the union of oxygen with sulphur dioxide and water; the oxides of nitrogen combine with the oxygen of the air present in the chambers, then give up this oxygen to the sulphur dioxide and water or steam to form sulphuric acid, again combine with more oxygen, and so on.  The exact processes or reactions are of course much more complicated, but the above represents what is practically the ultimate result.  It is evident that the gases leaving the last lead chamber in which the formation of vitriol is effected, must still contain nitrous fumes, and it becomes a matter of importance to recover them, so that they can be used over again.  To effect this object, use is made of the solubility of nitrous fumes in strong vitriol.  The gases from the last lead chamber of the series are passed through what is called a Gay-Lussac tower (the process was invented by the eminent French chemist Gay-Lussac), which is a tower made of lead, supported by a wooden framework, and filled with coke or special stoneware packing, over which strong vitriol is caused to flow.  The vitriol dissolves the nitrogen oxides, and so-called “nitrous vitriol” flows out at the base of the tower.  The recovery of the nitrogen compounds from the nitrous vitriol is effected in Glover towers (the invention of John Glover of Newcastle), which also serve to concentrate to some extent the weak acid produced in the lead chambers, and to cool the hot gases from the sulphur burners or pyrites kilns.  The weak chamber acid is mixed with the nitrous vitriol from the Gay-Lussac tower, and the mixture is pumped to the top of the Glover tower, which is of similar construction to the Gay-Lussac tower, but is generally packed with flints.  This Glover tower is placed between the sulphur burners or pyrites kilns and the first lead chamber.  The nitrous vitriol passing down the tower meets the hot gases from the kilns, and a threefold object is effected:  (1) The nitrous fumes are expelled from the nitrous vitriol, and are carried into the chambers, to again play the part of oxygen-carriers; (2) the weak chamber acid which was mixed with the nitrous vitriol is concentrated by the hot kiln gases; and (3) the hot gases themselves are cooled.  The acid from the Glover tower is purified by special treatment—­for example, the arsenic may be removed, after precipitation with sulphuretted hydrogen, in the form of insoluble arsenic sulphide,—­and the purified acid is concentrated by heating in glass or platinum vessels.

A considerable amount of sulphuric acid is now made by the so-called “contact process,” in which sulphur dioxide and oxygen unite to form sulphuric acid in presence of a heated “contact” substance, usually some form of finely-divided platinum.

Nitric Acid.—­This acid is usually prepared by distilling a mixture of sodium nitrate and vitriol in cast-iron retorts or pots, the nitric acid being collected in stoneware vessels connected one with another, or, as is more generally the case at the present time, in condensing apparatus consisting of stoneware pipes or coils cooled by water.  The effluent gases are passed through a scrubber in order to free them from the last traces of acid before discharging them into the atmosphere.