The metal copper, as obtained from copper ore, contains many impurities of various kinds. For most purposes these impurities greatly affect the value of the copper, and before the metal can be of much commercial use they must be got rid of in some way. In the previous chapter, in describing how to copperplate an old spoon, we saw that the anode need not consist of pure copper, because in any case nothing but the pure metal would be deposited upon the spoon. This fact forms the basis of the important industry of electrolytic copper refining. The process is exactly the same as ordinary copperplating, except that the cathode always consists of absolutely pure copper. This is generally in the form of a sheet no thicker than thin paper, but sometimes a number of suspended wires are used instead. A solution of copper sulphate is used as usual for the electrolyte, and the anode is a thick cast plate of the impure copper. The result of passing a current through the solution is that copper is taken from the anode and carried to the cathode, the impurities falling to the bottom of the vat and accumulating as a sort of slime. In this way thick slabs of pure copper are obtained, ready to be melted down and cast into ingots.
The impurities in the raw copper vary according to the ore from which it is obtained, and sometimes gold and silver are found amongst them. When the copper is known to contain these metals the deposit at the bottom of the refining vats is carefully collected, and from it a considerable quantity of gold and silver is recovered. It is estimated that about half a million tons of copper are refined every year. An immense amount of this pure copper is used for electrical purposes, for making conducting wires and cables, and innumerable parts of electric appliances and machinery of all kinds; in fact it is calculated that more than half of the copper produced all over the world is used in this way.
A similar method is employed to obtain the precious metals in a pure state, from the substance known as “bullion”; which consists usually of an intermingling of gold, silver, and copper, with perhaps also lead. Just as in copper refining, the raw material is used as the anode, and a strip of pure gold or silver, according to which metal is required, as the cathode. A silver solution is used if silver is wanted, and a gold solution if gold is to be deposited.
The metal aluminium has come into general use with surprising rapidity, and during the last twenty-five or thirty years the amount of this metal produced annually has increased from two or three tons to many thousands of tons. Aluminium occurs naturally in large quantities, in the form of alumina, or oxide of aluminium, but for a long time experimenters despaired of ever obtaining the pure metal cheaply on a commercial scale. The oxides of most metals can be reduced, that is deprived of their oxygen, by heating them with carbon; but aluminium oxide holds on to its oxygen with extraordinary tenacity, and absolutely refuses to be parted from it in this way. One process after another was tried, without success, and cheap aluminium seemed to be an impossibility until about 1887, when two chemists, Hall, an American, and Héroult, a Frenchman, discovered a satisfactory solution of the problem. These chemists, who were then scarcely out of their student days, worked quite independently of one another, and it is a remarkable fact that their methods, which are practically alike, were discovered at almost the same time. The process is an interesting mixture of electrolysis and electric heating. An iron crucible containing a mixture of alumina, fluorspar, and cryolite is heated. The two last-named substances are quickly fused, and the alumina dissolves in the resulting fluid. When the mixture has reached the fluid state, electrodes made of carbon are dipped into it, and a current is passed through; with the result that oxygen is given off at the anode, and metallic aluminium is produced at the cathode, in molten drops. This molten metal is heavier than the rest of the fluid, and so it falls to the bottom. From here it is drawn off at intervals, while fresh alumina is added as required, so that the process goes on without interruption. After the first fusing of the mixture no further outside heat is required, for the heat produced by the passage of the current is sufficient to keep the materials in a fluid state. Vast quantities of aluminium are produced in this way at Niagara Falls, and in Scotland and Switzerland.
Most of us are familiar with the substance known as caustic soda. The chemical name for this is sodium hydrate, and its preparation by electrolysis is interesting. Common salt is a chemical compound of the metal sodium and the greenish coloured, evil smelling gas chlorine, its proper name being sodium chloride. A solution of this in water is placed in a vat or cell, and a current is sent through it. The solution is then split up into chlorine, at the anode, and sodium at the cathode. Sodium has a remarkably strong liking for water, and as soon as it is set free from the chlorine it combines with the water of the solution, and a new solution of sodium hydrate is formed. The water in this is then got rid of, and solid caustic soda remains.
Amongst the many purposes for which caustic soda is used is the preparation of oxygen and hydrogen. Water, to which a little sulphuric acid has been added, is split up by a current into oxygen and hydrogen, as we saw in [Chapter V]. This method may be used for the preparation of these two gases on a commercial scale, but more usually a solution of caustic soda is used as the electrolyte. If the oxygen and hydrogen are not to be used at the place where they are produced, they are forced under tremendous pressure into steel cylinders, and at a lantern lecture these cylinders may be seen supplying the gas for the lime-light. Although the cylinders are specially made and tested for strength, they are covered with a sort of rope netting; so that if by any chance one happened to burst, the shattered fragments of metal would be caught by the netting, instead of flying all over the room and possibly injuring a number of people. A large quantity of hydrogen is prepared by this process for filling balloons and military airships.
CHAPTER XXIV
THE RÖNTGEN RAYS
In the chapter on electricity in the atmosphere we saw that whereas air at ordinary pressure is a bad conductor, its conducting power increases rapidly as the pressure is lowered. Roughly speaking, if we wish to obtain a spark across a gap of 1 inch in ordinary air, we must have an electric pressure of about 50,000 volts. The discharge which takes place under these conditions is very violent, and it is called a “disruptive” discharge. If however the air pressure is gradually lowered, the discharge loses its violent character, and the brilliant spark is replaced by a soft, luminous glow.
The changes in the character of the discharge may be studied by means of an apparatus known as the “electric egg.” This consists of an egg-shaped bulb of glass, having its base connected with an air-pump. Two brass rods project into the bulb, one at each end; the lower rod being a fixture, while the upper one is arranged to slide in and out, so that the distance between the balls can be varied. The outer ends of the rods are connected to an induction coil or to a Wimshurst machine. If the distance between the balls has to be, say, half an inch, to produce a spark with the air at normal pressure, then on slightly reducing the pressure by means of the air-pump it is found that a spark will pass with the balls an inch or more apart. The brilliance of an electric spark is due to the resistance of the air, and as the pressure decreases the resistance becomes smaller, so that the light produced is much less brilliant. If the exhaustion is carried still further the discharge becomes redder in colour, and spreads out wider and wider until it loses all resemblance to a spark, and becomes a luminous glow of a purple or violet colour. At first this glow seems to fill the whole bulb, but at still higher vacua it contracts into layers of definite shape, these layers being alternately light and dark. Finally, when the pressure becomes equal to about one-millionth of an atmosphere, a luminous glow surrounds the cathode or negative rod, beyond this is dark space almost filling the bulb, and the walls of the bulb between the cathode and the anode glow with phosphorescent light. This phosphorescence is produced by rays coming from the cathode and passing through the dark space, and these rays have been given the name of “cathode rays.”
Many interesting experiments with these rays may be performed with tubes permanently exhausted to the proper degree. The power of the rays to produce phosphorescence is shown in a most striking way with a tube fixed in a horizontal position upon a stand, and containing a light cross made of aluminium, placed in the path of the rays. This is hinged at the base, so that it can be stood up on end or thrown down by jerking the tube. Some of the rays streaming from the cathode are intercepted by the cross, while others pass by it and reach the other end of the tube. The result is that a black shadow of the cross is thrown on the glass, sharply contrasted with those parts of the tube reached by the rays, and which phosphoresce brilliantly. After a little while this brilliance decreases, for the glass becomes fatigued, and loses to a considerable extent its power of phosphorescing. If now the cross is jerked down, the rays reach the portions of the tube before protected by the cross, and this glass, being quite fresh, phosphoresces with full brilliance. The black cross now suddenly becomes brilliantly illuminated, while the tired glass is dark in comparison. If the tired glass is allowed to rest for a while it partly recovers its phosphorescing powers, but it never regains its first brilliance.