COMPOUNDS OF N IT I : < '< ; K N WITH HYDKOGEN AND < > X YGEN 285 



steam and gases necessary for the preparation of the sulphuric acid 

 passes through these chambers and tubes. The acid as it is formed falls 

 to the bottom of the chambers, or runs down their walls, and flows from 

 one chamber to another (from the last to the first, to permit of which 

 the partitions do not reach to the bottom of the chambers), and there- 

 fore the floor and walls of the chambers should be made of a material 

 on which the sulphuric acid will not act. Among the ordinary metals 

 lead is the only one suitable. The other metals, such as iron, zinc, or 

 copper, are corroded by the acid ; glass and earthenware are not acted 

 011, but would not withstand the changes of temperature which occur in 

 the chambers, and would be difficult to join closely ; whilst wood and 

 similar materials are destroyed by the acid. 



For the formation of the sulphuric acid it is necessary to introduce 

 sulphurous anhydride, steam, air, and nitric acid, or some oxide of 

 nitrogen, into the chambers. The sulphurous anhydride is produced by 

 burning sulphur or iron pyrites. This is carried on in the furnace with 

 four hearths to the left of the drawing. Air is led into the chambers 

 and furnace through orifices in the furnace doors. The current of air 

 and oxygen is regulated by opening or closing these orifices to a greater 

 or less extent. The ingoing draught in the chambers is brought about 

 by the fact that heated gases and vapours pass into the chambers whose 

 temperature is further raised by the reaction itself, and also by the 

 remaining nitrogen being continually withdrawn from the outlet (above 

 the tower K) by a tall chimney situated near the chambers. Nitric 

 acid is prepared from a mixture of sulphuric acid and Chili saltpetre, in 

 the same furnaces in which the sulphurous anhydride is evolved (or in 

 special furnaces). Not more than 8 parts of nitre are taken to 100 parts 

 of sulphur burnt. On leaving the furnace the vapours of nitric acid 

 and oxides of nitrogen mixed with air and sulphurous anhydride first 

 pass along the horizontal tubes T into the receiver B B, which is partially 

 cooled by water flowing in on the right-hand side and running out on 

 the left by o, in order to reduce the temperature of the gases entering 

 the chamber. The gases then pass up a tower filled with coke, and 

 shown to the left of the drawing. In this tower are placed lumps of 



They may be easily expelled from this solution by heating or by dilution with water, as 

 they are only slightly soluble in aqueous sulphuric acid. Besides which, sulphurous 

 anhydride acts on such sulphuric acid, being oxidised at the expense of the nitrous anhy- 

 dride, and forming nitric oxide from it, which again enters into the cycle of action. 

 Therefore the sulphuric acid which has absorbed the oxides of nitrogen escaping from 

 the chambers in the tower K (see fig. 50) is led back into the first chamber, where it 

 comes into contact with sulphurous anhydride, by which means the oxides of nitrogen 

 are reintroduced into the reaction which proceeds in the chambers. This is the use of 

 the towers (Gay-Lussac's and Glover's) which are erected at either end of the chambers. 



