44 NITROUS ACID. 



Nitrous acid gas is very easily absorbed by 

 water, rendering it a green, sour liquid. 



When nitrous gas is mixed with atmospheric air, 

 the same red fume appears, from the nitrous gas 

 uniting to the oxygen of the atmosphere, leaving 

 the nitrogen by itself. Hence this gas has been 

 used for measuring the quantity of oxygen gas con- 

 tained in air, and an instrument for this purpose 

 has been called an Eudiometer. A tall glass tube, 

 sealed at one end, is used for this purpose, filled with 

 water, and inverted in the pneumatic apparatus. 

 Send up a certain measure of the air to be examined 

 into this tube, and mark the space which it occu- 

 pies in the top of the tube. Then add to it a mea- 

 sure of nitrous gas, and observe the degree of 

 diminution. By comparing how much different 

 specimens of air will be di"minished by the same 

 quantity of nitrous gas, the relative quantities of 

 oxygen in each may be estimated. This method of 

 analysing atmosphere is not considered as very cor- 

 rect, and other modes are sometimes used, where 

 there are fewer sources of inaccuracy. 



Nitrous Acid. 



Considerable uncertainty prevails respecting this 

 acid. The yellow coloured fuming acid, to which 

 this name has been given, appears to be only nitric 

 acid holding nitrous gas in solution. The nitrous 

 gas may be expelled from it by the application of 

 heat, and then the nitric acid is left colourless. 



But if nitrous gas and oxygen gas be mixed 

 together, without the access of water, by intro- 

 ducing them into a vessel previously exhausted of 

 air, an union takes place ; the two gases diminish 

 to half the volume, and an acid gas is produced. 



