xx DISSOCIATION 517 



of silver and mercury were formed on a cold tube of 

 amalgamated silver and several cubic centimetres of 

 hydrogen were carried forward (Comptes rendus, 1865, 60, 



3177325)- 



Similar changes were produced by sparking. Sulphurous 

 anhydride was condensed completely, SO 2 -> S + 2SO 3 , by 

 sparking over sulphuric acid, which absorbed the sulphuric 

 anhydride as fast as it was formed. A mixture of sulphurous 

 anhydride with oxygen was absorbed still more readily : 



-> 2SO 3 . 



FIG. 55. DEVII.LE'S "HOT-COLD" TUBE. 



PP is a hot tube of porcelain. LL is a brass tube cooled with water. The gas to be 

 decomposed passes in at T and out at T'. 



Hydrogen chloride, which had been shown by Davy fifty 

 years previously (Ch. XI, p. 224) to give calomel and one- 

 half its volume of hydrogen when sparked over mercruy, 

 was brought into the category of dissociable substances by 

 Deville, who confirmed Davy's earlier observation. 



Dissociation of ammonia. In the course of these experi- 

 ments Deville discovered that the decomposition of ammonia 

 into nitrogen and hydrogen by sparking as used by Berthollet 

 to establish its composition (p. 262) was a reversible change. 

 By using an acid to absorb the ammonia as fast as it was 



