They usually take the form of cast iron open stoves fitted with a number of Bunsen burners which heat perforated lumps of asbestos.
A water pipe of copper or wrought iron is passed through a cylinder in which gas or oil heating burners are placed.
The supply of the nitric acid required to make up this loss is obtained in England by "potting" that is, by decomposing solid nitrate of soda by sulphuric acid in a flue between the pyrites burners and the chambers.