NITRIC ACID. 297 



Nitric acid in a solution cannot be detected by precipitating 

 that acid in combination with any base, as the nitrates are all 

 soluble, so that tests of another nature must be had recourse 

 to, to ascertain its presence. A highly diluted solution of sul- 

 phate of indigo may be boiled without change, but on adding to 

 it at the boiling temperature, a liquid containing free nitric acid, 

 the blue colour of the indigo is instantly destroyed. If it is a 

 neutral nitrate which is tested, a little sulphuric acid should be 

 added to the solution, to liberate the nitric acid, before mixing 

 it with the sulphate of indigo. It is also necessary to guard 

 against the presence of a trace of nitric acid in the sulphuric 

 acid. Another test of the presence of nitric acid has lately been 

 proposed by de Richemont. The liqaid containing the nitrate 

 is mixed with rather more than an equal bulk of oil of vitriol, 

 and when the mixture has become cool, a few drops of a strong 

 solution of protosulphate of iron are added to it. Nitric oxide 

 is evolved, and combines with the protosulphate of iron, pro- 

 ducing a rose or purple tint even when the quantity of nitric 

 acid is very small. One part of nitric acid in 24,000 of water 

 has been detected in this manner. Free nitric acid also is 

 incapable of dissolving gold-leaf, although heated upon it, but 

 acquires that property when a drop of hydrochloric acid is added 

 to it. But in testing the presence of this acid, it is always 

 advisable to neutralize a portion of the liquor with potash, and 

 to evaporate so as to obtain the thin prismatic crystals of 

 nitre, which may be recognized by their form, by their cooling 

 nitrous taste, their power to deflagrate combustibles at a red 

 heat, and by the characteristic action of the acid they contain, 

 when liberated by sulphuric acid, upon copper and other metals, 

 in which ruddy nitrous fumes are produced. If nitric acid be 

 rigidly pure, it may be diluted with distilled water, and is not 

 disturbed by nitrate of silver, nor by chloride of barium,' the 

 first of which detects the presence of hydrochloric acid by 

 producing a white precipitate of chloride of silver; the last 

 detects sulphuric acid by forming the white insoluble sulphate 

 of barytes. The fuming nitric acid may be freed from hydro- 

 chloric acid, by retaining it warm on a sand-bath for a day or 

 two, when the chlorine of the hydrochloric acid goes off as gas. 

 To free it from sulphuric, it should be diluted with a little 

 water, and distilled from nitrate of barytes ; but the process for 



