SULPHURIC ACID. 323 



But sulphuric acid is prepared, in vastly greater quantity, by 

 the oxidation of sulphur. When burned in air or oxygen, 

 sulphur does not attain a higher degree of oxidation than 

 sulphurous acid, but an additional proportion of oxygen may 

 be communicated to it by two methods, and sulphuric acid 

 formed. 



1. When a mixture of sulphurous acid and air is made to 

 pass over spongy platinum at a high temperature, the sulphu- 

 rous acid is converted into sulphuric acid at the expense of the 

 oxygen of the air. Mr. Peregrine Phillips, who first made 

 this observation, has founded upon it a method of preparing 

 sulphuric acid on the large scale, which, although not yet suffi- 

 ciently tried to establish its advantage as a manufacturing pro- 

 cess, is still of great interest in a scientific point of view, and 

 deserves consideration. Sulphur is burned, or iron pyrites in 

 place of it, and the sulphurous acid produced, is mixed with an 

 excess of air, by a blowing apparatus, and carried through a 

 tube filled with the platinum sponge or balls of fine platinum 

 wire. The vapours of sulphuric acicl formed, which are mixed 

 with the nitrogen of the air, are condensed in a long and narrow 

 vessel of lead, in an upright position, filled with pebbles, which 

 are kept constantly wet by a small stream of water, admitted at 

 the top and w r hich percolates downwards. 



2. Sulphurous acid mixed with air may likewise be con- 

 verted into sulphuric acid, by the agency of nitric oxide, which 

 is the process generally pursued in the manufacture of that 

 acid. The theory of this latter method, which is by no means 

 obvious, was established by the researches of Clement De- 

 sormes and of Sir H. Davy. When nitric oxide mixes with air 

 in excess, it instantly combines with oxygen, and becomes in 

 a great measure peroxide of nitrogen, or NO 4 . If dry sulphu- 

 rous acid gas, SO 2 , be mixed with that compound, no change 

 occurs, the two gases when dry having no action upon each other. 

 But if a little moisture, in the state of vapour be admitted to 

 the mixture, then oxygen is transferred from the peroxide of 

 nitrogen to the sulphurous acid, the former becoming nitrous 

 acid NO 3 , and the latter sulphuric acid SO 3 ; and these two 

 acids, in combination with each other and with a portion of 

 water, precipitate as a crystalline solid, a kind of sulphate of 

 nitrous acid, of which the exact composition has been already 

 given (page 289). The effect of an additional small quantity of 



y 2 



