450 NITROUS ACIDS. 



time, is astonished at the deep blood red color of the fumes, and 

 the rapid absorption, especially when oxygen gas is employed. In 

 this case, the hand laid upon the jar, in which the combination is 

 going on, is sensible of considerable heat. 



3. Lift out of the pneumatic cistern, a large air jar, filled with ni- 

 tric oxide gas, having previously slipped under it a pane of window 

 glass ; reverse its position, and suddenly remove the glass plate ; im- 

 mediately a dense cloud of red nitrous acid vapor will rise from the 

 mouth of the jar, and the hand placed in the current, will be warm- 

 ed. The acid will soon disappear, being absorbed by the watery 

 vapor in the atmosphere. 



NITROUS ACIDS. 



1. General Explanation. It is obvious, from the statements that 

 have been made, that whenever nitric oxide gas is mingled with free 

 oxygen gas, nitrous acid is produced, and thus these gases become 

 very delicate tests of the presence of each other. It is also true, 

 that nitric oxide willl sometimes detach oxygen gas from combina- 

 tion, and form with it nitrous acid. 



(a.) Prepare a flask with a tube bent twice at right angles, thus : 

 in the flask A, place the copper and diluted ni- 

 tric acid : in the bottle B, some pale colorless 

 nitric acid. As soon as the nitric oxide gas be- 

 gins to be evolved, the pale acid will change its 

 color, and pass rapidly through many shades of 

 yellow, ending with deep green, while blood red 

 fumes will rise from the surface. These chang- 

 es are owing to the absorption of the nitric ox- 

 ide gas, by the nitric acid ; this is at the same 

 time partially decomposed, giving oxygen to 

 the nitric oxide gas, which is thus converted into nitrous acid, and in 

 this state mingles with the still undecomposed nitric acid, and thus 

 presents a variety of shades of color ;* " even a little more than 1 

 per cent, being sufficient to impart a pale yellow color." 



(b.) In the process for nitric acid from the nitrate of potash and 

 sulphuric acid, as already described, p. 447 it is now obvious that the 

 red fumes which appear slightly at the beginning, and abundantly at 

 the end of the distillation, are owing to the decomposition of a por- 

 tion of nitric acid, giving both nitric oxide, and oxygen gas, which 

 again unite in different proportions from the original ones, and thus 

 produce fuming nitrous acid. 



* Heat, gradually and long applied, will discharge this color, and dilution with 

 water does it instantly, while red fumes are emitted. 



