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oxygen consists of only two. Ozone, therefore, is a polymer of oxygen ; its molecule is more condensed, three atoms occupying two unit volumes. This conclusion as to the constitution of ozone has been arrived at from the consideration of a number of experimental facts.

(1.) When oxygen is subjected to the action of the electric discharge, it is found to undergo a diminution in volume.* This was shown by Andrews and Tait by means of the tube seen in Fig. 38. The tube was filled with dry oxygen, which was prevented from escaping by means of the sulphuric acid contained in the bent portion of the narrow tube, which served as a manometer. When the

silent discharge was passed through the oxygen, a
contraction in the volume took place, indicated by
a disturbance of the level of the acid in the syphon,
When the tube was afterwards heated to about
300° C. and allowed to cool, the gas was found to
have returned to its original volume, and to be
devoid of ozone. This could be repeated inde-
finitely, the gas contracting when ozonised and re-
expanding when the ozone was converted by heat
into ordinary oxygen. As only a very small propor-
tion of the oxygen was converted into ozone, this
experiment alone afforded no clue as to the rela-
tion between the change of volume and the extent
to which this conversion took place.

(2.) A small sealed glass bulb, containing a solution of potassium iodide, was placed in the tube

before the experiment. The oxygen was ozonised, ses and the usual contraction noticed. The bulb was then broken, and on coming in contact with the ozone present the potassium iodide was decomposed, iodine being liberated. No further contraction, however, followed ; and, further, when the tube was subsequently heated to 300° and cooled, the gas suffered no increase in volume. By carefully estimating the amount of iodine that was liberated by the ozone, the actual amount of oxygen which had caused this liberation could be determined according to the equation

2KI + H,0+0=1, +2KHO, and it was found that the volume of oxygen so used up was exactly

* " Chemical Lecture Experiments," new ed., Nos. 63, 64.

Fig. 38.

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equal to the contraction which first resulted on the ozonisation of the oxygen.

These facts proved that when potassium iodide was oxidised by ozone a certain volume of ordinary oxygen was liberated, which was equal to the volume of ozone ; and a certain volume was used up, which was equal to the original contraction.

These facts were explained by the supposition that ozone was represented by the molecular symbol 03 ; and its action upon potassium iodide may be expressed as follows2KI+H,0+02=0, +19+2KHO.

(3.) To prove the correctness of this supposition, however, it was necessary to learn the exact relation between these two volumes. This Soret did, by making use of the property possessed by turpentine (and a other essential oils) of absorbing ozone without decomposing it; and he found that the diminution in volume which took place by absorbing ozone from ozonised oxygen was exactly twice as great as the increase in volume that resulted when the same volume of ozonised oxygen was heated.

This fact may be shown by means of the apparatus, Fig. 39.* The oxygen to be ozonised is contained in the annular space between the elongated hollow stopper, which reaches nearly to the bottom, and

Fig. 39. the outer tube. The turpentine is contained in a little sealed thin glass tube d, almost capillary in bore, which is held in position between four little projecting glass points a and ó upon the stopper and outer tube. The temperature is main

* Newth, Trans. Chem. Soc., 1896, p. 1298.

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tained constant throughout the experiment by placing the apparatus in melting ice. One wire from the induction coil is dipped into the ice water, while the other passes into the dilute acid contained in the stopper. When the electric discharge is passed a portion of the oxygen is ozonised, resulting in a contraction in the volume which is indicated by a rise of the liquid in the gauge ; when sufficient contraction has taken place the discharge is interrupted, and the contents of the capillary tube brought into contact with the gas. This is done by a slight twist of the stopper, which thereby crushes the little tube and throws out the turpentine. Immediately a further contraction takes place, due to the absorption of the ozone by the reagent, and if the gauge be graduated it will be seen that this second contraction is twice as great as the first.

(4.) If the molecule of ozone be correctly represented by Oj, its density will be 24, as against 16 for oxygen ; and its rate of diffusion will be proportionately slower in accordance with the law of gaseous diffusion (see Diffusion of Gases, p. 84). Soret found that this was actually the case, and from his experiments the number 24 for the density of ozone receives conclusive confirmation.

CHAPTER III

COMPOUNDS OF HYDROGEN WITH OXYGEN THERE are two oxides of hydrogen known, viz. :

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WATER. Formula, H,0. Molecular weight = 18.02. Until the time of Cavendish, water was considered to be an elementary substance. Priestley had noticed that when hydrogen and oxygen were mixed and inflamed, moisture was produced, and he had also observed that the water so obtained was sometimes acid. Cavendish showed that the water was actually the product of the chemical union of hydrogen with oxygen, and he also discovered that the acidity which this water sometimes possessed was due to the presence of small quantities of nitric acid ; and he traced the formation of this acid to the accidental presence of nitrogen (from the atmosphere) with which the gases were sometimes contaminated.

Cavendish filled a graduated bell-jar with a mixture of hydro

Fig. 40. gen and oxygen, in the proportion of two volumes of the former to one of oxygen, and he attached to the bell-jar a stout glass vessel resembling the pear-shaped apparatus shown in Fig. 40, which was perfectly dry and rendered vacuous. On opening the stop-cocks, gas entered the exhausted tube, which was furnished at the top with two platinum wires. The cocks were again closed and an electric spark passed through the mixed gases, thereby causing their explosion, when the interior surface of the previously dry glass vessel was found to be dimmed with a film of moisture. On again opening the stop-cocks more gas was drawn into the upper vessel, the same volume passing in as originally entered the evacuated apparatus. This showed that the two gases in their combination with each other had entirely disappeared. By repeatedly filling the vessel with the mixed gases and causing them to unite in this way, Cavendish succeeded in collecting sufficient of the water to identify the liquid, and prove that it was in reality pure water.

The more exact volumetric proportion in which oxygen and hydrogen combine to form water has been determined by modern eudiometric methods which have been developed from Cavendish's experiment. Accurately measured volumes of the two gases are introduced into a long graduated glass tube standing in the mercurial trough and provided with two platinum wires, by means of which an electric spark can be passed. The gases are caused to unite by means of the spark, and the contraction in volume is carefully observed. Fig. 41 shows the apparatus for this purpose. The long glass tube A having a millimetre scale graduated upon it, and having two platinum wires sealed into the glass near the upper and closed end, is completely filled with mercury and inverted in the trough of the same liquid : this tube is known as a eudiometer. A quantity of pure oxygen is then introduced into the tube, and the volume occupied by the gas carefully read off upon the graduations. Seeing that the volume occupied by a given mass of gas is dependent both upon the temperature and the pressure, each of these factors has to be taken into account in the process of this experiment. The temperature is ascertained by the attached thermometer T. The pressure under which the gas is, will be the atmospheric pressure at the time (ascertained by the barometer B placed near the apparatus) minus the pressure of a column of mercury, equal to the height of the mercury within the eudiometer above the level of that in the trough. This height is obtained in millimetres by carefully reading upon the graduated scale the level of the mercury in the trough and the top of the column in the tube, and the number of millimetres so obtained is deducted from the barometric reading. These observations are made by means of

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