178 CHEMISTRY. 



into water it hisses like red-hot iron, and forms sulphuric 

 acid : 



Sulphuric anhydride. Water. Sulphuric acid. 

 SO, + H a O = H 3 SO 4 



This anhydride dissolved in common sulphuric acid forms 

 the so-called Nordhausen or fuming sulphuric acid, gen- 

 erally obtained by distilling ferrous sulphate (sulphate of 

 iron) : 



Ferrous sulphate. Ferric oxide. Sulphuric Sulphurous 



anhydride. anhydride. 



2(FeSOJ Fe 3 3 + SO 3 + SO 3 



When water is present, and this is generally the case, some 

 of the sulphuric anhydride dissolves in it, forming sulphuric 

 acid (as above), and some of this anhydride dissolves in the 

 acid thus formed. This acid is called Nordhausen acid, after 

 the town in Saxony where it has been made for many years. 

 This was one of the earliest ways of making sulphuric acid, 

 but common oil of vitriol is manufactured differently. 



242. Manufacture of Sulphuric Acid. This acid has one 

 more atom of oxygen in it than sulphurous acid, and there- 

 fore can be made from the latter by adding this amount of 

 oxygen to it. When sulphur burns it forms sulphurous an- 

 hydride, and will not take up any more oxygen from the 

 air; hence this oxygen must be added in an indirect manner, 

 as shown in the following sentences : Sulphurous anhydride 

 produced by burning sulphur, or by roasting iron or copper 

 pyrites, is conducted together with steam into an apartment 

 lined with lead, on the floor of which is some water. But 

 there is sent in with this some nitric acid; this acid on 

 meeting the sulphurous anhydride gives up to it a part of 

 its oxygen, and the sulphuric anhydride thus formed dis- 

 solves in the watery vapor present, making sulphuric acid. 

 Meanwhile the nitric acid, having parted with some of its 



