NITRIC ACID. 45 



This is the nitrous add in a gaseous form. This 

 acid gas is extremely absorbable by water, which is 

 at first rendered green : then, as the absorption goes 

 on, it becomes blue, and, finally, of an orange 

 colour : by adding more water, it may be brought 

 back to the green colour. 



If dry nitrate of lead be distilled, an . orange- 

 coloured liquid comes over; which is considered as 

 nitrous acid nearly pure. 



Nitric Acid. 



This acid is one of those which have been longest 

 known to chemists. It is so named from nitre, 

 from, which it was procured. The corrosive acid 

 called aqua fortis is an impure and weak nitric 

 acid: but it was long used before its analysis was 

 known: this we owe to Cavendish. 



Nitre, called also saltpetre, consists of this acid 

 united with potass, and the process for procuring the 

 acid depends upon decomposing this salt. For this 

 purpose, some substance is added to the nitre that 

 has an attraction for the potass sufficiently strong to 

 overcome that of the nitric acid, and, consequently, 

 to allow it to be expelled by heat. Sulphuric acid 

 is used for this purpose. During its expulsion, 

 however, the nitric acid suffers a partial decompo- 

 sition ; for though it is nitric acid that exists in the 

 salt, it is nitrous acid that condenses in the receiver 

 in the form of orange fumes. This is the common 

 nitrous acid of commerce. 



To convert this into nitric acid, another process 

 is necessary. By distillation, nitrous acid gas is 

 driven off, leaving the nitric acid colourless. Or, 



