This change in volume suffered by the metal may be strikingly demonstrated by employing two strips of palladium foil, protected on one side by a varnish, as the electrodes in the electrolytic cell. On passing the current the negative electrode immediately begins to bend over towards the varnished side: when the current is reversed it again uncurls; and the other, being now the negative pole, at once begins to perform the same curling movements.

Hydrogen, which is thus occluded in the metal palladium, is capable of bringing about a number of chemical changes which ordinary hydrogen is unable to effect: thus, when a strip of hydrogenised palladium is immersed in a solution of a ferric salt a portion of the iron is reduced to the ferrous state.*

* See "Chemical Lecture Experiments," Nos. 27, 28, 29.

CHAPTER II

OXYGEN

Symbol, O. Atomic weight 16.00. Molecular weight = 32.

History.-Oxygen was discovered by Priestley (1774). He obtained it by heating the red oxide of mercury (known in those days as mercurius calcinatus, per se) by concentrating the sun's rays upon it by means of a powerful lens. Priestley applied to the gas the name dephlogistigated air. Oxygen was independently discovered by Scheele. Scheele's discovery of oxygen was published in 1775, but recent research among his original papers has brought to light the fact that the discovery was actually made in 1773, prior therefore to Priestley's discovery. Scheele called the gas empyreal air, on account of its property of supporting combustion. Lavoisier subsequently applied to this gas the name "oxygene" (from ¿§ús, sour; and yevváw, I produce), to denote the fact that in many instances the products obtained by the combustion of substances in the gas were endowed with acid properties. Oxygen, indeed, came to be regarded as an essential constituent of acids, and was looked upon as the "acidifying principle." The subsequent development of the science has shown that this idea is erroneous, and that oxygen is not a necessary constituent of all acids.

Occurrence. In the free state oxygen occurs in the atmosphere, mechanically mixed with about four times its volume of nitrogen. In combination with other elements it is found in enormous quantities. Thus it constitutes eight-ninths by weight of water, and nearly one-half by weight of the rocks of which the earth's crust is mainly composed.

The following table (Bunsen) gives the average composition of the earth's solid crust, so far as it has been penetrated by man. It must be remembered, however, that the greatest depth to which man has examined, when compared with the diameter of the earth, is after all only, as it were, a mere scratch.

Modes of Formation.-(1.) Oxygen may readily be obtained by a slight modification of Priestley's original method, namely, by heating mercuric oxide in a glass tube, by means of a Bunsen flame. The red oxide of mercury first darkens in colour, and is decomposed by the action of the heat into mercury and oxygen,

thus

2HgO=2Hg+0.

The evolved oxygen may be collected over water in the pneumatic trough, while the mercury condenses in the form of metallic globules upon the cooler parts of the tube. This method of obtaining oxygen is never employed when any quantity of the gas is required-it is chiefly of historic interest.

(2.) For experimental purposes oxygen is best prepared from potassium chlorate. When this salt is heated it melts, and at about 400° decomposes with brisk effervescence due to the evolution of oxygen, while potassium chloride remains :*

If the potassium chlorate be previously mixed with about onefourth of its weight of manganese dioxide, it gives up the whole of its oxygen at a temperature considerably below the melting-point of the salt, and at a greatly accelerated rate. When, therefore, the oxygen is not required to be perfectly pure, a mixture of these two

* The mechanism of this reaction is more complex than is represented by this equation. It has been shown that during the decomposition potassium perchlorate, KCIO,, is continuously being formed, and again resolved into KCIO, and O.

substances is usually employed. The mixture may be conveniently heated in a " Florence" flask, supported in the position shown in the figure, and gently heated with a Bunsen flame. The gas is washed by being passed through water, and then collected either at the pneumatic trough or in a gas-holder.

The manganese dioxide is found at the end of the reaction to be unchanged the part it plays in the decomposition belongs to a class of phenomena to which the name catalysis is applied; the inanganese dioxide, in this instance, being the catalytic agent. It was at one time supposed that by its mere presence, itself undergoing no change, the manganese dioxide enabled the potassium chlorate to give up its oxygen more readily and at a lower temperature; but the accumulated evidence which has been collected by the study of an increasing number of similar cases of catalytic action leads to the conclusion that the manganese dioxide is here

មា

FIG. 32.

playing a more distinctly chemical part in the reaction. So far as is known, in all phenomena of this order, the catalytic agent is a substance which possesses a certain degree of chemical affinity for one of the constituents of the body to be decomposed, and the influence of this attraction is a necessary factor in determining the splitting up of the compound. Owing, however, to certain conditions which are present, such, for example, as the particular temperature at which the reaction is conducted, the catalytic agent is unable to actually combine with the constituent for which it has this affinity, or if it combines, the combination it forms is unable to exist and is instantly resolved again: hence the catalytic agent comes out of the action in the same state as it was at the commencement.

In the case before us, it is believed that a cycle of changes takes place* in which the power possessed by manganese to enter into

* M'Leod.

higher states of oxidation results first in the formation of potassium permanganate, KMnO4; with the simultaneous production of chlorine and oxygen, thus

(1) 2MnO2+2KClO3=2KMnO4+Cl2+Og

The potassium permanganate then passes into potassium manganate, K,MnO4, with evolution of oxygen and partial reformation of manganese dioxide, thus—

(2) 2KMnO4 = K„MnO4+MnO2+02

And this is decomposed, by the chlorine evolved by the first reaction, into potassium chloride, manganese dioxide, and oxygen, thus

(3) K,MnO4 + Cl2=2KCl + MnO2+0.

(3.) When manganese dioxide itself is heated to bright redness, it parts with one-third of its oxygen and is converted into trimanganic tetroxide.

(4.) Other peroxides, when heated, similarly yield a portion of the oxygen they contain. One of these, namely, barium peroxide, is now largely employed for the preparation of oxygen upon a manufacturing scale. This method, known as Brin's process, from the name of the inventor, is based upon the fact that when barium oxide (BaO) is heated in contact with air, it unites with an additional atom of oxygen, forming barium peroxide, thus

BaO+O=BaO

And that when this substance is still further heated, it again parts with the additional oxygen and is reconverted into the monoxide

BaO.=BaO+0.

The process, therefore, is only an indirect method of obtaining oxygen from the air, the same quantity of barium monoxide being employed over and over again. In practice it was found that instead of effecting the two reactions by altering the temperature, which involved loss of time and considerable expense, the same result could be obtained by altering the pressure and keeping the temperature constant. If the monoxide be heated to the lower temperature, at which the first reaction takes place, and air be passed over it at the ordinary atmospheric pressure, atmospheric oxygen is taken up and barium peroxide is formed. If the pressure