The value of nitrite of soda in the improved methods of dyeing fabrics is increasing. Below is given a brief but authentic account of the manufacture of that chemical. The raw material, from which nitrite of soda is manufactured, is purified Chile saltpetre the sodic chloride present in the latter lowers the value of the nitrite, but the elimination of the sodic chloride is an expensive operation not generally practised. The saltpetre is melted in large cast-iron vessels, and this involves the evaporation of the water and the decomposition of a part of the iodides and iodates which are in the saltpetre. The lead necessary for the decomposition of the saltpetre must be pure, as the presence of small quantities of other metals, especially of antimony, might cause the decrepitation of the whole charge. When the saltpetre, which melts at 310oC, has reached a temperature of 420°C, 11 parts of sheet lead are gradually added for every 5 parts of saltpetre, the whole being constantly stirred to obtain an intimate mixture. If the charge is too strongly heated the vessel might hi pierced; if there appears a likelihood of the latter happening, add a quantity of cold saltpetre or withdraw the fire.

Continue stirring after the lead has been added, and then, by means of a large cast-iron ladle, run the melted mass into cold water and assist the solution by constant stirring. The decomposition of the saltpetre by the lead at from 420° 0. to 500o C. produces, besides the nitrite, about 1 per cent, of caustic soda, which dissolves some of the oxide of lead formed; to remove the latter, neutralise the solution with nitric acid. In this manner saltpetre is re-formed, the oxide of lead being precipitated as insoluble hydroxide. The neutralising may be effected either with nitrate of lead or with dilute sulphuric acid instead of nitric acid; of the two former, sulphuric acid is the cheaper, but by its use sulphate of soda is deposited in the concentrating vessels in the form of anhydrous salt. There are now in aqueous solution (1) nitrite, (2) unde-ciimposed saltpetre, (3) caustic soda holding oxide of lead in solution, and (1) the soluble impurities of the saltpetre, such as chloride of sodium, etc. The insoluble residue which was precipitated consists of (1) oxide of lead, (2) a very small quantity of metallic lead which has escaped oxidation, and (3) peroxide of lead.

The solution, diluted to from 6° B. to 8o B., is neutralised again with the same agent as was used before; the oxide of lead in solution is precipitated, and the neutralising agent is added as long as a precipitate will form. It may here be mentioned that it is commonly supposed, and most authors state, that nitrite of sodium has an alkaline reaction, but this is not the case, the pure nitrite being absolutely neutral. The neuti-alised solution is separated from the insoluble precipitate by any convenient method, and is then concentrated in cast-iron paus until it has a density of from 42° B. to 45o B. when warm. The insoluble precipitated residue is thrown upon a large filter of coarse sacking, where it is washed with warm water and the wash waters are added to the principal solution. The concentrated solutions are mixed together in cast-iron vats and left to crystallise; if the crystals thus obtained are not pure, they must be re-dissolved and re-crystallised. The pure crystals are separated in a centrifugal machine, washed, and dried. The desiccation takes place in an oven at a temperature of about 5o0C, and the crystals are packed in parchment-paper cylinders of double thickness.

The residuary oxide of lead may be melted and cast as it is, reduced to the metallic state, or transformed into minium, a heavy, brilliant red pigment which is used as a cement and paint, and in the manufacture of flint glass. The lead oxide can also be used in the preparation of white lead, of lead nitrate, lead acetate, and other plumbic compounds.