ALTERATIVE ACTION OF IODINE. 149 



the sulphurous acid, which is of itself unable to combine directly 

 with atmospheric oxygen, instantly robs the peroxide of nitrogen 

 of the oxygen which it had absorbed directly from the atmosphere, 

 as in the ordinary manufacture of sulphuric acid, thus : 



Sulphurous Nitric perox. Sulphuric Nitric oxide 



H a S0 3 + N0 a = H a S0 4 + NO. 



(157.) Nitric oxide and peroxide, then, are the types of chemi- 

 cally mobile compounds. Eegarded as a reducing agent, there 

 are many more powerful de-oxygenants than the oxide ; regarded 

 as an oxidising agent, there are many more energetic oxyge- 

 nants than the peroxide ; but there are no two associated bodies 

 known to chemists which respectively absorb and evolve oxygen 

 with so much facility. As remarked by Laurent, ' There is no 

 substance which presents such singular properties as nitric oxide. 

 i . - . It is, perhaps, the only body which, in the dry state and 

 at the ordinary temperature, can combine suddenly with oxygen. 

 The combination, moreover, takes place without the evolution of 

 heat, and the body which results, far from retaining the oxygen 

 that it has so readily absorbed, is perhaps of all bodies the one 

 which is deoxidised most easily.' Now, while chlorine may be 

 compared to peroxide of hydrogen, it is the sort of chemical 

 mobility manifested by the oxides of nitrogen, which is charac- 

 teristic of iodine, and, I believe, of most mineral alteratives. 

 Iodide of hydrogen or potassium is, like nitric oxide, a facile 

 reducing agent, and free iodine or hypiodite of potassium, like 

 peroxide of nitrogen, a facile oxygenant so that wherever the 

 iodine travels it is capable of influencing the processes of oxida- 

 tion there going on absorbing oxygen where there is excess, 

 delivering active oxygen where there is deficiency just as our 

 nitric compound absorbs oxygen from the air and delivers it up 



