199. Sodium hyposulphite may be made by passing sulphurous acid (sulphurous anhydride) and air through a solution of sodium sulphide, produced by fusing together five parts of sodium sulphate (a common product of the marshes in the interior) and one part of pulverized charcoal, and extracting with water.

The sodium polysulphide may also be used. It is made by boiling a solution of caustic soda on excess of sulphur, or by fusing together sodium carbonate and sulphur, and dissolving in water. Caustic potassa (concentrated lye) may be used in place of caustic soda, making potassium polysulphide, and then potassium hyposulphite with sulphurous acid and air. The potassium salt answers the same purpose as that of sodium.

Sodium hyposulphite may also be made by heating a solution of sodium sulphite with sulphur. The latter is dissolved. The sulphite is made by passing sulphurous acid through a solution of sodium carbonate, or by exposing the latter, in a moist condition, to the fumes of burning sulphur (sulphurous anhydride) in a rever-beratory furnace, or in the flue of a roasting furnace.

For all the purposes mentioned, the sodium carbonate or bicarbonate may be used indifferently.

It has been asserted, by some writers, that the solution of sodium hyposulphite does not dissolve any gold, even in those cases in which the calcium salt does so, referring to that lower gold chloride which sometimes exists in ore which has been roasted with salt (30). This however is a mistake, as has been proved by direct experiment, more than half of the contained gold having been extracted from a sample of such ore, by leaching it with the solution in question.