2NO2 + 2SO2 + 2H2O = 2H2SO4 + 2NO.
This deportment of the nitric oxide being
continuous, it will be seen it acts the part of a carrier of oxygen from the atmospheric air contained in the chamber to the sulphurous acid, and by so doing (theoretically) renders any further supply of nitrate of soda than that required to start the process unnecessary.
As soon as the water, or rather liquid acid on the floor of the leaden chambers, has acquired the sp. gr. of 1·35 to 1·50, it is drawn off, and concentrated by boiling in shallow leaden pans to the density of about 1·72, after which it is further concentrated in green-glass or platinum retorts, until the sp. gr. reaches 1·842 to 1·846. When of sp. gr. from 1·35 to 1·50 it is called chamber acid, and when of the last strength, is used in the manufacture of salt-cake, sulphate of ammonia, some kinds of manure, and nitric acid. Sulphuric acid of sp. gr. 1·720 is mostly employed in the preparation of superphosphate of lime. After concentration to 1·842 or 1·846, the clear acid is put into large globular bottles of green glass (carboys), surrounded with straw and basket-work, and is sent into the market under the name of ‘oil of vitriol.’
The leaden chambers in which the chemical changes take place, that result in the formation of the acid, vary greatly in dimensions in different works, being sometimes as much as 12 or 15 feet high, 15 or 20 wide, and from 150 to 300 feet long. They are mostly partially divided by incomplete leaden partitions, known as curtains, so arranged on the roof and the floor as to cause the currents of mixed gases to come into collision, and thus cause their admixture. Where there are a number of small separate chambers they are connected by means of leaden tubes. A chamber having a capacity of 25,000 cubic feet will yield 10 tons of acid weekly.
The sheets of lead used in the construction of the chambers are united by fusion, or melting together of their edges. If cement were used it would be speedily attacked and destroyed by the acid and gaseous products.
The process for the manufacture of sulphuric acid above described, was devised in 1774 by a calico printer of Rouen, and improved by Chaptal.
In 1776 the first vitriol factory was set up at Prestonpans, by Dr Roebuck, of Birmingham, with whom originated the idea of the leaden chambers.
Various attempts have been at different times unsuccessfully made to supersede the old process. Of these we may mention:
1. The proposal to oxidise sulphurous acid by means of chlorine in the presence of steam.