It is not known with any degree of certainty how the nitric acid acts in bringing about this remarkable change. It has been suggested that reduction to nitrogen peroxide first takes place, and that sulphur dioxide takes oxygen from this body, reducing it still further to nitric oxide, which at once combines with the free oxygen present to form nitrogen peroxide again. So the cycle of changes goes on, the nitrogen peroxide playing the part of oxygen carrier to the sulphur dioxide; and since it is continually regenerated, it remains at the end mixed with the residual gases. Recovery of the Nitrogen Peroxide. If the gases from the last chamber passed directly into the chimney shaft, there would be a total loss of the oxides of nitrogen, and the consequence of this would be that more than 2 cwt. of nitre would have to be used for the production of 1 ton of sulphuric acid. This would be a serious item in the cost of production, and it is therefore essential that this loss should be prevented. The recovery of the oxides of nitrogen is effected in the Gay Lussac tower, a structure about 50 ft. in height, built of sheet lead and lined with acid-resisting brick. It is filled with flints, over which a slow stream of cold concentrated sulphuric acid is delivered from a tank at the top. As the gas from the last chamber passes up this tower, it meets the stream of acid coming down. This dissolves and retains the nitrogen peroxide. The acid which collects at the bottom of the tower is known as nitrated vitriol. The next step is to bring the recovered nitrogen peroxide again into circulation. The nitrated vitriol is raised by compressed air to the top of the Glover tower, and as it trickles down over the flints in this tower it is diluted with water, while at the same time it meets the hot gases coming from the pyrites burners. Under these conditions, the nitrogen peroxide is liberated and carried along by the current of gas into the first lead chamber. The stream of cold acid coming down the Glover tower also serves to cool the hot gases before they enter the first chamber. In order to complete the description of the works, it is necessary to add a note on the lead chambers themselves. The sheet lead used in their construction is of a very substantial character; it weighs about 7 lb. per square foot. The separate strips are joined together by autogenous soldering, that is, by fusing the edges together. In this way the presence of another metal is avoided; otherwise this would form a voltaic couple with the lead, and rapid corrosion would take place.