"To catch the gold. If you spread it on the riffles it seems to grab a hold o' 'color' like glue, an', what's more, nothin' but gold'll stick to it."
"Why is that?"
"I don't know," Jim answered, a bit irritably, "it does, that's all."
Owens interposed.
"You can't blame Jim for not knowing why, Clem," he said. "So far as that goes, I don't believe any chemist in the world can tell you exactly why quicksilver catches gold. It does, though, sure enough. But I can show you how it does it, in a way.
"You know that if iron is exposed to damp air, it turns red with rust? That is due to the chumminess or the affinity of iron with oxygen. You know if silver is exposed to city air, where the burning of coal in furnaces and fireplaces sends a sulphurous smoke into the air, it turns black? That's due to the fact that silver is a natural chum of sulphur. Chemically speaking, they make compounds easily.
"It's the same way with mercury, or, as it is generally called, quicksilver. Gold and quicksilver are chums, and the minute they get together they join to form a mixture which is called an amalgam. That's one of the great discoveries of the age. Gold-mining has taken a big jump forward since that was found out.
"You can see yourself how that would work. Whether with a pan, a cradle, or a sluice, the only thing that enables a miner to separate the gold from the worthless dirt is that the gold is smaller and heavier. But suppose the gold dust is so fine as to be invisible, it will be so light as to wash away easily; if it is in fine flakes, the flakes will almost float. All that light gold would be lost in the dirt that flows out of the bottom of the sluice, the tailings, as they are called.
"In the days that Jim is describing, two-thirds of the gold was lost that way. Every one, absolutely every single one of the forty-niners would have made a fortune, if the chemistry of gold had been as far advanced then as it is to-day. Even now, men are working over with profit the tailings that the forty-niners threw away.
"Suppose, now, you make your sluice, cover the bottom of it and the riffles with copper plates to hold the quicksilver better, and then cover your copper with quicksilver. What happens when the dirt and water come flowing down the sluice? The riffles will catch your heavy gold, just as well as before, and the quicksilver will catch a lot of the light gold that used to escape. You've got your gold in the riffles, then, and you've got a mixture of gold and quicksilver which has formed an amalgam.