18

CHLORINE --ITS ELECTROLYTIC DISENGAGEMENT;

limb has no such bleaching powers, but leaves the blue colour of the liquid unchanged. So soon as a sufficient quantity of gas has been collected in the sealed limb (a result usually obtained in eight or ten minutes), the electric current is interrupted, and the gas is transferred to the open limb, previously filled up with water and closed with the thumb (Fig. 21). It is found to be inflammable, and we recognize it without difficulty as hydrogen.

The experiment is now reversed by connecting the positive pole with the sealed, and the negative with the open, limb (Fig. 22);

and immediately, as might be expected, hydrogen escapes in abundance from the open mouth of the bent tube, while the liquid becomes decolorized in the closed limb. But this altered disposition of the experiment enables us to acquire some further information regarding the second gas,-that, namely, which is evolved in small quantity, but marked by its powerfully irritating odour, and by its bleaching action upon vegetal colours. In the course of ten or fifteen minutes the decolorized liquid in the sealed limb begins to assume a yellowish-green colour, and the evolution of gas (at the outset scarcely perceptible) becomes gradually more and more copious; so that, in thirty or forty minutes, the greater portion of the tube is filled with a transparent yellowish-green gas.

The

ITS ORDINARY PREPARATION.

19

battery current is now interrupted, and the gas transferred to the open limb for examination. The approach of a light proves it to be uninflammable: the mouth of the tube is no sooner unclosed than its suffocating odour, already mentioned, becomes most plainly perceptible, while its powerful bleaching property is shown by its decolorizing, in the very act of transference to the open limb of the tube, the portion of the acid which had retained its blue colour. This peculiar gas has received the name of Chlorine, derived from the Greek word XAwpós (yellowish-green).

Chlorine may be obtained from muriatic acid by another process, which has the great advantage of evolving it without giving rise to a simultaneous disengagement of hydrogen. Muriatic acid, when heated with powdered black oxide of manganese, in a flask (Fig. 23), yields abundance of chlorine, which

may be collected in inverted gas-jars, over tepid water. The manganese process is universally adopted when large quantities of chlorine are required. Hereafter, when we come to study more minutely the nature of this important body, we shall have occasion to dwell on the details of this process: at present it

20

CHARACTERS OF CHLORINE.

only claims our attention, in passing, as an easy and economical mode of evolving chlorine in abundance, for the purpose of studying its remarkable properties.

Chlorine is soluble in about one-third of its volume of cold water: a property which explains its slow and scanty evolution, in the form of visible bubbles, at the outset of the experiment, and the more rapid disengagement of the gas at a later period, when the liquid is saturated therewith. This solvent power of water for chlorine is, however, greatly diminished by heat; whence the recommendation to collect the gas over tepid water. A lighted taper, when introduced into a vessel containing chlorine, burns with a dim flame, depositing a good deal of soot. Of this circumstance advantage may be taken to make manifest the high volumeweight of this gas relatively to that of air and, à fortiori, of hydrogen. When a cylinder filled with chlorine is held mouth downward, the colour of the gas rapidly disappears, and its odour is no longer perceptible. If a lighted taper be now introduced into the cylinder, its unimpeded combustion indicates that the chlorine has been displaced by air. Again, if a taper be allowed to burn at the bottom of an air-filled vessel

FIG. 24.

(Fig. 24), and the mouth of a cylinder filled with chlorine be inclined towards it, as if water were being poured, the flickering

REPRODUCTION OF MURIATIC ACID.

21

176

and smoky flame shows that chlorine is falling on the taper. Thus we learn that chlorine is heavier than air, and consequently very considerably heavier than hydrogen. Exact experiments have proved that, bulk for bulk, chlorine is 35 times heavier than hydrogen; in other words, that, if the volumeweight of hydrogen be expressed by 1, that of chlorine is represented by 35.5.

From these experiments we have learned that the action of the electric current upon muriatic acid gives rise to the evolution of two essentially different gases; of which, one, hydrogen, familiar to us from our previous experience, is disengaged at the negative pole, while the other, chlorine, our new acquaintance, makes its appearance at the positive pole of the battery. We know, moreover, that each of these gases may be separately evolved from muriatic acid; the hydrogen by sodium, the chlorine by black oxide of manganese; and we are therefore justified in considering hydrogen and chlorine as constituents of muriatic acid.

That muriatic acid contains no other than these two constituents remains to be proved by a further experiment. For this purpose it is necessary to obtain a mixture of the two gases in the proportions in which they combine to form muriatic acid. Such a mixture is most readily obtained by the decomposition of muriatic acid itself into its elements; a decomposition readily effected by the action of the electric current, or, as it is termed, by electrolysis (the terminal, lysis, being derived from the Greek Xów, I loosen). Let this gaseous mixture be collected over warm water in a glassstoppered gas-cylinder (Fig. 25), care being taken to allow a considerable quantity of the gas to escape before commencing the collection. Let the cylinder, when full, be closed, and allowed to stand for several hours in diffuse daylight, and ultimately be exposed to the direct in

FIG. 25.

fluence of the solar beams. Under this treatment the gas will

22 INFLUENCE OF LIGHT AND HEAT ON THE MIXED GASES.

be found to have entirely lost its yellowish tint, and to be no longer capable either of taking fire, or of bleaching vegetal colours. In contact with common air the colourless gas forms white clouds; blue litmus paper exposed to its influence turns red; and if the cylinder containing it be opened under water the gas is rapidly absorbed. These are the characters of muriatic acid; and it is evident, therefore, that the hydrogen and chlorine have, in this experiment, reunited to form once more the substance from which they were originally separated. We arrive, of course, at exactly the same result if we make use of hydrogen and chlorine developed from muriatic acid by other means than electricity; as, for instance, by the action of sodium and of oxide of manganese.

By whichever of these methods the two gases are obtained, let them be collected in two separate cylinders; let these be placed one over the other, the lower one upright, the upper one inverted, so that their orifices may meet; then let the glass plates by which they are closed be withdrawn (Fig. 26), and the

FIG. 26.

cylinders be shaken, so as to promote the intermingling of the gases; lastly, let the cylinders be separated, so as to admit of a flame being presented to their orifices (Fig. 27), and then it will be seen that the mixed hydrogen and chlorine, at the touch of fire, instantaneously combine. A sort of hissing explosion