1892.] Formation and Decomposition of Nitrous Acid. 41 



excess of the peroxide separates out (the red oil). According to 

 the experiments of Peligot, nitric oxide passed into this gives a 

 green liquid. The absorption and retention of the nitrogen per- 

 oxide accounts for the gain in weight (about one-sixth) of the acid. 

 The reduction then proceeds further, and nitrous acid (the blue 

 liquid) is produced, thus : 



N 2 4 +2JTO-i-2H 2 = 4HN0 2 , 



and this is evaporated by the continued current of the nitric oxide, 

 leaving finally only the water originally present, both free and com- 

 bined, in the nitric acid. This destruction of the nitric acid accounts 

 for the loss in weight. As Priestley wrote : " Towards the end of the 

 process (of absorption of nitrous air by pale-yellow spirit of nitre), 

 the evaporation of the acid was perceived to be very great, and when 

 I took it out the quantity was found to be diminished by one-half . 

 Also, it had become, by means of the process and evaporation 

 together, exceedingly weak, and was rather blue than green." 



Reaction between Nitric Oxide and Mercuric Nitrate in Nitric Acid 



Solution. 



In my former paper* it was shown that when a current of hydro- 

 gen was passed through a solution of mercuric nitrate in nitric 

 acid solution, the mercuric was reduced to a mercurous salt with- 

 out any considerable formation of nitrous acid, which was, however, 

 produced in abundance when cupric nitrate was subjected to the 

 same action. As it seemed probable that the nitrous acid which 

 might have been produced in the former operation was in reality used 

 up in the reduction of the mercuric salt, the experiment was re- 

 peated in precisely similar manner, nitric oxide being substituted 

 for hydrogen. It was found, as before, thab the mercuric salt was 

 reduced ; from O5 gram mercuric oxide dissolved in nitric acid 

 (1 c.c. containing 0*2549 gram acid) after passage of nitric oxide 

 for 3f hours at a temperature of 35, 0*019 gram of mercurous 

 chloride was obtained, while from a similar portion, through 

 which the nitric oxide had not been passed, 0*0018 gram was pre- 

 cipitated. It is probable, then, that the hydrogen in the former 

 experiments reduced at first the free nitric into nitrous acid, which 

 in its turn reduced the mercuric nitrate, thus : 



2Hg(N0 3 )H-2HN0 2 = Hg 2 (N0 3 ) 2 + 2HN0 3 . 



No free nitrous acid would therefore appear in the course of the 

 operation. 



* ' Phil. Trans.,' 1891, A, p. 315. 



