To free the nitric acid of the accompanying lower oxides of nitrogen (as well as chlorine, compounds of chlorine and other impurities) air is blown into the hot acid. The mixture of sodium sulphate and sodium bisulphate remaining in the retorts is either converted into sulphate by addition of salt or used in the manufacture of glass. The nitric acid obtained is used either as such or mixed with sulphuric acid or with hydrochloric acid. Pure nitric acid cannot at ordinary atmospheric pressure be distilled unaltered, becomes coloured on distillation, and turns red when exposed to light. It is extremely dangerous to handle, as it sets light to straw, for example, if long in contact with it. It must be packed, therefore, in kieselguhr earth, and when in glass carboys forwarded only in trains for transport of inflammable material. Red, _fuming nitric acid_, a crude nitric acid, contains much nitrous and nitric oxides. It is produced if in the distillation process less sulphuric acid and a higher temperature are employed or (by reduction) if starch meal is added. The successful production of nitric acid from the air must be referred to. It is effected by electric discharges in special furnaces from which the air charged with nitrous gas is led into towers where the nitric oxide is further oxidised (to tetroxide), and finally, by contact with water, converted into nitric acid. Nitric acid is used in the manufacture of phosphoric acid, arsenious acid, and sulphuric acid, nitro-glycerin and nitrocellulose, smokeless powder, &c. (see the section on Explosives), in the preparation of nitrobenzenes, picric acid, and other nitro-compounds (see Tar Products, &c.). The diluted acid serves for the solution and etching of metals, also for the preparation of nitrates, such as the nitrates of mercury, silver, &c.