OXIDATION BY PEROXIDES. 147 



attention to the chemical properties of iodine in comparison 

 with those of its intimate congener, chlorine. As I shall pre- 

 sently show you, both elements possess in a striking degree the 

 property of oxidising various substances which resist the action 

 of ordinary oxygen ; and this observation leads me to make a 

 few preliminary remarks upon the process of oxidation in 

 general. 



(155.) It is well known that many oxidisable bodies which 

 are unable or scarcely able to combine directly with free oxygen, 

 can nevertheless combine on the instant with oxygen that is 

 already in a state of combination. It seems, indeed, as if the 

 fact of previous combination conferred upon the transferable 

 oxygen a greater activity or tendency to unite with other bodies. 

 Of this peculiarity of behaviour, the non-oxidation or slow oxi- 

 dation of sulphurous acid H 2 S0 3 , into sulphuric acid H 2 S0 4 , by 

 mere exposure to oxygen or air, and its rapid oxidation by means 

 of certain hyper-oxygenised compounds, such as the peroxides 

 of hydrogen and nitrogen, affords us an excellent illustration. I 

 have here a freshly made solution of sulphurous acid, and to it I 

 add a little chloride of barium, which, you observe, does not in 

 the least disturb its transparency, thereby showing its freedom 

 from any trace of sulphuric acid. I now draw a rapid current of 

 air through the mixed liquid, but with no obvious effect. The 

 sulphurous acid and oxygen, despite their agitation together, 

 remain sulphurous acid and oxygen, instead of combining with 

 one another to form sulphuric acid. Accordingly, we have not 

 enough sulphuric acid produced to afford even a turbidity 

 with the previously added barium-salt, although, by a pro- 

 longed agitation with one another, some sulphuric acid would 

 be slowly formed. I now divide the mixed solution of sul- 

 phurous acid and chloride of barium, and to one portion add a 

 little peroxide of hydrogen, when immediately we get a copious 

 white precipitate of sulphate of barium, showing that the sul- 

 phurous acid, which would not unite with the free oxygen of the 

 air, has united at once with the combined oxygen of the peroxide, 

 thus : 



L2 



