GOLD. —Sodium Amalgam.

84

to a furnace, by means of which the mercury isdriven off as presently to be described.

Sodium Amalgam .—It has been already mentionedthat the amalgamation is frequently hindered, or alto-gether prevented, by the fine particles of metal beingcovered by a thin, film of a protective covering. Thistarnish generally consists of compounds of sulphur,arsenic, antimony, bismuth, or tellurium, or simplygrease. Dr. Wurtz, in America , and Mr. Crookes,in England, at about the same time (1864-65), dis-covered that, by the addition of a small quantity ofsodium to the mercury, amalgamation is very much

facilitated, even if the gold be admixed with any ofthe above-mentioned substances. Dr. Wurtz recom-mends two amalgams, one containing 2 per cent., andthe other 4 per cent, of sodium. The latter is a solid,very brittle, crystalline substance. By the additionof 1 per cent., or even less, of either of these to themercury used for amalgamation, it is said to adhereto metals which otherwise would not have been“ wetted ” by it. Crookes recommends three kinds,which he calls the A, B, and C amalgams. Each con-tains 3 per cent, of sodium. In A, however, it ismixed with 97 per cent, of mercury: in B with 77

Fig. 20.

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per cent, of mercury and 20 per cent, of zinc; and inC, with 77 per cent, of mercury, 10 per cent, of zinc,and 10 per cent, of tin. In preparing these amalgamsthe mercury is heated to about 300° Fahr., and caremust be taken to add the sodium in very smallquantities at a time, as the reaction which takes placeis very violent. In the case of B and C the sodiumis added to one half of the mercury, while the otherhalf is mixed with the other metals; the two halvesare then fused together.

Concentration of Tailings .—The impoverished sandsand ore which pass off from the sluices, &c., are sub-jected to a process of concentration, in order to collect

the small particles of gold inclosed in pyrites andother metallic sulphides, which would otherwise belost. There have been many contrivances employedfor this purpose, but it is unnecessary here to do morethan briefly describe the plan generally employed inCalifornia at the present day. On leaving the sluicesthe sands pass into a simple apparatus called a pointedbox, by means of which they are sized, as subsequentoperations are found to be facilitated by the sandsoperated on being of a uniform size. They are nextroughly concentrated by being passed over sluiceswith movable gates at their lower ends, which act thepart of riffles. The final concentration is performed,