and on the Crystalline Solid formed in that Process, 121 



escapes during the effervescence ; and during the boiling, no 

 red vapours appear if free oxygen gas is excluded from the 

 vessel. The above experiments were reversed by passing the 

 vapour from sulphuric acid and nitre over a weak solution 

 of sulphurous acid in water. No effervescence took place, ex- 

 cept when a drop of strong nitric acid fell from the beak of 

 the retort into the liquid. The nitrous acid vapour was ra- 

 pidly absorbed by the sulphurous acid solution, and whatever 

 nitric acid was formed by the action of the water remained 

 undecomposed. 



It is evident that by adding oil of vitriol to weak nitric 

 acid in a solution of sulphurous acid gas in water, the nitric 

 acid can be decomposed ; the oil of vitriol acting to concen- 

 trate the nitric acid. Now, in the manufacture of oil of vitriol, 

 either by intermitting or continuous combustion, nitric acid 

 is formed by the action of the house-acid water, either on the 

 white solid, or on the absorbed nitrous vapour. As the lead- 

 house acid increases in gravity, it is equivalent to adding oil of 

 vitriol to weak nitric acid : hence, if the process be continued 

 long enough, all the nitric acid will be decomposed : the pro- 

 per remedy is, to burn sulphur only, to add sulphurous acid gas 

 alone. The decomposition of the nitric acid seems to be per- 

 fected when the house-acid is of a dark-bromi colour, a change 

 owing to the absorption of sulphurous acid gas. I made a 

 solution of the crystalline solid in water, and passed a current 

 of sulphurous acid gas through it, to test the truth of the 

 above opinion. By continuing the action of the sulphurous 

 acid gas till all effervescence was over, — from green, the liquid 

 became brown, and showed no trace of nitric acid. Pale yel- 

 low acid is not uncommon in a lead-house. I have seen it 

 also deep green and deep yellow. At the sp. gr. 1*58 or 1*53, 

 all these coloured acids effervesce briskly : when a current of 

 sulphurous acid gas is passed through them, they become 

 light-coloured, white, and then speedily turn brown ; the ac- 

 tion ceasing, there are no traces of nitric acid in them. The 

 dark colour of lead-house acid is then evidently due to ab- 

 sorbed sulphurous acid gas. When all the nitric acid is decom- 

 posed, the house-acid is tinged only brown, and by boiling, 

 the sulphurous acid, which it holds in solution, is driven off; 

 as the workmen say, " the kettle clears off," the liquid be- 

 comes pellucid. As long as there are any bubbles in the li- 

 quid from the chamber, the nitric acid is not wholly decom- 

 posed ; the colour and appearance then are important guides 

 to the manufacturer at what period to draw oft' his acid. A 

 quiet, still, pale-brown liquor is the desirable point. If the 

 acid is drawn from the lead-house after a few days' burning, 



Third Scries. Vol. 3. No. 14. Aug. 1833. R 



