if previously kindled, burn in the gas with great vehemence: many metals also burn vividly in it; a piece of potassium of the size of a pea, if placed in a small copper spoon, c, fig. 267, and heated strongly by a spirit lamp, bursts into flame when plunged into the gas; if a piece of German tinder be attached to a piece of watch-spring or thin steel wire and be

lighted, to start the combustion before it is introduced into the

of which amount to double the combining volume of hydrogen, if H=1; as for example:

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Equal
vols.

Molecular
weight.

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Oxygen belongs to a class of elements including sulphur, selenium, and tellurium, which may be termed electro-negative dyads. They are characterized by their power of uniting each with twice its volume of hydrogen, furnishing a gaseous compound in which the three volumes are condensed into the space of two: one atom of the elements belonging to this group may be said to be equivalent in combination to two atoms of hydrogen or of the halogens. The molecule of these elements contains 2 atoms; e.g. SS or S2=64, the molecule of sulphur; Se, = 159, the molecule of selenium.

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PREPARATION OF OXYGEN.

oxygen, the wire will burn with brilliant scintillations; and zinc foil, tipped with sulphur and kindled, burns in oxygen with an intense bluish white light.

Oxygen is essential to the support of animal life, and hence by the older chemists was termed vital air. A small animal will live in a confined space filled with oxygen for a longer period than in an equal bulk of air; but the gas is of too stimulating a quality to be breathed undiluted with impunity for any considerable time, and before long it produces death from over-excitement of the system.

Oxygen, like air, is destitute of colour, taste, and smell. Of all known substances it exerts the smallest refracting power upon the rays of light. Hitherto all attempts to reduce it to the liquid form by the combined application of pressure and of cold have proved fruitless. Oxygen has been proved to possess weak but decided magnetic properties, like those of iron, and like this substance its susceptibility to magnetization is diminished, or even temporarily suspended, by a sufficient elevation of temperature (325). It is heavier than the atmosphere, its specific gravity, according to Regnault, being 110563;* 100 cubic inches weighing 34 203 grains; it is only slightly soluble in water, which takes up about of its bulk, at 32°, and at 60°; 100 cubic inches of water, according to Bunsen, dissolving 4'11 cubic inches at 32°, and 2.99° cubic inches at 59° F.

Preparation. There are several methods of procuring oxygen gas, the simplest of which consist in the exposure of certain metallic oxides to a high temperature, by which they are made to give up, more or less completely, the oxygen with which the metals had combined.

1. The original method of Priestley, by which he first isolated pure oxygen, ín 1774, consisted in heating the red oxide of mercury to 700° or 800°, 2 Hg yielding 2 Hg+0,; but there are other modes of procuring it which are more convenient and economical.

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2. For the supply of large quantities of oxygen it is usual to employ the black oxide of manganese (MnO,), a mineral which at a red heat parts with one-third of the oxygen which it contains, whilst a reddish-brown oxide of manganese remains behind; 3 MnO, giving Mn,,+0. The mineral must be reduced to small fragments of about the size of a pea, and introduced into an iron

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* Dumas and Boussingault found the density of oxygen almost exactly the same as that given above-viz. 11057; De Saussure states it to be 1.1056.

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