Returning now to our imaginary apparatus, let us turn on the current. At first nothing seems to be happening, although suitable instruments would show that current was flowing. Soon, however, little bubbles appear upon the electrodes, and these grow larger and larger, until they detach themselves from the platinum to which they have been adhering, float up to the surface and burst. The question which naturally arises is, What do those bubbles consist of ? Are they air?
If we take means to collect the gases which formed them we get an unmistakable answer. The bubbles which arise from the in-electrode are oxygen, those from the other hydrogen. If we allow our apparatus to work for some time, and collect all the gas which arises, we shall find that there is twice as much hydrogen as oxygen. We shall also find, as the process goes on, that the quantity of water diminishes.
Perhaps I may be allowed at this point to remind my readers that water is a collection of minute ultra-microscopic particles called "molecules," each of which is formed of three smaller particles still called "atoms." Of the three atoms two are hydrogen and one oxygen. Water therefore consists of hydrogen and oxygen, there being twice as much of the former as there is of the latter.
We see, therefore, that electrolysis gives us hydrogen and oxygen in exactly those proportions in which they occur in water, and since we also see that as these gases appear the water itself disappears, we are led to conclude that the current is splitting up the water into the gases of which it is formed.
But the strange thing is that this will not work with pure water. We have to add something to it. In the case of our imaginary experiment it was sulphuric acid. What part does that play?
[57]This is not fully understood, but we may be able to form a mental picture of what is believed to happen as follows.
The in-electrode is surrounded by a vast assemblage of these tiny molecules, most of them those of water, but a few those of the acid. The latter are more complex in their structure than the former, but they too contain hydrogen. Current flows into the electrode and instantly hydrogen atoms from the acid molecules crowd round it, like boatmen at the seaside anxious to secure a passenger. Each takes on board a quantity of electricity and with it darts across the intervening space to the other electrode. Arrived there, it
gives up its load and, its work done, remains lying upon the electrode until enough others like unto itself have gathered there to form a bubble and so escape. These hydrogen atoms are thought to be the craft which carry the current through the liquid and enable it to pose, as it were, as a conductor of electricity, which in reality it is not.
But where does the oxygen come from?
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To find the answer to that we must add a second chapter to our story. When the hydrogen "boats" took on board their load of electricity they left their former associates, and these forthwith "set upon" neighbouring water molecules and with the audacity of high-waymen stole from them enough hydrogen atoms to take the place of those they had lost. Thus the acid molecules became complete once more, while the scene of the conflict near the in-electrode was strewn with the remains of the water molecules from which the hydrogen atoms had been stolen. These remains, of course, would be oxygen, and they, collecting together on the electrode, would eventually be in numbers sufficient to form bubbles and so escape.
Thus it may be the acid which really does the work, yet because of its subsequent raid upon the water it is the latter which disappears, and it is the materials of the latter which are bought to the surface in the bubbles.
And there we see the mechanism whereby, so it is believed, electric current can pass through otherwise non-conducting[58] liquids. And the important point, as far as practical utility is concerned, is that the passage of the current is accompanied by a splitting up of something or other, either the water or something in it, the materials of which are deposited, one on one electrode and the other on the other.
And now we can proceed to those useful applications of electrolysis, the commonest of which, perhaps, is electro-plating.
We have seen how electrolysis causes hydrogen, probably out of the acid, to be deposited upon one electrode. Suppose that, instead of an acid, we put in the water one of those substances known to chemists as a "salt," the commonest example of which is ordinary table salt. This well-known condiment is caused by the interaction of hydrochloric acid and the metal sodium and will serve to illustrate what all salts are.
All acids are compounds of hydrogen and something else, and their biting action is due to the readiness with which the "something else" evicts the hydrogen and takes in a metal in its place. Thus hydrochloric acid, given the opportunity, gets rid of its hydrogen and takes in sodium, thereby forming chloride of soda or common salt.
Another example is the gold chloride familiar to photographers. This is the result of the action of certain acids upon gold, wherein the acids throw out their hydrogen and take in gold instead.
To sum up, then, a salt is just the same sort of thing as an acid, like the sulphuric acid which we used in our "experiment," except that some metal has taken the place of the hydrogen.
It is not surprising, then, to find that if we put a salt in the electrolyte instead of an acid we get a similar result. In the one case hydrogen is deposited upon the out-electrode, in the other the metal. In the former case, since hydrogen is a gas, it forms bubbles and floats away, but in the latter the solid metal remains a thin, even coating upon the electrode. That is the principle of electro-plating.
[59]The electrolyte consists of a suitable solution containing a salt of the metal to be deposited, and it is placed in an insulating ves-sel or vat. The articles to be plated form the out-electrode, so that they have to be suspended in some convenient way from a metal conductor by conducting wires. Of course they are entirely immersed in the liquid. The in-electrode is sometimes a plate of platinum (the reason that expensive metal is used being that it is unaffected by the chemicals) or else a plate of the metal being deposited.
In the former case, the solution becomes weaker as the work proceeds, and more salt has to be added. In the latter, however, the strength of the solution remains unchanged, for by an interesting interchange the in-electrode adds to it just what it loses by deposition upon the other one. The effect is therefore just as if the current tore off particles from the one and placed them upon the other.
This is believed to be due to the agency of the oxygen which in the case of the electrolysis of water becomes free, but which in this case forms with the metal electrode a layer of oxide upon its surface, this oxide being then dissolved away by the liquid. Thus as fast as the metal is deposited upon the out-electrode its place is taken by more metal from the in-electrode.
In some processes it is desired to deposit metal upon a non-conducting surface, and it is evident that such cannot be used as an electrode. Nor is it any use to attempt to deposit upon anything except an electrode. The only thing to do, then, is to make the object a conductor by some means. Models in clay, wax and plaster, once-living objects like small animals, fruit, flowers or insects, can, however, have a perfect replica made of them by electrical deposition, by the simple method of coating the surface to be plated with a thin layer of plumbago. This skin, although extremely thin, is a sufficiently good conductor to make the process possible. Process blocks for printing are copied in this way, so that a particularly delicate example of the blockmaker's art need not be worn down by much pressing,[60] copies or "electros" being made off it for actual use in the press.
The original block is a plate of copper on which the picture is represented by minute depressions and prominences. On this a layer of soft wax is pressed, so as to obtain a perfect but reversed copy. Having been coated with plumbago, this is then put into a vat
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containing a solution of copper salts and is used as the out-electrode, the other being a plate of copper. When the current is turned on the copper is thus deposited on the wax until a thin sheet of copper is formed which is an exact but reversed copy of the wax, a direct copy, that is, of the original block.
The back of this thin sheet is then covered