COMPOUNDS >K MTKot.KN WITH II YIHIOCKN AND OXYGEN 263 



be in excess (although but small) to the end of the reaction. 

 According to the equation expressing the reaction, it is required that 

 there should be 98 parts of sulphuric acid to 85 parts of sodium 

 nitrate ; but if these quantities be taken, the nitric acid is not entirely 

 displiu ('(! by the sulphuric acid. It is necessary that an excess of the 

 latter should be taken ; generally, 80 parts of nitre are taken to 

 98 parts of sulphuric acid, and therefore a portion of the sulphuric 

 acid remains in a free state to the end of the reaction. Thus, in 

 the reaction of sulphuric acid on nitre there is formed a n on- volatile 

 salt of sulphuric acid, which remains, together with an excess of this 

 acid, in the distilling apparatus, and nitric acid, which is converted 



Fin. 17. Method of preparing nitric acid on a large scale. A cast-iron retort, C, is fixed into the 

 furnace, and heated by the fire, B. The flame and products of combustion are at first lei along 

 the flue. M(in unler to heat the receivers), and afterwards into L. The retort is charged with Chili 

 .-altpctre and sulphuric acid, and the cover is luted on with clay and gypsum. A clay tube, a, is 

 fixed into the nock of the retort (in order to prevent the nitric acid from corroding the ca>t iron ), 

 and a bent glass tube, D, is luted on to it. This tube carries the vapours into a series of earthen- 

 ware receivers, K. Nitric acid mixed with sulphuric acid collects in the first. The purest nitric 

 acid is procured from the second, whilst that which condenses in the third receiver contains hydro- 

 chloric acid, and that in the fourth nitrous oxide. Water is poured into the last receiver in order 

 to condense the residual vapours. 



into vapour, and may be condensed, because it is a liquid and volatile 

 substance. On a small scale, this reaction may be carried on in a glass 

 retort with a glass condenser. On a large scale, in chemical works, the 

 process is exactly similar, only iron retorts are employed for holding 

 the mixture of nitre and sulphuric acid, and earthenware three-necked 

 bottles are used instead of a condenser, :i as shown in fig. 47. 



50 It must be observed that sulphuric acid, at least when undiluted ((50 Baume*), 

 corrodes cast iron with difficulty, so that the acid may be heated in cast-iron boilers. 



