HYPO-NITROUS ACID. 457 



In the middle of the process, when the first effects of the sulphu- 

 ric acid are over, and the materials have not as yet become very hot, 

 the nitric acid passes, without decomposition, and by changing 

 the receiver, we obtain it nearly colorless. If any combustible is 

 mixed with the materials, the red fumes are much increased, as the 

 acid is then decomposed more rapidly than before. 



(c.) In all cases where nitric acid acts on combustibles, or on 

 metals, it becomes colored, and emits red fumes, especially if in con- 

 tact with the atmosphere ; this is owing to the generation of nitrous 

 acid, in consequence of the extrication of nitric oxide gas, and its 

 subsequent reoxigenation to produce nitrous acid. 



(d.) It follows, that nearly all the acids of the nitric family, 

 found in the shops, and in the arts, and all that are colored, are 

 mixtures of nitric and nitrous acids ; but the nitric acid usually pre- 

 vails, and such acids by uniting with bases, form true nitrates ; still it 

 is true that the purest and strongest nitric acid, and the purest and 

 strongest nitrous acid are scarcely known, except in the hands of the 

 philosophical chemist ; the pale acid of the shops is usually a nitric 

 acid, diluted, more or less, with water ; and all the colored acids, 

 may, by additional dilution, or by the proper application of heat, be 

 brought to the condition of nitric acid. 



(e.) Still, although there are many varieties in the weight, color, 

 fuming properties, and energy of the nitrous acids of the arts, we 

 must not suppose that there is a great diversity of real nitrous acids, 

 and that the nitric oxide and oxygen " can unite in every propor- 

 tion" within certain limits. " The true explanation is, that the 

 mixture of these gases may give rise to three compounds, the hyponi- 

 trous, the nitrous, and the nitric acids, and that if certain precautions 

 are adopted, either of them may be formed, almost if not entire- 

 ly, to the exclusion of the others."* 



HYPO-NITROUS ACID. 



1. NAME AND HISTORY. Called by some per-nitrous, but hypo- 

 or sub-nitrous seems the most proper name, since it is less energetic 

 as an acid than the nitrous and nitric, and also contains less oxygen. 

 First obtained by Mr. Dalton,f and Gay-Lussac.f 



2. PROCESS. 



(a.) Mingle over mercury, in a glass tube, containing a strong so- 

 lution of pure potassa, 400 measures of nitric oxide gas, with 100 

 of oxygen. The compound thus formed, will be absorbed by the 

 alkali, and is supposed to be the hypo-nitrous acid. 



(6.) If 100 measures of nitric oxide gas be exposed for three 

 months to a solution of pure potassa, over mercury, 25 measures of 



* Turner. 



t Thomson's Ann. Vol. X. * Ann. de Chim. et de Phys. Vol. I, p. 400. 



58 



