DECOMPOSITION OF SULPHUROUS ACID. 479 



carbon, gaseous sulphur reduced to its normal molecular weight, 

 instead of sulphur of triple density volatilisable towards 448). 



It may be first noted that sulphur burning in dry oxygen 

 produces sulphurous acid, mixed with a considerable proportion 

 of anhydrous sulphuric acid. Sulphur vapour directed upon 

 charcoal at a red heat combines with it, producing carbon 

 sulphide. 



Carbon burnt in oxygen produces carbonic acid, always 

 mixed with a little carbonic oxide. 



Carbonic acid directed upon red-hot charcoal is changed into 

 carbonic oxide ; but the transformation is never complete. 



2. Decomposition of the sulphurous gas. A series of electric 

 sparks decompose sulphurous acid gas into sulphur and sulphuric 

 acid (Buff and Hoffman) 



3S0 2 = 2S0 3 + S. 



Operating in a sealed tube without mercury, with platinum 

 electrodes, several hours are needed to decompose the half of 

 the gas, and decomposition ceases at a certain point, as was 

 observed by Deville. It does not yield free oxygen, but a 

 portion of the sulphur unites with the platinum. 



The greater portion of the sulphur forms with anhydrous 

 sulphuric acid a special viscid compound, which, moreover, 

 absorbs a certain quantity of sulphurous gas. This compound 

 is the real medium of the reaction. Being inversely decom- 

 posable, the tension of sulphurous and sulphuric gases which it 

 gives off limits the reaction. 



3. Decomposition of the carbonic oxide. Carbonic oxide under 

 the influence of the spark, or even of a white heat, partly 

 decomposes into carbon and carbonic acid 



2CO = C0 2 -f C. 



But the reaction is limited to a few thousandth parts. It was 

 found that it takes place at a bright red heat, and even at the 

 temperature of the softening of glass. The carbon is deposited 

 at the point where the porcelain tube issues from the furnace, 

 and undergoes a lowering of temperature, even without having 

 recourse to the artifice of the hot and cold tube. It may be 

 still better shown by placing fragments of pumice-stone in this 

 region of the tube. A trace of carbonic acid produced at the 

 same time may be observed in the gases collected by adopting 

 certain precautions. 



Though so slight and inappreciable, this reaction is, never- 

 theless, of great importance ; for it intervenes, together with the 

 dissociation of the carbonic gas into carbonic oxide and oxygen, 

 in the reduction of the metallic oxides and in a great number of 

 other reactions, brought about by heat. We will now place 

 sulphur and carbon, whether free or combined, together. 



4. Sulphurous acid gas and carbon (baker's embers calcined 



