There is a more modern form of it, although the whole process is quite novel, having been introduced only in the nineties of last century. This development, it is almost wearying to repeat, is electrical. Instead of the zinc shavings being used to precipitate the gold out of the solution, the process is electrolytic. A lead anode is used while the process is carried on in a box the bottom of which is covered with mercury, which forms the cathode. The precipitated gold is thus amalgamated, the amalgam being removed at intervals, retorted, and the gold recovered.
The idea of recovering gold from the waters of the sea is certainly a most attractive one. To some, it is true, the suggestion may bring thoughts the reverse of pleasant, for there have been several partially successful attempts to delude the public with specious promises of vast dividends to be gathered in the form of pure gold from the inexhaustible sea. Still, there is something in it, and some day the dreams may be realised.
The quant.i.ty of gold dissolved in sea-water is so small that in 200 cubic centimetres it is impossible to detect it, even by the most delicate tests known. The quant.i.ty needs to be multiplied threefold before the quant.i.ty of gold becomes even detectable, to say nothing of being recoverable.
A writer in _Ca.s.sier"s Magazine_, a few years ago, related how he had actually obtained gold from the water of Long Island Sound. But whereas he got two dollars" worth, it cost him over 4000 dollars to do it. No company will ever be floated on results such as that. From the mud of a creek near New York, however, he did a little better, for there ten dollars" worth of gold only cost 379 dollars. A company promoter would still look askance at even that comparatively successful undertaking.
As usual, authorities differ, but there is a consensus of opinion that in every ton of sea-water there is from one-half to one grain of gold, besides silver and iodine.
It seems as if the water were able to dissolve that amount and no more.
If, as has been suggested earlier in this chapter, all the gold which is now found in mines and in gravel beds was carried there in water, it is probable that the sea obtains its gold from the same original sources, and that, just as the hot ocean of ages ago carried its burden of gold in solution, so the colder water of to-day has its share, the cold water naturally carrying less than the hot did.
It is quite likely, then, that, could we find out how to rob the sea of its precious metal, it could replenish its store from some secret h.o.a.rd of its own. But even if it could not, it would make little difference to us, since what it holds is far more than we could ever use. Put it at half-a-grain per ton: there are 4205 million tons in every cubic mile of ocean, and 300 million cubic miles of water in the ocean. If all the gold that man has ever handled were to be dissolved in the sea, no chemist would be able to discover the fact. On the other hand, if that half-grain per ton which we believe to be in the ocean now were to be recovered we should have about 40,000 million tons of gold, a prospect which is enough to make the political economist turn pale with apprehension.
What is required is some substance which, on being added to sea-water, will combine with the gold, and then be precipitated--that is to say, fall to the bottom. The precipitate--that which falls to the bottom--would need to be heavy, so that it would fall quickly and not necessitate the water being left standing for long periods. It would need to be cheap, too, or easily recoverable, so that it could be used over and over again. And, finally, it would need to be such that the gold, having been captured by it, could be easily obtained from it.
Given such a precipitant, the process of recovering the gold would be simple and cheap. Tanks would be formed in sheltered bays and inlets. At every tide these would be filled, and when full the precipitant would be added. The tide falling, the water would run out again and leave the precipitate on the floor of the tanks, whence it could be removed by sc.r.a.ping. Simple treatment would release the gold from its partner, which would then be returned to the tanks to act as the precipitant once more. Thus by simple means, the tide itself a.s.sisting, the gold could be obtained from the sea.
And there is nothing inherently impossible about this suggestion. The necessary precipitant may exist, awaiting discovery. A large works operating in this manner would produce, it is estimated, about thirteen tons of gold per annum. It looks as if it would be a bad day for the Rand when that discovery is made.
And there is yet another possibility, though less alluring than what has just been described. The American writer mentioned a little while back got a better return from the mud of a creek than from the water itself.
In all probability this is due to the action of organic matter carried down by streams, or in some other way introduced into the waters of the creek whence the mud was obtained. This organic matter would possibly have an effect as a precipitant upon the dissolved gold, causing it to be thrown out of solution and deposited in the mud. Thus the mud around our sh.o.r.es, and particularly in the creeks and estuaries, may be potential gold mines whence in time to come we may draw supplies of the precious metal. The cyanide or some similar process may be needed in order that we may extract the metal from its enclosing mud, but the time may not be so very far distant when dredging for gold may be a regular occupation at, for example, the mouths of the Thames and the Hudson.
CHAPTER X
INTENSE HEAT
Many of the useful and interesting manufacturing processes of to-day are based upon the intense heat which science has taught the manufacturer how to produce. Tasks which our forefathers dreamed of, but were unable to accomplish, are easy to-day because of the facility with which great heat can be generated. The "burning fiery furnace" "seven times heated"
is as nothing to some of the temperatures which are now obtained in the ordinary course of things.
The greatest heat of all is that of the electric arc. Two conductors, generally rods of carbon, are placed with their ends touching, and the current is turned on so that it pa.s.ses from one to the other. Then they are gradually drawn apart. As the gap widens the current experiences more and more difficulty in pa.s.sing over this non-conducting gap, and great electrical energy has to be employed to keep it going. Now that wonderful law of the Conservation of Energy decrees that no energy can ever be lost. It can only be changed from one form into another.
Therefore the energy expended upon the arc is not lost, but is converted into heat. It is that heat, acting upon the small particles of carbon which are torn off the ends of the rods, which gives us the arc light.
As a matter of fact nearly all artificial light (and natural light too for that matter[1]) is due to heat. The heat sets the molecules in violent agitation, which, acting upon the corpuscles in the atoms, sets them in violent motion too, so that light is often the companion of heat. Some substances give light more readily than others, under the influence of heat, and we may reasonably believe that they are those whose corpuscular arrangements are such that they can be readily accelerated by the molecular action.
[1] The glow-worm is an example of the few exceptions.
To take a familiar instance, coal-gas is mainly "methane," one of the many combinations of carbon and hydrogen, and when it is burnt in air the hydrogen and oxygen combine, liberating heat, which causes the carbon liberated at the same time to glow. As each methane molecule breaks up the carbon atoms are thrown out, forming solid particles of carbon, and it is they really which give the light. It is therefore the combustible gas heating the solid particles of carbon which forms the luminous part of the gas flame. The non-luminous part of the flame, near the burner (I am now speaking of the old-fashioned burner), is the burning gas before the carbon particles have had time to heat up.
And the old gas flame, as we know, is now being rapidly displaced by the incandescent mantle, the reason being simply that Von Welsbach discovered how certain rare minerals gave a more brilliant light when heated than particles of carbon do. In other words, it is easier to accelerate the motion of the corpuscles in ceria, thoria and the other ingredients of the mantle, than it is those of carbon. Consequently, they sooner reach that degree of agitation which will send forth electro-magnetic waves of the high frequency necessary to produce the sensation of light.
For this reason the mantle heated by gas gives as bright a light as the carbon particles in the electric arc, although the latter are subjected to a much more intense heat.
But the arc can be, and often is, used as a source of heat, apart altogether from the light which it gives. In Sweden, for example, where coal is rare, but water-power plentiful, the power of the waterfalls is made to smelt iron. Hence the waterfalls are sometimes termed the "white coal" of that country. Needless to say, it is the ubiquitous electricity which performs the change from the force of falling water into heat.
The furnaces are in shape much like those in which iron is smelted with coal--namely, tall chimney-like structures at the bottom of which is the fire. In the "arc furnaces" there are, pa.s.sing in through the side, near the bottom, a number of electrodes, and between these a series of arcs are formed. c.o.ke and ironstone are thrown in from the top into this region of intense heat, and there the iron is liberated from the oxygen with which it is combined in the ore. Liberated, it flows out through a spout at one side of the furnace.
But the question will arise in the reader"s mind: Why is c.o.ke needed in an electric furnace? It is for metallurgical reasons. The heat of the arc loosens the bonds between the iron and oxygen, but it needs the presence of some carbon to tempt the oxygen atoms away. Therefore c.o.ke, as the most convenient form of carbon, has to be there. It is there, however, in much smaller quant.i.ty than it would be in an ordinary furnace. It is not there as fuel, but simply as the "counter-attraction"
to draw the oxygen atoms away from their old love.
The arc is also used for welding pieces of iron together, for which purpose it is eminently suitable, since what is wanted is intense heat at a particular point. But perhaps the reader will be wondering by this time what the heat of the arc is. It has been repeatedly referred to as "intense," but something more definite may be demanded. In theory it is unlimited. Apply more pressure--more volts, that is--thereby driving more current across, and the temperature will rise. It is only a question of making dynamos large enough, and driving them fast enough, and any temperature is possible. But there are practical difficulties which limit the degree of heat. One is the melting-point of the furnace itself. Fire-clay melts at about 1700 to 1800 C. So in a furnace which has to be lined with fire-clay that is about the limit.
In welding two pieces of iron together, the iron, of course, defines what the limit shall be. It needs to be heated to "welding heat" and no more--that is, a little short of melting--so that the parts to be joined are soft, and, with a little hammering, will join thoroughly together.
If too much heat were to be applied the parts would melt away. But the heat of the arc can be controlled by simply varying the current, and so the right heat can be applied at the right place, than which little more is wanted.
One very simple way of doing this is for the workman to hold one of the "electrodes"--a rod of carbon suitably insulated--in his hand. The current is led to it through a flexible wire. The iron itself is made the other electrode by being gripped in a vice which is itself insulated but connected to the source of current. Thus on bringing the point of his rod near to the part to be heated the man causes an arc to be created there. By moving the rod he can move the arc about, heating one part more than another, distributing his heat if he wants to do so over a larger area, or keeping it to a small one, just as he wills. On reaching the right heat the rod is withdrawn, the arc destroyed, and the iron can be hammered just as if it had been heated in a fire.
Yet another way still is known as "resistance" welding. In it an enormous current at an extremely low voltage is used. The fundamental principle is the same, since the heat is formed by forcing current past a point over which it is reluctant to pa.s.s. That point of poor conductivity is the ends of the two bars to be joined. They are placed just touching, but since an imperfect contact like that always offers considerable resistance to the flow of a current, the pa.s.sing current needs only to be made large enough for great heat to be generated.
This is exceedingly pretty to watch. We will suppose that the article to be operated upon is the tyre of a wheel. The bar of iron has already been bent by rollers into the correct curve and the two ends are touching. Brought to the machine, it is gripped, each side of the junction, in the jaws of an insulated vice and the current is turned on.
In a few seconds the place where the two ends are just touching begins to glow. Rapidly it increases in brightness until in about half-a-minute it is at welding heat. Then one vice, which is movable, is forced along a little by a screw, so that the ends are pressed firmly together, a little judicious hammering meanwhile helping to complete the job. Then the current is switched off and the complete tyre taken out of the machine. The current used has a force comparable with that which operates domestic electric bells, but in volume it is thousands of amperes. Alternating current is used, and it is obtained from a transformer or induction coil. In such a case the primary part of the coil is made of many turns of fine wire, so that little current pa.s.ses through it, while the secondary part is but one or two turns of thick bar. Thus the voltage generated in the secondary is very little, but since the secondary has an almost negligible resistance the current caused by that small voltage is enormous. Such an arrangement is in industrial realms generally called a transformer, the term induction coil being employed more for those things of a similar nature intended for the laboratory. The one just described is, moreover, a "step-down"
transformer, since it lowers the voltage, to distinguish it from "step-up" transformers, which raise the voltage.
And the "resistance" principle is also applied in another way to large furnaces, such as those for refining iron. In these the resistance of the iron itself is utilised to generate the heat. Of course, it should be well understood, heat is always generated in everything through which current flows. There is no perfect conductor, and so every conductor is more or less heated by the pa.s.sage of current through it. Some energy needs to be expended to drive current, even along large copper wires, and that energy must be turned into heat in the wires. If the same volume of current be forced along iron wires of the same size, the heat will be greater, since iron is but a poor conductor compared with copper, the relation being about as one to six. And if the iron be hot the resistance will be still more, for it stands to reason that when heated the molecules, being farther apart, will be the less easily able to exchange corpuscles. We have the best reasons for believing, as has been suggested already, that a current of electricity is but a flow of corpuscles, and so we are not surprised to hear that, as a general rule, the hotter a thing is the less does it conduct electricity.
[Ill.u.s.tration:
_By permission of Cambridge Scientific Inst. Co., Ltd., Cambridge, Eng._
MEASURING HEAT AT A DISTANCE
This wonderful instrument, the Fery Radiation Pyrometer, although itself some distance away from the furnace, is telling the temperature of its hottest part.]
So imagine a circular trough of fire-clay or other heat-resisting material filled with fragments of iron, or, it may be, with iron barely above melting-point, which has come from another furnace, where it underwent the previous process. Circling inside or outside this trough is an enormous coil of wire through which currents of electricity are alternating. That is the "primary" of a transformer, and the "secondary"
is--the iron itself, in the trough. If it be, as it often is, in the form of sc.r.a.p, or broken pieces, the heat will begin to show itself where the pieces touch each other. The currents generated in the trough, by the coil outside, will, of course, pa.s.s from piece to piece and the points of contact, since they offer the greatest resistance, will show signs of heat. This will increase until the pieces begin to melt. As the separate fragments merge into the molten ma.s.s the resistance will in one way decrease, for the imperfect contacts between the pieces will give place to the perfect contact throughout the ma.s.s of liquid metal. But for another reason--namely, the increase in heat--the resistance will increase. And all the while the alternations in the primary coil will be pumping currents, as it were, round and round the ring of molten iron.
Whether the resistance increase or decrease, the current will do the opposite, so that heat will be generated whatever happens. For as resistance decreases current increases, and vice versa. And the slightest variation in the strength of the primary current will have its effect upon the secondary, and therefore on the heat generated. So, by simply regulating the primary current, the temperature of the metal can be controlled to a nicety. And such furnaces have the immense advantage that there is no possibility of deleterious substances in the fuel getting into and spoiling the metal, a thing which may very easily happen during the manufacture of high-cla.s.s steels, alloys of iron in which the exact quant.i.ties, purity and proportions of the ingredients are of the utmost importance.
Hence these "induction furnaces," as they are called, are frequently used quite apart from any question of utilising water-power. And they will probably be used still more as time goes on.
For one thing, they may become valuable adjuncts to the older form of iron and steel furnaces, from which they will obtain their power free, gratis and for nothing. In districts such as Middlesbrough they could generate more electricity than they have any use for. The ordinary iron furnaces belch forth flames which are really good useful gas (carbon monoxide) burning to waste. Many of the furnaces are covered in at the top, and this gas is led away to heat boilers for the steam-engines or to drive large gas-engines, but in a large works there is more of this waste gas than they know what to do with. Now that could, and probably will ere long, be turned into electricity by means of gas-engines and the current used for making steel in induction furnaces.
It will probably surprise many to know that these enormous currents which can thus heat great ma.s.ses of metal until they melt are no danger at all to the men who work with them. A man might dip an iron rod into the trough of metal and he would scarcely feel the shock. And the same is true of the welding machine, which can be touched in any part without fear. The reason, of course, is that, broadly speaking, it is volume of current which does harm, and the resistance of the human body is so great that with the small voltages used, the volume which can pa.s.s is negligible. It should be mentioned, however, that the volume of current in lightning is also small, but we know that it is capable of inflicting terrible injury. Lightning, however, is in a cla.s.s by itself. Our terrestrial voltages are baffled by an air-gap of a few inches, but lightning springs across a gap miles wide. Its voltage must, therefore, amount to millions, and the ordinary rules relating to earthly currents do not apply.
But other sources of heat besides electricity are at the disposal of our manufacturers nowadays. Pre-eminently there is the flame of some gas burning with pure oxygen. The oxyhydrogen jet has been known for many years as the best means of producing the light for a magic lantern. Such a jet impinging upon a pencil of lime causes the latter to glow with a dazzling white light.
But the oxyhydrogen jet is now employed in many factories for the welding of metals. This is known as fusion welding, since the two parts are actually reduced to liquid. The usual way to go about this work is to bevel off the ends or edges to be joined. Suppose, for instance, that we wanted to weld two pieces of bra.s.s pipe together. We should first file or otherwise trim the edges to be joined until when put together they form a groove practically as deep as the metal is thick. Then with a stick of bra.s.s wire in the left hand, and an oxyhydrogen blowpipe in the right, we should direct the flame from the pipe on to the metal until, at one point, the sides of the groove were beginning to melt.
Then, inserting the point of the wire into the groove, we should melt a little off it. Thus we should work all round the joint, melting the sides of the groove and filling in with melted metal from the wire, until the whole groove had been filled up and the metal added had been thoroughly amalgamated with that on either side.
As a matter of fact, if it were bra.s.s which we were working on we should probably use the cheaper though less pure form of hydrogen--coal-gas--so that it would really be "oxycoal-gas" that we should use and not oxyhydrogen. The latter is used, however, notably for the fusion-welding of lead, or "lead-burning," as it is termed.
The blowpipe is a bra.s.s tube about a foot or eighteen inches long, with two pa.s.sages in it, one for the oxygen and the other for the other gas.
The gases are brought to one end of it through rubber pipes, while at the other end there is a nozzle in which the gases mingle and from which they emerge in a fine jet.
The oxyhydrogen flame has a temperature of about 2000 C., hot enough to melt fire-clay. That does not matter in the case of welding, however, since the molten metal is very small in quant.i.ty at any given moment, and is allowed to cool before it can run away. It would be an awkward temperature to deal with, nevertheless, in a furnace. It seems strange that it does not burn the nozzle of the blowpipe, but the fact that it does not is, it is believed, explained by the fact that the expansion of the gas, as soon as it emerges from the hole out of which it shoots, causes a comparatively cool s.p.a.ce just there, shielding it from the intense heat farther on.