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a telescope placed at such a convenient distance that the heat of the body may not introduce disturbances.

The data obtained give the volume of gas at a particular temperature, and under a pressure less than that of the atmosphere. By the process of calculation explained under the general properties of gases (p. 69), this is reduced to the standard temperature and pressure, viz., oo and 760 mm.

A quantity of hydrogen is then introduced into the eudiometer, considerably in excess of that required for complete combination

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with the oxygen, and the volume again ascertained with the above precautions and corrections.

The difference between the first and second reading will give the volume of hydrogen which has been added.

The eudiometer is then firmly held down against a pad of caoutchouc upon the bottom of the trough, and the gases fired by an electric spark from a Ruhmkorff coil. A bright flash of light passes down the tube, and on releasing it from the indiarubber bed, mercury enters to fill the space previously occupied by the gases which have combined.

On allowing the instrument to once more acquire the temperature of the surrounding atmosphere, the residual volume is read off and corrected for temperature and pressure.

The following data have now been obtained :-

(1.) The volume of oxygen, corrected for temperature and

pressure. (2.) The volume of mixed oxygen and hydrogen, corrected for

temperature and pressure. (3.) The volume of residual hydrogeri, corrected for tempera

ture and pressure.

A concrete example will explain how the result is deduced from these observations :

Corrected volume of oxygen used . . . . . 45.35
Corrected volume after the addition of hydrogen . . 256.05

Corrected volume of residual hydrogen . . . . 120. 10
256.05 - 45.35=210.70=total volume of hydrogen employed.
210.70 - 120.10= 90.60=volume of hydrogen which has combined with
45.35 volumes of oxygen.

1:45.35:1::90.60 : 1.997. .. One volume of oxygen has combined with 1.997 volume of hydrogen

to form water. * The volume composition of water may be shown by analytical

processes, as well as the synthetical method described above. This decomposition of water is most conveniently effected by means of an electric current. If the two terminals from a galvanic battery are connected to two pieces of platinum wire or foil, and these are dipped into acidulated water, bubbles of gas make their appearance upon each of the wires. If these two

strips of platinum be so arranged in a Fig. 42.

bottle that all the gas evolved escapes

by a delivery-tube (Fig. 42), it will be found that the gas explodes violently on the application to it

* In accurate experiments the volume occupied by the minute quantity of water formed has to be taken into account, and a number of other correcuon5 have to be made that are not mentioned in this outline description of the process.

of a lighted taper, showing it to be a mixture of oxygen and hydrogen. By modifying the apparatus in such a way that the gas from each platinum plate shall be collected in separate tubes, so arranged that the volumes of the gases can be measured, it is found that twice as much hydrogen is evolved, in a given time, as oxygen. A convenient form of voltameter is seen in Fig. 43, where the two measuring tubes are suspended over the platinum plates contained in a glass basin. The electrode, which is connected with the negative terminal of the battery, is the one from which the larger volume of gas, viz., the hydrogen, is evolved, while the oxygen is liberated at the positive plate.

When the volumes of the gases are carefully measured, it is found that they are not exactly in the proportion of two of hydrogen to one of oxygen, but that the oxygen is in deficit of this proportion. This is due, in the first place, to the greater solubility of oxygen in water than of hydrogen ; and, secondly, to the formation of a certain quantity of ozone during the electrolysis, whereby there is a shrinking of volume in the proportion of three to two.

Fig. 43.

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The “ electrolysis of water," as this process is usually called, is not the simple phenomenon that at first sight it might appear to be. In the first place, pure water is not an electrolyte, and it is necessary either to acidulate it or to render it alkaline by the addition of sodium or potassium hydroxides. Solutions of these substances in water are electrolytes, and the substances are dissociated into their respective ions (see Electrolysis, Chap. xi.) to an extent depending upon the degree of dilution of the solution. In the case of water acidulated with sulphuric acid, these ions are 2H and SO, The first action of the electric current, therefore, is to cause these ions to travel to their respective electrodes and there to discharge their electricity; the hydrogen ions going to the cathode, and the SO, ions to the anode. The hydrogen ions at the cathode, after becoming discharged of their positive electricity, unite to form molecules of hydrogen which then escape as gas. At the anode the SO4 jon, after discharging its negative electricity, undergoes chemical interaction with the water with which it is in contact, with the re-formation of sulphuric acid and liberation of oxygen. The effect of the curient may be represented thusAt the Cathode.

At the Anode. 2H12

250, +2H,0=2H.SO, +0.. Indirectly, therefore, the water is decomposed. As already stated, some of the nascent oxygen is converted into ozone, some also unites with water to form hydrogen peroxide, H,02, and probably a still larger quantity is employed in oxidising the sulphuric acid to persulphuric, which is always formed in solution at the anode. Berthelot considers that the variation of the proportion of hydrogen and oxygen evolved during the electrolysis of dilute sulphuric acid, from the proportion of two volumes of hydrogen to one volume of oxygen, is mainly, if not entirely, due to the formation of persulphuric acid.

It has been recently shown * that the purest “electrolytic gas," as this mixture of hydrogen and oxygen is called, is obtained by the electrolysis of pure barium hydroxide. Under these circumstances the oxygen contains no ozone or hydrogen peroxide.

The Volumetric Composition of Steam.- When a mixture of oxygen and hydrogen is exploded in a eudiometer, we have seen that a certain contraction of volume follows, due to the formation of water by the uniting gases. The oxygen and hydrogen that have entered into combination have disappeared as gases, the volume of the resultant water being practically negligible. It is important to know what relation exists between the volume of the uniting gases and the volume of the product of their combination when in a state of vapour—that is to say, what volume of steam is produced by the union of one volume of oxygen with two volumes of hydrogen ; in other words, whether there is any molecular contraction in the formation of steam.

To ascertain this, the mixed gases, in the exact proportions to form water, must be made to combine under such circumstances that the product shall remain in a state of gas or vapour, so that its volume and that of the mixed gases may be measured under comparable conditions. For this purpose a mixture of oxygen and hydrogen, obtained by the electrolysis of acidulated water, is introduced into the closed limb of the U-shaped eudiometer shoun in Fig. 44.1 This tube is graduated into three equal divisions, indicated by the broad black bands, and is furnished with two platinum wires at the closed end. It is also surrounded by an outer tube, so that a stream of vapour from some liquid, boiling

* Baker, Jour. Chem. Soc., April 1902.

+ See Experiments Nos. 74 and 75, “ Chemical Lecture Experiments, new. ed.

bove the boiling-point of water, can be made to circulate. The nost convenient liquid for the purpose is amyl alcohol, which poils at 130°. In this way the eudiometer and the contained gases vill be maintained at a constant temperature high enough to keep he water formed by their combination in the state of vapour.

The amyl alcohol is briskly boiled in the flask, and its vapour is led into the tube surrounding the eudiometer. The temperature of the mixed gases is thereby raised to 130°, and they occupy the

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three divisions of the tube when the mercury in the open limb is at the same level, that is, when the gases are under atmospheric pressure. The amyl alcohol vapour leaves the apparatus by the glass tube at the bottom, and is conveyed away and condensed. An electric spark is then passed through the gases by means of the induction coil. (In order to prevent the mercury from being forcibly ejected from the open limb of the U-tube at the moment of explosion, an additional quantity of mercury is poured in, and the open end is closed by the thumb when the spark is passed.) On bringing the enclosed gas again to the atmospheric pressure, by

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