It was next discovered that by mixing with the sulphur some
nitre, much more sulphuric acid could be produced per given quantity of
brimstone. At first large glass carboys were used, but in 1746 the
carboys were replaced by chambers of lead containing water at the
bottom, and in these lead chambers the mixture of sulphur and nitre was
burnt on iron trays. Next, although gradually, the plant was divided
into two portions--a furnace for burning the sulphur, and a chamber for
receiving the vapours. The system was thus developed into the one
followed at the present time. The sulphur, or, in most cases, cupreous
iron pyrites (a combination of iron and copper with sulphur), is burned
in specially constructed kilns or furnaces, and the hot gases,
consisting essentially of sulphur dioxide with the excess of air, pass
through flues in which are placed cast-iron "nitre pots" containing a
mixture of nitre (sodium nitrate) and vitriol. The gases thus become
mixed with nitrous fumes or gaseous oxides of nitrogen, and, after
cooling, are ready for mixing with steam or water spray in the lead
chambers in which the vitriol is produced.
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