292 NITROGEN.' 



ascertained by Cavendish, in 1785. He succeeded in forming 

 nitric acid from its elements, by transmitting a succession of 

 electric sparks during several days, through a small quantity of 

 air, or through a mixture of 1 volume of nitrogen and 2J volumes 

 of oxygen, confined in a small tube over water, or over solu- 

 tion of potash ; in the last case the absorption of the gases was 

 complete and nitrate of potash was obtained. A trace of this acid 

 in combination with ammonia has been detected in the rain of 

 thunder storms, produced probably in the same manner. It was 

 also observed by Gay-Lussac to be the sole product when nitric 

 oxide is added, in a gradual manner to oxygen in excess over 

 water ; the gases then unite and disappear in the proportion of 4 

 volumes of the former to 3 of the latter. It is also a consti- 

 tuent of the salt, nitre or saltpetre, found in the soil of India and 

 Spain, which is a nitrate of potash, and also of nitrate of soda 

 which occurs in large quantities in South America. 



Preparation. This acid cannot exist in an insulated state, 

 but is always in combination with water, as in aqua fortis or 

 the hydrate of nitric acid, or with a fixed base, as in the 

 ordinary nitrates. The hydrate, (which is popularly termed 

 nitric acid,) is eliminated from nitrate of potash by means of oil 

 of vitriol, which is itself a hydrate of sulphuric acid. That acid 

 unites with potash, in this decomposition, and forms sulphate 

 of potash, displacing nitric acid, which last brings off in com- 

 bination with itself the water of the oil of vitriol. There is a 

 great advantage, first pointed out by Mr. Phillips, in using two 

 equivalents of oil of vitriol to one of nitrate of potash, which is 

 97 of the former to 100 of the latter, or nearly equal weights. 

 The acid and salt, in these proportions, are introduced into a 

 capacious plain retort, provided with a flask as a receiver. 

 Upon the application of heat, a little of the nitric acid first 

 evolved undergoes decomposition, and red fumes appear, but 

 soon the vapours become nearly colourless, and are easily con- 

 densed in the receiver. During the whole distillation, the tem- 

 perature need not exceed 260. The mass remains pasty till all 

 the nitric acid is disengaged, and then enters into fusion ; red 



O O ? * 



vapours again appearing towards the end of the process. If the 

 neck of the retort now be heated, the residuary salt, while still 

 fluid, may be poured out into a bason ; it is the bisulphate of 

 potash, which may be used for different purposes after it has 

 solidified. The rationale of this important process is exhibited 

 in the following diagram : 



