And hence its value to humanity as the great medium of exchange. When a tailor wants bread, as has been pointed out by a great political economist, he does not go searching for a baker who happens to need a coat. If he did, he might starve before he found one. Instead, he gives his coat to anyone who needs one, no matter what his trade may be, taking gold in exchange. Then he goes with confidence to the baker, knowing full well that he, in turn, will be perfectly ready to give bread in exchange for gold. That is the principle upon which gold, and in a few cases silver, has become the foundation of trade. We use it for toning photographs and a few other things, but, practically speaking, it is useless stuff, yet certain special circumstances have given it a special function in civilised society, and so governments now make it up into little flat discs, putting their own special stamp upon them as a guarantee of size and quality, and it is by handing those little discs about that we carry on our trade. Or even where we use no actual disc, we pretend that we do, and use a piece of paper the value of which we say is so many discs, but that value depends entirely upon the fact that someone has guaranteed, on demand, to give so many discs for it.
And the strange thing about it is that although this usefulness of gold depends upon its rarity, we lose no opportunity of looking for new sources of supply, and so diminishing that rarity. As has been said, gold is present in sea-water, although no one knows how to get it out, except at a cost which makes it not worth while. But suppose that some genius found a way, and gold thus became twice as plentiful as it is now, the world would be no better off. Everything would cost twice as much as it does now; that is all. A pound is merely so much gold. If gold be twice as plentiful people will want twice as much of it in exchange for what they have to sell. Yet, all the same, the man who could solve that problem of getting gold from sea-water, or from anywhere else, in fact, would be hailed as a benefactor, and for a time at least he would reap a generous harvest.
Even as it is, science has done much for the production of gold. Not, as in other metals, in finding ways for extracting it from its ores, for, strictly speaking, it has none, but in finding ways of catching the tiny particles of metal from the "gangue," as it is called, the rock or earth in which they are embedded. The trouble is that they are so small, so infinitesimally small, almost.
There are two great types of place where gold is found. In the alluvial deposits, the beds of old rivers, the gold is quite loose. The convulsions of ages ago have, in many cases, elevated these beds, until now they are on the sides of mountains. In such cases the loose, gravelly stuff of which they are composed is washed down by a powerful stream of water from a huge hose-pipe terminating in a nozzle called a "monitor." This process, called "hydraulicing," brings down everything into a pond formed at the foot of the hill, and in some cases a boat or raft is floated upon the pond with machinery on board for dredging up the material. Often a powerful centrifugal pump sucks up the water through a pipe reaching to the bottom of the pond, bringing gravel and gold with it. Arrived in this way upon the raft, it all goes on to separating tables, by which the gold, being heavier, is divided from the gravel, which is lighter. These tables will be referred to again later.
In non-alluvial workings the gold is embedded in rock of some kind, such as that called quartz. This is hard, somewhat of the nature of granite, and before the gold can be liberated it has to be crushed to the likeness of fine sand, so that the tiny grains of gold can be captured. The quartz is found in veins or lodes, fissures, evidently, in the original crust of the earth, produced probably as the earth cooled. These have been gradually filled up by hot volcanic streams of water, which carried not only the gold in solution but also the materials of which the quartz is formed. It used to be thought that the veins were the result of hot liquids forced up from below by volcanic action, the rock and metal being themselves in the liquid state through intense heat. It is now more generally held that water was the vehicle by which the materials were brought in, and the vein formed. The gold in the alluvial deposits, too, is now thought to have come there in solution in water, and not by the erosion and washing down of rocks higher up the original river.
However that may be, and it is the subject of discussion among geologists and metallurgists, there the gold is to-day, firmly fixed in the hard rock, and the problem which confronts the metallurgist is to get it out with the least expense. The old historic way of breaking up the quartz rock is with what are called "stamps," pestles and mortars on a huge scale. There are a number of vertical beams of wood, each shod with iron, fixed in a wooden frame, so that they are free to slide up and down. Running along behind these stamps is a horizontal shaft with projections upon it called cams. There is one cam for each stamp, and as the shaft turns slowly round this projection catches under a projection on the stamp, and after lifting it up a short distance drops it suddenly. Thus, as the machine works, the stamps are lifted and dropped in rapid succession. The rock is fed into a box into which the feet of the stamps fall, and thus it is pounded until it is quite small. Meanwhile a stream of water flows through the box and carries away the finely broken particles through a kind of sieve which forms the front of the box, and which allows the fine, small pieces to escape, while holding back the larger ones and keeping them until they too have been crushed.
An average stamp will weigh 600 to 700 lb., and the repeated blows of such a hammer are enough to pulverise the hardest rock.
Machines such as these have been employed since the sixteenth century, at all events, and the improvements of modern times are only as regards details. It may well be wondered, then, why such an old device is still in use and how it comes about that it has not been displaced by something newer and better. The answer, which is an instructive one, well worth bearing in mind by many inexperienced inventors, is that it is so simple. It can be shipped in comparatively small parts, and so taken cheaply to any outlandish place. A good deal of it can be made roughly of wood, so that if native timber is available it can be made partly at the mine, and carriage costs saved. Finally, it is so easy to work and to understand that the most inexperienced workman can handle it, and there is so little that can go wrong that the most careless attendant cannot damage it.
In the bottom of the boxes there is placed some mercury, for which gold has a curious affinity. If a particle of gold once gets into contact with the surface of the mercury it will not get away again easily. Thus the mercury catches and holds many of the gold particles which are liberated when the rock is broken up.
As it reaches the required fineness, then, the crushed rock escapes from the stamp machine and flows away in the stream of water, and although much gold is caught by the mercury, it is by no means all. The stream is therefore directed over tables formed of copper sheets coated with mercury, so that additional opportunities are given to mercury to catch the grains of gold. Moreover, the table, which, by the way, is placed at a slight incline, is broken at intervals by little troughs of mercury called riffles, which assist in the depositing and catching of the metal particles.