268 



acid, and by means of compressed air this acid is forced through 

 a pipe up to a vessel at the top of the Glover tower G. A neigh- 

 boring vessel at the top of this tower is filled with dilute sulphuric 

 acid, and when the contents of both vessels are mixed by allow- 

 ing their contents to trickle down through the tower, nitrous 

 anhydride is once more set free by the interaction of the water in 

 the dilute acid (action (2)). The Glover tower is filled with 

 broken flint or tiles, in order that the descending liquid may offer 

 a large surface to the hot gases ascending the tower, and thereby 

 facilitate the acquisition by these gases of a sufficient quantity 

 of nitrous anhydride. Their high temperature also causes a con- 

 siderable concentration of the diluted sulphuric acid as it trickles 

 downward. This acid, after traversing this tower, is sufficiently 

 strong to be used once more for the absorption of nitrous anhy- 

 dride. 



To replace the part of the nitrous anhydride which is inevitably 

 lost, fresh nitric acid is furnished by small open vessels N, contain- 

 ing sodium nitrate and sulphuric acid, placed in the flues of the 

 pyrite-burners. About 4 kg. of the nitrate are consumed for every 

 100 kg. of sulphur. 



The acid which accumulates upon the floors contains but 60 to 

 70 per cent of sulphuric acid, and has a specific gravity of 1.5- 

 1.62. 



This crude sulphuric acid is applicable directly in some 

 chemical manufactures, such as the preparation of superphos- 

 phates (p . 4 1 2) . For many purposes, however, a more concentrated 

 acid is required. Concentration is effected by evaporating off 

 water from the chamber acid in pans lined with lead, which are 

 frequently placed over the pyrite-burners in order to economize 

 fuel. The evaporation in lead is carried on until a specific grav- 

 ity 1.7, corresponding to 77 per cent concentration, is reached. 

 Up to this point the sulphate of lead formed by the action 

 of the sulphuric acid produces a crust which protects the metal 

 from further action. When a still more concentrated acid is 



